A community for students.

Here's the question you clicked on:

55 members online
  • 0 replying
  • 0 viewing

anonymous

  • 5 years ago

physics question, suppose two people pull on rope. person A is 50 kg and pulls towards the right (+) with 30 N force. person b who is 40 kg pulls towards the left with a 20 N force. assume the rope is massless. I dont get why the tension in the rope is the same magnitude. For the rope the forces on it are F1 - F2 = 0*a. but F1 and F2 should be the 30N and 20N , so it should be not zero but 10 N to the right for tension. something doesnt make sense here

  • This Question is Closed
  1. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You are suggesting that at one part of the rope, there is 10 newtons of tension, and at the other end there is 0 tension (i.e. slack)?

  2. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'm not sure why you're looking at the forces on the rope. You're assuming it's massless, so you can't really talk about the rope's acceleration.

  3. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hi

  4. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    well i was using my book. it says the forces on the rope are F1 - F2 = 0. isnt tension the force on the rope?

  5. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    What are you being asked to find?

  6. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    A rope is one element. It can't have variable force along it. So whatever the tension is in the rope, it has to be the same all along the rope.

  7. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Here is the full paragraph. "Suppose two people pull the ends of a rope with oppositely directed forces F1 and F2 (in bold). The rope also exerts forces -F1 and -F2 on the people (in bold)From the second law, we know F1 - F2 = ma , where m is the mass of the rope and a is its acceleration. If that acceleration is zero , or if the mass of the rope is so small that we can idealize the rope as having zero mass, then F1 - F2 = 0. In this special situation the forces exerted by the rope on the two people are equal in magntitude

  8. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I agree with that.

  9. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    well im wondering, what is the force on the rope , if it says F1 - F2 = 0, then F1 = F2, but 30 N does not equal 20 N

  10. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    What it's actually saying is that it's impossible for person A to be pulling with 30N of force and person B to be pulling with 20N of force and for there to be zero acceleration. But you know that already since: $$F=ma$$ If F were not 0, you'd have an acceleration.

  11. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    in my example, one guy pulls 30 N right and the other guy pulls 20 N left

  12. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I don't see where you got 30N and 20N

  13. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i made up an example

  14. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    In your example, there would be acceleration towards the guy pulling with 30N.

  15. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    you mean 10 N

  16. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    There would be a net force of 10N which would result in an acceleration in the direction of the guy pulling with 30N force.

  17. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Yes, but you are contradicting the hypothisis

  18. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    my question is, what is the force on the rope? and why does it say the force exerted by the rope on the two people is equal in magnitude. and which force is the tension. so there are 3 questions

  19. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    They are saying that If the rope is not accelerating, we know the forces on both ends are equal

  20. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    how do you get the forces at both ends are equal, and what is this force? the tension?

  21. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    and which force is it, what is exerting on what

  22. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    you said the forces at both ends are equal. what do you mean by that. the force that the rope exerts on the two people are equal?

  23. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No, the force on the rope.

  24. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but there are different forces, one is 30 N and one is 20 N

  25. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    which we stipulated

  26. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Yes, you made up that example, where the conclusion is false, and trying to make a claim about the implication.

  27. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the conclusion is false?

  28. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    you said the force on the rope at both ends is equal. clearly this is false, it is 20 N and 30 N , they are not equal

  29. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    So the acceleration is not 0

  30. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    wait, youre saying the net force on both ends is equal in magnitude?

  31. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but my rope is massless

  32. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'm saying IF the acceleration is 0, then the NET force on the rope is 0.

  33. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    why does the book say F1-F2 = 0, so F1 = F2,

  34. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You can't talk about force acting on massless things.

  35. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    what is F1 and F2 ?

  36. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    It says F1-F2 is 0 BECAUSE it claimed already that the acceleration is 0

  37. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so then F1 and F2 are zero?

  38. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No.

  39. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hmmm

  40. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    is tension the F1 and F2

  41. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    \[\sum F = ma\]

  42. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If a =0 then \[\sum F = F1-F2 = 0 \implies F1 = F2\]

  43. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    But if the forces do not sum to 0 than the acceleration of the rope will not be 0.

  44. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes but in my example, what is F1 and F2

  45. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    is the acceleration equal to 0?

  46. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    nomo, if you cant type , you should restart.

  47. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    www.openstudy.com start over

  48. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    A review of massless rope mechanics: A rope carries a tension. This tension is the force felt at the endpoints of the rope by whatever the rope is attached to. So, if you pull against a rope caught tied to a wall: |------ -> 30N the tension in the rope is 30N. But, by action/reaction, the wall is also pulling on the rope. And since there is no acceleration, it is pulling with an equal and opposite force of -30N. Now let's look at your example. You now have two people, pulling on the rope. One is pulling harder than the other. Without doing the math, it's obvious that whoever is pulling harder is going to move the other person. That's exactly what happens: 20N <--------> 30N --> everything moves this way So your next question is, what's the tension in the rope? Well, the tension in the rope has to be 20N, because it's the force the weaker puller is pulling with. The rest of the force of the stronger puller, 10N, goes into acceleration. So: 1. The tension in the rope is 20N. 2. The net external force on the rope is 10N. Hope this helps.

  49. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    lets see

  50. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i get a different answer

  51. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    30 N - T = 50kg * a -20 N + T = 40 kg * a

  52. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Heh. Cantorset, you are in fact correct.

  53. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Those are the equations of motion, and I should have written them out before posting.

  54. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so 10 N = 90 kg *a , so a = 1/9 m*s^-2

  55. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Yes, I agree.

  56. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    So what is T?

  57. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    one sec, my browser is crashing

  58. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    weird, one sec

  59. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    plug back in

  60. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so 30 - 50/9 N = T

  61. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    24.444 N is the tension

  62. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so natho was wrong

  63. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The problem with this exercise is that in the absence of any frictional forces, you 're going to have that both people accelerate toward each other which will remove the tension.

  64. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so what is F1 and F2, i still dont see

  65. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    So your acceleration is not constant.

  66. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    F1 and F2 are the tension forces?

  67. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    F1 is the force of one person pulling, F2 is the force of the other.

  68. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but thats false, because 30 - 20 is not 0

  69. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Who said it was = 0?

  70. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    is the a = 0?

  71. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The equations of motion that cantorset wrote are valid at the instant when both people start pushing, as are the results. However, polpak is correctly pointing out that this is a dynamic problem, not a static problem. Odds are you'll never actually find a problem like this in a problem set, because there is too much information missing to be able to give a complete answer.

  72. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    nomo, so do you agree that 24.444 is the tension?

  73. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    pol, the book answer

  74. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    In the instant when they both pull, (and in the absence of any friction) The tension is 24.444

  75. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Is the acceleration given to be 0?

  76. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    "Suppose two people pull the ends of a rope with oppositely directed forces F1 and F2 (in bold). The rope also exerts forces -F1 and -F2 on the people (in bold)From the second law, we know F1 - F2 = ma , where m is the mass of the rope and a is its acceleration. If that acceleration is zero , or if the mass of the rope is so small that we can idealize the rope as having zero mass, then F1 - F2 = 0. In this special situation the forces exerted by the rope on the two people are equal in magnitude, and the rope can be thought of as simply transmitting a force from one person to the otherl the force at any point in the rope is referred to as tension. It is the same everywhere in the rope only if the rope is unaccelerated or if is idealized as massless

  77. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No, but the rope is given to be massless

  78. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    So the acceleration is 0.

  79. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    But that's not a problem, it's a lesson.

  80. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You CANNOT have forces acting on massless things.

  81. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Your example is a problem, and it's a problem not covered by this lesson, since it clearly stipulates that the acceleration is 0; in your case, it isn't.

  82. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hmmm, so the book is contradicting itself?

  83. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No.

  84. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You are trying to contradict the book

  85. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    But not making a reasoned argument.

  86. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No, you're contradicting the book. You are constructing a situation where the acceleration cannot be zero, and trying to analyze the problem as if it could.

  87. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    as if it were 0.

  88. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok , i know the acceleration is not zero

  89. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    So then the book doesn't apply anymore.

  90. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Then the forces cannot be the same.

  91. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok lets look at the problem again. the forces on the rope is F1 - F2 = ma , the mass of the rope is neglible. so we have F1 - F2 = 0

  92. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Therefore the F1 = F2

  93. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If you contradict by saying \[F1 \ne F2\] then \[ma \ne 0.\]

  94. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the end.

  95. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but didnt we stipulate earlier that F1 = 30 N and F2 = 20 N

  96. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You cannot say that ma = 0 if f1 not equal f2

  97. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The sum of the forces IS ma.

  98. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok, hmmmm

  99. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If the sum is non-zero ma is non-zero. If ma is 0 the sum is 0. If the sum is 0 ma is 0. If ma is non-zero the sum is non-zero.

  100. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but in my problem, i started with 30 N and 20 N , and they applied the forces at either end, correct?

  101. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You can *only* assume the rope is massless *IF* F1=F2. Otherwise, the discussion in the book doesn't apply.

  102. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    So while there are real-world situations where F1 is not equal to F2, you can't analyze them using what the book says. It *only* applies for the special case where F1=F2.

  103. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but didnt we assume that the two guys are applying two different forces

  104. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    We did. But then you tried to use what the book told you. You can't do that.

  105. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If forces are non-zero then ma is non-zero.

  106. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Pick one or the other, but you can't just pick both to be different.

  107. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok let me try again, one sec

  108. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    person A is 50 kg and pulls towards the right (+) with 30 N force. person b who is 40 kg pulls towards the left with a 20 N force. assume the rope is massless.

  109. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You can't do that.

  110. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You cannot say the sum of the forces is not zero and ma is 0. You cannot say that ma is not zero but the sum of the forces is 0. Because the sum of the forces IS ma. If you pick one you are also choosing the other.

  111. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You can't assume the rope is massless.

  112. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    they do that in problems though, i thought

  113. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No.

  114. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    They do that when the forces are equal.

  115. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You are trying to do something like: Let \(1 = 0 \implies 2+1 = 2\) And then complaining because you know 2+1 should be 3.

  116. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i dont understand , what is wrong with my problem

  117. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You have ma = 0, but the sum of the forces is not 0.

  118. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You cannot do that.

  119. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If it is not accelerating then there is no net force on the object.

  120. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Things do not accelerate in the absence of a net force.

  121. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    im confused

  122. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Things with net forces acting on them MUST accelerate.

  123. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so what exactly is the contradiction in the question

  124. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    nothing wrong with assuming the rope is massless person A is 50 kg and pulls towards the right (+) with 30 N force. person b who is 40 kg pulls towards the left with a 20 N force. assume the rope is massless.

  125. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You try to claim \(m = 0 \implies ma=0\) But \[\sum F = ma\] And you pick two forces also that don't sum to 0.

  126. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    We are telling you THERE IS SOMETHING WRONG with that assumption.

  127. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok

  128. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so what is the force on the rope

  129. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Because what happens in reality is that ma is not 0, You will get a tension, the tension will pull the two people together

  130. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh you said there is no force on the rope, since its massless

  131. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'm not saying anything. You are choosing two sides of the equation that are non-equal

  132. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    wait, you cant have tension with a massless rope?

  133. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Which do you want, a rope that doesn't accelerate? or a net force?

  134. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Ive seen many tension problems where we assume the mass of the rope is negligible

  135. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Was the rope accelerating?

  136. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i dont know

  137. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I do.

  138. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    one sec, let me get a problem

  139. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Good.

  140. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    An important property of massless ropes is that the total force on the rope must be zero at all times. To prove this, we go back to Newton's Second Law. If a net force acts upon a massless rope, it would cause infinite acceleration, as a = F/m , and the mass of a massless rope is 0.

  141. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    http://www.sparknotes.com/physics/dynamics/newtonapplications/section3.rhtml

  142. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Right. So if you assume the rope is massless you have to have it not accelerating. If it is not accelerating the net force must be 0. If the net force is zero, each force acting on the rope must be cancelled by another foce.

  143. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so youre saying, if you have a 40 kg guy and a 50 kg guy and they are pulling on a rope in opposite directions, and the rope is massless, then

  144. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    then the force exerted by the rope on the two people must be equal

  145. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  146. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but then the two people, what about the force of the two people exerting on the rope , or the forces on the rope

  147. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The net force they exert on the rope must be 0.

  148. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but they can have different forces as well

  149. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Yes

  150. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but then by newtons third law, then that would force them to have the same force

  151. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the reaction force

  152. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Wait, no

  153. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    They cannot be exerting different forces.

  154. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If they were then the sum would not be 0.

  155. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok lets say we have a pulley and two masses

  156. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Ok.

  157. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    one mass is 50 kg, the other mass is 30 kg

  158. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    m1 = 50 kg, m2 = 30 kg

  159. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    they go up and down, like vertically

  160. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  161. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    simple pulley. the force on m1 are 9.8 *50 - T , where + is down direction

  162. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Certainly.

  163. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the force on m2 is T - 30 *9.8

  164. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but the problem says the mass of the cable and the pulley is neglible

  165. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    that means don't try to use it for finding the mass

  166. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    , wait, you said then the acceleration is zero

  167. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but the pulley is accelerating, i mean the cable is accelerating

  168. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You just keep talking in circles. What exactly is the question?

  169. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the force on the cable should be zero

  170. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    since it is massless, but there is force on the cable

  171. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    It's not massless.

  172. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    thats what the problem states

  173. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    It says ignore the mass, not that it's massless.

  174. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    In all these cases you are going to assume that the mass of the rope is ignorable, and that the tension in the rope is uniform.

  175. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i think it says the mass is zero

  176. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    It's telling you to ignore it because otherwise it becomes impossible to solve with your current tools. IT CANNOT BE 0 everything (even light) has mass. Forces cannot act on objects with no mass.

  177. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    it says let it be zero

  178. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'm not going to continue this further. If you want to argue further you can contact the authors, or your professor.

  179. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If the rope has mass m, then Newton’s second law applied to the rope gives T - T'= ma. If the mass m is taken to be zero, however, as in the upcoming examples, then T = T'.

  180. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    What is the acceleration of a massless object with a force of 1 newton acting on it?

  181. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    here i will show you a link , one sec http://www.wadsworthmedia.com/marketing/sample_chapters/0495106194_ch04.pdf page 70

  182. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I don't disagree with what they state there.

  183. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    All they are saying with the 'massless rope' thing is that the tension in both ends is the same. It's a trick used to simplify your problems

  184. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    in reality that cannot happen.

  185. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    But if you violate the precepts of the 'trick' then you cannot use the rules they infer from it.

  186. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but if the tension is the same, then it wont move

  187. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'm not talking about the external force on the rope.

  188. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the tension is the internal force.

  189. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The point is that it is not slack, it does not break, and it is not elastic.

  190. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You can have it move, but you cannot try to find it's acceleration in the absence of other objects because it has no mass.

  191. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    It's a tool, it is not real.

  192. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    It is a way to magically give direction to forces acting on objects.

  193. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    here is a diagram of a pulley http://www.sparknotes.com/physics/dynamics/newtonapplications/section3.rhtml

  194. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    it says specifically the pulley and cable are massless

  195. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Do you have a question, or are you just going to keep repeating the same thing and pasting examples from the book.

  196. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I have read the book. I don't need to see it again.

  197. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok if the pullies and cable are massless then

  198. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    how can it have acceleration

  199. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'm done. Until you actually read what I have written I have no reason to continue to write.

  200. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so your saying the cable experiences no acceleration, ok i agree

  201. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'm not saying that.

  202. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    well if it is massless then it experiences no acceleration. the cable i mean

  203. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'm saying massless cables do not goddamn exist. If you want to talk about them then you have to obey very specific rules.

  204. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right, its idealized

  205. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    well actually kevlar ropes exist which can carry so much weight, that in comparison to the weight it is massless

  206. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    They are a figment of your imagination. A tool to make it easy for you to solve these problems without a hell of a lot of integrals.

  207. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok , so then my original problem was impossible, having a massless rope and 20 N left and 30 N right, force

  208. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    On a rope by itself, yes.

  209. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Because F = ma therefore F/m = a and m = 0 so F/m is not defined.

  210. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right i agree

  211. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the accelleration is not 0, it's exploding.

  212. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The only time it is ok to talk about external forces on massless objects is when those forces cancel out (a = 0)

  213. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok then can you think of an example where we want a massless rope?

  214. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The pulley example is fine.

  215. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    As long as you talk about the a of the masses, not the a of the rope.

  216. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok

  217. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    All you need to consider about the rope is that the tension is uniform.

  218. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so the point of having a massless rope is to make or force the tension to be the same throughout , it simplifies the problem

  219. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  220. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    even though intuitively, there are different forces on the ends of the rope

  221. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so you might think there are two different tensions on the ends of the rope

  222. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    like in my example, there would be a tension of 20 N opposite to the guy pulling left 20N , and a tension of 30 opposite to the guy pulling to the right 30 N ... well i guess in this case we need to know the mass of the rope to determine the tension err, i mean .... the tension does not hold because it is accelerating , so there isnt tension , in the case where there is mass in the rope

  223. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    look at the pully example. You have that 50*9.8 - T = (50+30)a 30*9.8 + T = (50+30)a So we can find T, and we can find a because we assume that T and a are the same (one block accelerates up while the other accelerates down. We know the rope is accelerating at the same rate as the blocks. So there is a net force on the rope but the tension is uniform. Also the rope is not massless, we just ignore it.

  224. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    supposing that in my example the mass of the rope is 1 kg. so a guy is pulling 20 N to the left and 30 N to the right someone else pulls.

  225. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    what is the tension in the rope

  226. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    you're making up impossible scenarios again.

  227. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    just to make it simple , 1 kg rope

  228. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Are you assuming the rope stays taut?

  229. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  230. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh i see what you mean

  231. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Are you assuming that the guys are standing on the ground and don't fall over?

  232. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  233. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Does the ground have friction?

  234. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    or with a pulley example , even

  235. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  236. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so its much more complicated

  237. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok pulley, 50 kg moving down, 40 kg moving up. the cable is 1 kg

  238. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    what is the tension in the cable,

  239. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    this will be my last example, sorry for keeping ya

  240. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i was wondering if that made sense

  241. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Yes, that's fine.

  242. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    also ignore gravity for the cable,

  243. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  244. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    how do you calculate the tension of the rope , if it has mass 1 kg

  245. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The same was you would if it had no mass.

  246. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok so the forces are 50* 9.8 - 40 * 9.8 , the forces on the cable

  247. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No.

  248. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    50* 9.8 - 40 * 9.8 = 1kg * a

  249. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Stop that

  250. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the net force on the cable

  251. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No.

  252. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If you do that the rope will fly away.

  253. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Stop obsessing about the stupid rope. It doesn't work like that in reality and you don't have the tools to look at it that way. Look at the masses.

  254. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    We MUST assume that they all accelerate at the same rate. Otherwise we have a dynamics system and you don't have the PDE's to solve that.

  255. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The blocks, and the ropes all move with the same a.

  256. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok

  257. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok so dont treat the rope independently of the masses

  258. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Yes, because if you have a 40N force acting on a 1kg rope it will go shooting off into space.

  259. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    And if the mass of the rope is smaller, it will fly away faster.

  260. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    They just want you to ignore the rope, not make any calculations based on its mass or acceleration.

  261. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh actually its (50-40)* 9.8=98 Newtons force on the cable, ok . bad way of looking at it, also for my other example, two guys 30 N and 20 N right and left respectively, rope is 1 kg. the tension is still 24.4444, doing it the right way. dont treat the cable seperately

  262. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok got it. ignore the rope

  263. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Yes, exactly.

  264. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    how come its not obvious the tension is 24.444

  265. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the other guy assumed the tension was 20N.

  266. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so with my other rope example, it would be wrong to do (30 - 20) = 1kg * a , 10N/1kg = a , or 10 m/s^2. it will fly off , ok i see

  267. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    it would shoot off in the direction of the stronger person

  268. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right.

  269. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    interesting

  270. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so this is why we dont deal with the rope directly

  271. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If you say something like Two men are having a tug of war. One man (weighs 50kg) is pulling with a force of 20N, the other weighing 40kg is pulling with a force of 10N. What is the acceleration? and the tension in the rope.

  272. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok good question,

  273. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Again you must assume the acceleration is a constant for the whole 2 men system

  274. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If it werent then the tension in the rope would change

  275. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so 10 N in the right direction?

  276. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    you can do 10 N = ( 40 + 50) * 1kg ?

  277. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    errr, 10 N = (40 + 50) kg * a

  278. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Actually that's still a bad example because we have to assume some friction under their feet or they would just go toward each other. (and lose tension in the rope)

  279. myininaya
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok i don't know anything about physics but it sounds like cantorset is trying to use a theorem. And it sounds like you guys are saying just because the conclusion is true doesn't mean the is. p->q is not equivalent to q->p or are you guys talking about somtehing different. You might have already figure this out so sorry if you have. if you don't have any clue what i'm talking about ignore this post

  280. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so a = 1/9 or .1111 m/s^2

  281. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yeah, that's what I was complaining about earlier myininaya. But I think we worked through it.

  282. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but i ignored the tension

  283. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    in fact i ignored the rope altogether

  284. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I like the pully problem better ;)

  285. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    This one still has too many unknowns.

  286. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    you can do 20 N - T = 50 * a T - 10 N = 40 * a

  287. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh right, we solved for a , so 20 N - T = 50 * 1/9

  288. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    T = 14.4444

  289. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Sure, that will tell you how fast they fly together.

  290. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hmmm

  291. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    And how tight the rope was at the instant they both pulled (before they start moving)

  292. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh right, so we would have friction , and...

  293. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Though it's not quite right because you did the forces wrong

  294. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes this only works for an instant at the beginning before they actually start moving

  295. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i did ?

  296. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Two men are having a tug of war. One man (weighs 50kg) is pulling with a force of 20N, the other weighing 40kg is pulling with a force of 10N. 20 N - T = 50 * a (force on bigger guy) T - 10 N = 40 * a (force on smaller guy )

  297. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    The net acceleration of the system will be 0, and you'll do stuff like this when you get to center of mass, but the system you have there doesn't correctly model what happens.

  298. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    you dont agree the tension is 14.444 N , in the idealized case

  299. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    what do you mean net acceleration is zero. oh you mean that the people are staying in their places and not moving (so the string is taut)

  300. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Imagine for a moment that it's an astronaut(75kg) in space, pulling (20N) on a cord attached to a free floating 50 kg backpack. The astronaut pulls the rope. The rope pulls the astronaut.

  301. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    This will work.

  302. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    why not 2 different astronauts in space

  303. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I'd rather start with this one.

  304. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok so whats the question?

  305. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    what is tension in rope

  306. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    What is the acceleration of the backpack?

  307. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    isnt it just 20 N = 50kg * a

  308. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    20 /50 m s^-2

  309. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yep.

  310. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok good

  311. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    And what is the acceleration of the astronaut?

  312. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    we never said the astronaut is accelerating

  313. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hes just pulling, so maybe zero

  314. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    he's floating in space, pulling on a cord attached to a 50kg backpack.

  315. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    he's definitely gonna accelerate.

  316. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    because the bookbag pulls back on him ?

  317. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    is he exerting a force?

  318. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    he is pulling on a rope

  319. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    then the rope must also be pulling on him

  320. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right with equal and opposite force

  321. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so 20/75 m*s^-2

  322. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    in the opposite direction to bookbag

  323. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    20 N = 75 kg * a

  324. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Right, so they start flying toward eachother.

  325. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right

  326. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    And the tension in the rope?

  327. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hmmm

  328. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    (for one instant anyway)

  329. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh

  330. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    one sec

  331. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    20 N - T =

  332. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    20 N - T = 75 * a , and a = 20 /75

  333. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so T = 40 N ?

  334. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Hold up.

  335. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    You are listing the forces on the astronaut?

  336. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes, but there is tension in that force, so im solving for the tension

  337. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    What forces are on the astronaut?

  338. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh ,

  339. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    lets see

  340. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    we know astronaut is accelerating 20/75 m * s^-2

  341. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    20 N

  342. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    duh, equal and opposite force

  343. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right.

  344. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Now lets do the same thing, but with 2 astronauts.

  345. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so 20 N - T = 75* a

  346. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    wait, whats the tension?

  347. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No.

  348. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Name the forces on the astronaut. (don't bother with the number)

  349. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the forces on the astronaut are 20 N ....

  350. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes. And what is exerting that force?

  351. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ummm, just the tension ?

  352. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  353. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    and the tension comes from the equal and opposite force

  354. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  355. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok so the tension is , instantaneously 20 N

  356. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  357. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    we dont even have to solve for that

  358. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right.

  359. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    So now lets analyze two astronauts. If one is just floating and the other pulls, we have the same situation as we do in the backpack case right?

  360. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok 2 astronauts

  361. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  362. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so lets do both pulls >

  363. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Will the tension in the rope be greater, the same, or less if both astronauts pull?

  364. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i dont know

  365. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hmmm, less ?

  366. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    well if they both pull 20 N say

  367. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Hold up.

  368. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Imagine only one pulls.

  369. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok

  370. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    They both fly together right?

  371. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  372. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    like our bookbag case

  373. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Will they fly together just as fast if they both pull, or will it be slower, or faster?

  374. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    dont know

  375. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    without thinking about it, faster >?

  376. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    it depends on the pulls, are they equal

  377. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    even if they are equal.

  378. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh much faster

  379. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    indeed

  380. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If they accelerate faster than they would if just one pulled, what can we say about the tension?

  381. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    then the tension must be greater

  382. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Also correct.

  383. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so lets say that they both pull 20 N force

  384. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ok.

  385. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    20 N - T = 75 * a . and

  386. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hold on

  387. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    What is that 20N?

  388. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    they are both pulling on each other 20 N

  389. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No, they are both pulling on the rope with 20N

  390. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so the m1 = 75 kg, and m2 = 50 kg , the astronauts

  391. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes but i will examine the forces on the astronauts, and glean out the tension

  392. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    indirectly

  393. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    True, but the force on the astronaut from the rope is not 20N.

  394. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If it were, they would accelerate just as if only one was pulling.

  395. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the 20 N is from the opposite reaction ,

  396. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    ohhh

  397. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    theres only tension then

  398. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i keep thinking of the pulley example

  399. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes For astronaut 1, the only force on them is the tension.

  400. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right theres no gravity

  401. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so if we assume the tension is the same throughout, dunno

  402. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the rope doesn't fly away.

  403. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    how to calculate tension

  404. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    well we add the forces and the reaction forces

  405. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes.

  406. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so 2 * 20 + 20/75 + 50/75 ?

  407. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    no it's easier than that.

  408. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Are you familiar with hanging scales?

  409. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so i did it wrong >

  410. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yeah

  411. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    you know like hanging scales in a grocery store?

  412. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes

  413. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    what was wrong with my analysis

  414. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Ok well it tells how much the thing in the basket weighs by the tension pulling down.

  415. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right

  416. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If I put a 1kg mellon in the basket the scale will read 1kg. If I push down on the scale with a 9.8N of force what will the scale read?

  417. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    9.8 kg ?

  418. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    2kg actually. but that's ok. Point is my force is added to the tension from the mellon

  419. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    you mean if you push down with the melon in the scale, oh

  420. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right, 1 kg is getting pushed down with 9.8 N of force

  421. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    and if you push down 9.8 N and keep the force there, its like adding another kg

  422. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    right.

  423. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so how does that help us with the tension when two astronauts are pulling on each other 20 N

  424. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    If I accelerate the scale upwards at a rate of 9.8m/s^2 the scale will also read 2kg

  425. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    how can you accelerate a scale upwards?

  426. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    a rocket? my arms?

  427. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    a pully?

  428. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    we're talking about a scale in a store. it doesnt register upward forces

  429. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh you mean downwards, same thing, ok

  430. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    upwards meaning pushing towards the ground, ok

  431. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No I mean upwards because accelerating the scale up will change the tension in the chain

  432. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    no I mean blasting the whole apparatus into the sky.

  433. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    and the melon is inside the scale ?

  434. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I think I'm just confusing you further here. The point with the astronauts is this. If they both pull, the tension in the rope will be the sum of their individual tensions.

  435. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    and its not 20 + 20 + 20/75 + 20 / 50 ?

  436. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    No, it's 20 + 20 = 40

  437. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    what about those reaction forces

  438. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    the newtons third law pairs

  439. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    oh

  440. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    that effects the astronauts

  441. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    http://www.twiddla.com/530517

  442. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    It's no good without pictures

  443. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    so the astronauts are accelerating, one is accelerating 20/75 and other is accelerating 20/50 m/s^2

  444. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    40

  445. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    40 , yes

  446. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    40/75 and 40/50, because the reaction to tension

  447. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    yes.

  448. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    hmm, and why do you add the tensions to get 40

  449. anonymous
    • 5 years ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Hrm?

  450. Not the answer you are looking for?
    Search for more explanations.

    • Attachments:

Ask your own question

Sign Up
Find more explanations on OpenStudy
Privacy Policy

Your question is ready. Sign up for free to start getting answers.

spraguer (Moderator)
5 → View Detailed Profile

is replying to Can someone tell me what button the professor is hitting...

23

  • Teamwork 19 Teammate
  • Problem Solving 19 Hero
  • You have blocked this person.
  • ✔ You're a fan Checking fan status...

Thanks for being so helpful in mathematics. If you are getting quality help, make sure you spread the word about OpenStudy.

This is the testimonial you wrote.
You haven't written a testimonial for Owlfred.