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alffer1 Group Title

Hey I need help figuring out the free body diagram for a block and tackle system. Problem will be in the comments.

  • 9 months ago
  • 9 months ago

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  1. alffer1 Group Title
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    • 9 months ago
  2. alffer1 Group Title
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    I've worked with pulley systems before, but I'm not sure how to go about this one.

    • 9 months ago
  3. AnthonyStark Group Title
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    take the boy as the system....forces acting on him are 1) tension by rope 2) normal force exerted by the beam he is sitting on

    • 9 months ago
  4. AnthonyStark Group Title
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    + 3) the weight of the boy downwards

    • 9 months ago
  5. alffer1 Group Title
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    So the whole pully system doesn't affect the problem? I'm sure it does...

    • 9 months ago
  6. AnthonyStark Group Title
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    can u explain what u meant?

    • 9 months ago
  7. alffer1 Group Title
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    Sorry I'm really bad at explaining myself. In order to determine the force he's exerting we need to figure out how much force is required by this pulley system to lift 120 lbs right?

    • 9 months ago
  8. alffer1 Group Title
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    So perhaps the question is rather what are all the relevant free body diagrams in this problem.

    • 9 months ago
  9. AnthonyStark Group Title
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    since it moves at a constant velocity, i guess the force exerted by him = tension in the rope

    • 9 months ago
  10. alffer1 Group Title
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    yeah that is true.

    • 9 months ago
  11. AnthonyStark Group Title
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    and u are right,this is all about taking free body diagrams and finding unknowns

    • 9 months ago
  12. alffer1 Group Title
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    So how can I take the free body diagrams at the pulleys? the boy's is relatively simple as you mentioned.

    • 9 months ago
  13. AnthonyStark Group Title
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    as long as the pulley is massless, u neednt take it into consideration

    • 9 months ago
  14. alffer1 Group Title
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    I mean the whole mess of ropes around the pulleys

    • 9 months ago
  15. AnthonyStark Group Title
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    next you can take the beam as the free body, and the forces acting on it,,,1) normal force from the boy downwards, 2) weight downwards 3) tension upwards,,,,,equate to zero

    • 9 months ago
  16. alffer1 Group Title
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    the tension in the rope goes around the top pulley so the tension in the rope from the top to the bottom pulley is the same as the tension in the rope....does the tackle system provide a mechanical advantage of 2?

    • 9 months ago
  17. alffer1 Group Title
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    or is it an advantage of 4 since the rope goes around the bottom pulley twice?

    • 9 months ago
  18. AnthonyStark Group Title
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    the pulley thing is rather complex,,,i guess the advantage is 4,,,perhaps @LastDayWork may help u better,,,expert at physics

    • 9 months ago
  19. alffer1 Group Title
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    @LastDayWork is it 4? It would seem so looking at the picture, since the weight of the boy/beam system is 120lb, the boy only needs to pull with 1/4 of the force since 4 ropes are supporting that weight no?

    • 9 months ago
  20. LastDayWork Group Title
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    Is there something below the diagram; because I can't clearly makeout how the rope is attached to the beam (in which the boy is sitting).

    • 9 months ago
  21. alffer1 Group Title
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    No. but I don't think it matters, since they just say the system and ignore the angle at which the rope is.

    • 9 months ago
  22. alffer1 Group Title
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    Assuming that the tension in the hook in the bottom is 120lbs, and the tension in the other rope is whatever pull the boy is applying, wouldn't the pull be a quarter of the weight? not sure.

    • 9 months ago
  23. LastDayWork Group Title
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    I am not considering the angle; its just that the tone of the question implies that the boy is lifting the beam along with himself (hence the combined weight).

    • 9 months ago
  24. LastDayWork Group Title
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    ^^ But I cant see how

    • 9 months ago
  25. alffer1 Group Title
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    yeah, but doesn't that mean we can pretend the boy/bar system is just a box of weight 120lb? I think the bar slides up the vertical column, but that doesn't affect the problem.

    • 9 months ago
  26. LastDayWork Group Title
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    A box climbing a rope ?? :P

    • 9 months ago
  27. alffer1 Group Title
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    Well I have seen physics problems that say to work with perfectly spherical elephants whose mass can be ignored soo....why not?

    • 9 months ago
  28. JoannaBlackwelder Group Title
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    It looks like to me that the problem states that 120 lbs is the combined weight.

    • 9 months ago
  29. alffer1 Group Title
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    exactly. from the picture we need to determine the force exerted by the boy. I'm thinking that the tackle system provides a mechanical advantage of4 but i'm not sure.

    • 9 months ago
  30. alffer1 Group Title
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    Since the weight is supported by 4 ropes that lead to the top pulley, and the rope the boy is pulling on is equal to one of these ropes, then the force he uses to pull is a quarter of the weight right? or do I have faulty reasoning?

    • 9 months ago
  31. LastDayWork Group Title
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    As far as I can understand - There is some sort of spring attached to the hook of upper pulley (above the picture). By pulling the rope, the boy decreases the distance between the pulleys; and as the lower pulley can't move the (hypothetical) spring will get stretched to account for the displacement of upper pulley. There is a slag in the rope below the point where the boy is holding it (implies no tension). So the question is how is the boy lifting himself??

    • 9 months ago
  32. alffer1 Group Title
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    Ok this is getting ridiculously complicated. This shouldn't be that convoluted....

    • 9 months ago
  33. wolfe8 Group Title
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    Wouldn't it be the same as the weight of him and the beam? (Is he lifting the beam up as well?)

    • 9 months ago
  34. LastDayWork Group Title
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    Can he??

    • 9 months ago
  35. alffer1 Group Title
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    yeah he is. However, the force he's using isn't equal to the weight of the system. The pulleys provide mechanical advantage. I just looked up a double block and tackle system and yeah the advantage is 4, so I guess he pulls with a force of 30 lbs.

    • 9 months ago
  36. wolfe8 Group Title
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    The pulley just allows complete transfer of force though. Since we assume the weight of the pulley is negligible, it does not contribute to the force through torque as a pulley with mass would.

    • 9 months ago
  37. alffer1 Group Title
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    exactly. Well I think I have my answer. Thanks everyone for taking the time to break your heads on this one.

    • 9 months ago
  38. alffer1 Group Title
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    It's not a simple pulley. It's a double tackle

    • 9 months ago
  39. wolfe8 Group Title
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    Whoah. Didn't notice that. That's what happens when you jump into a question. Well good luck. Sorry I couldn't help more.

    • 9 months ago
  40. alffer1 Group Title
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    a single pulley would transfer, so he would pull with the same force as the combined weight. If it were a single block and tackle, the rope would loop once around the bottom ,providing a mechanical advantage of 2, etc. Thanks anyways Wolfe :)

    • 9 months ago
  41. alffer1 Group Title
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    good night everyone! Thanks.

    • 9 months ago
  42. Vincent-Lyon.Fr Group Title
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    If somebody else were pulling the rope and not the boy himself, I would divide the total weight by 4, as there are 4 ropes supporting him. But as the boy is pulling the rope himself, there is a 5th tension acting on him, so I would divide the total weight by 5.

    • 9 months ago
  43. LastDayWork Group Title
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    @Vincent-Lyon.Fr How can the boy pull himself (along with the beam) ??

    • 9 months ago
  44. Vincent-Lyon.Fr Group Title
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    Well, I understand he is holding and pulling the rope with his hands.

    • 9 months ago
  45. LastDayWork Group Title
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    That way he can pull himself but not the beam (why would question give us combined weight).

    • 9 months ago
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