Quantcast

A community for students. Sign up today!

Here's the question you clicked on:

55 members online
  • 0 replying
  • 0 viewing

buggiethebug

  • one year ago

A negative acceleration means that the speed of the object decreases. Explain why this statement can be incorrect????

  • This Question is Closed
  1. ghazi
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    the statement could be incorrect because just consider an example that still its acceleration so if its negative and if it persists then it will increase the speed in the reverse direction.

  2. ghazi
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    so speed basically is reduced by retardation , speaking of negative acceleration also depends on the frame of reference, if your assumption is somewhat in an opposite direction of the frame then also you may get negative acceleration.

  3. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but it all depends on the direction you choose as positive right, but I think the question is really asking why why is a negative acceleration means that the speed of the object is decreasing? how could this be wrong? it makes sense though

  4. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    Because speed isn't velocity. For example, drop a book the the floor. If we assign vertical up to be the positive y direction say, then the acceleration is negative. Yet the speed of the book constantly increases. Why? Because the velocity of the book becomes larger and larger negative values. But as speed is the absolute value or magnitude of velocity, speed is increasing, even as velocity is decreasing. Make sense?

  5. ghazi
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    in my first statement i stated that negative acceleration doesnt mean reduction in speed it is decided by frame of reference and your assumption of direction

  6. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    Explicitly suppose you hold a book 1 meter off the floor and drop it at time t = 0. The acceleration is a = -g m/s^2 The velocity of the book is v = -gt m/s which decreases as t marches on But the *speed* of the book is speed = gt m/s which increases as t marches on

  7. ghazi
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    by the way in case of free fall a= +g

  8. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    I get it, how speed isn't velocity, i think that makes more sense, as speed does not have direction , but if it were velocity, it would be a true statement right?

  9. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    Yes, it would be. @ghazi, as we have defined the coordinate system here, with the up direction being positive, the acceleration is negative, i.e., a = -g

  10. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    but isn't a=-9.8m/s^2 if it's free falling? direction is up(+)

  11. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    Yes, exactly. a = -g = -9.8 m/s^2

  12. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Also, when is the Displacement negative??

  13. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    Hence the velocity as a function of time is v(t) = -9.8t m/s

  14. ghazi
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    velocity has direction and magnitude but speed has just magnitude @JamesJ oh sorry i didnt notice that, thats what i am saying it all depends on the reference and a = +g if object is falling on earth not a=-g but as he has stated different coordinate so here we can consider a=-g

  15. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    It depends where we set displacement equal to zero. Suppose the floor is the zero and we drop the book from a height of 1 meter, you tell me: what is the displacement as a function of time t?

  16. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i mean aren't we usually finding the magnitude?

  17. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    The magnitude of what?

  18. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    displacement

  19. ghazi
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    when you are supposed to find just magnitude its, distance and when you get direction included its displacement

  20. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    Displacement like velocity is a vector quantity. Hence in one dimension like this book problem it can have a sign of positive of negative.

  21. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    *or

  22. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    A rock is thrown up with a speed of 32.5km/h from a tree that is 15.75 m tall. How long will it take to hit the ground?

  23. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    this is how i interpreted this

  24. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    You figure that out! Use the standard kinematic equations, in particular the one that gives displacement as sa function of time, initial velocity and acceleration.

  25. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    |dw:1360795426340:dw|

  26. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    Yes

  27. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    i dont get what the displacement will be though

  28. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    It's hard to picture it

  29. ghazi
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    well , displacement would be negative of the height of tree

  30. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Howww and why would it be negative :S

  31. ghazi
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    since the person is standing on tree so take that as the reference and below him take the height as negative so when stone goes up and comes back again it reaches the top of the tree , so basically the point from where it was thrown it got back there, therefore total displacement=0 but after that it keeps falling down so below that point height of the tree is negative for stone, as i said top of the tree is meant to be zero or starting point

  32. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    It depends where you define displacement to be equal to zero. You can make it zero where the ground is, or you can make it zero from the point on the tree where the rock is thrown. Or you can make displacement= 0 at 100 km/s above the ground. In general, it's best to choose the point where displacement = 0 to be as convenient as possible. In this case, I would choose it to be the level of the ground.

  33. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    OHHHHHHHH

  34. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    *100 km

  35. ghazi
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    |dw:1360795675315:dw|

  36. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    Ghazi in his diagram has chosen zero to be at the height where the rock is thrown. Nothing wrong with that choice.

  37. Vincent-Lyon.Fr
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    For linear motion, speed will increase if acceleration and velocity are both positive or if they are both negative. Speed will decrease if velocity and acceleration have opposite signs.

  38. buggiethebug
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 0

    Vincent: I'm confused

  39. JamesJ
    • one year ago
    Best Response
    You've already chosen the best response.
    Medals 1

    @vincent. Pottery barn rule --> OpenStudy rule: You answer it, you explain it! :-)

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

    Search OpenStudy
    • Attachments:

Ask your own question

Ask a Question
Find more explanations on OpenStudy

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.