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buggiethebug Group Title

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

  • one year ago
  • one year ago

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  1. ghazi Group Title
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    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.

    • one year ago
  2. ghazi Group Title
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    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.

    • one year ago
  3. buggiethebug Group Title
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    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

    • one year ago
  4. JamesJ Group Title
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    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?

    • one year ago
  5. ghazi Group Title
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    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

    • one year ago
  6. JamesJ Group Title
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    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

    • one year ago
  7. ghazi Group Title
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    by the way in case of free fall a= +g

    • one year ago
  8. buggiethebug Group Title
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    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?

    • one year ago
  9. JamesJ Group Title
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    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

    • one year ago
  10. buggiethebug Group Title
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    but isn't a=-9.8m/s^2 if it's free falling? direction is up(+)

    • one year ago
  11. JamesJ Group Title
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    Yes, exactly. a = -g = -9.8 m/s^2

    • one year ago
  12. buggiethebug Group Title
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    Also, when is the Displacement negative??

    • one year ago
  13. JamesJ Group Title
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    Hence the velocity as a function of time is v(t) = -9.8t m/s

    • one year ago
  14. ghazi Group Title
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    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

    • one year ago
  15. JamesJ Group Title
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    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?

    • one year ago
  16. buggiethebug Group Title
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    i mean aren't we usually finding the magnitude?

    • one year ago
  17. JamesJ Group Title
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    The magnitude of what?

    • one year ago
  18. buggiethebug Group Title
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    displacement

    • one year ago
  19. ghazi Group Title
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    when you are supposed to find just magnitude its, distance and when you get direction included its displacement

    • one year ago
  20. JamesJ Group Title
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    Displacement like velocity is a vector quantity. Hence in one dimension like this book problem it can have a sign of positive of negative.

    • one year ago
  21. JamesJ Group Title
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    *or

    • one year ago
  22. buggiethebug Group Title
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    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?

    • one year ago
  23. buggiethebug Group Title
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    this is how i interpreted this

    • one year ago
  24. JamesJ Group Title
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    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.

    • one year ago
  25. buggiethebug Group Title
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    |dw:1360795426340:dw|

    • one year ago
  26. JamesJ Group Title
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    Yes

    • one year ago
  27. buggiethebug Group Title
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    i dont get what the displacement will be though

    • one year ago
  28. buggiethebug Group Title
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    It's hard to picture it

    • one year ago
  29. ghazi Group Title
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    well , displacement would be negative of the height of tree

    • one year ago
  30. buggiethebug Group Title
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    Howww and why would it be negative :S

    • one year ago
  31. ghazi Group Title
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    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

    • one year ago
  32. JamesJ Group Title
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    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.

    • one year ago
  33. buggiethebug Group Title
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    OHHHHHHHH

    • one year ago
  34. JamesJ Group Title
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    *100 km

    • one year ago
  35. ghazi Group Title
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    |dw:1360795675315:dw|

    • one year ago
  36. JamesJ Group Title
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    Ghazi in his diagram has chosen zero to be at the height where the rock is thrown. Nothing wrong with that choice.

    • one year ago
  37. Vincent-Lyon.Fr Group Title
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    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.

    • one year ago
  38. buggiethebug Group Title
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    Vincent: I'm confused

    • one year ago
  39. JamesJ Group Title
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    @vincent. Pottery barn rule --> OpenStudy rule: You answer it, you explain it! :-)

    • one year ago
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