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Do Objects Ever Stop Moving?

  1. Dec 4, 2005 #1
    I know that I'm knew to these forums, so please excuse me if I'm not using the correct formatting for questions...

    I've just been contemplating a theory of mine for a while now, and would love to have some answers: Do objects ever stop moving (or more specifically, do they ever stop changing directions)?

    I'll give you a brief synopsis of why I think that the answer is No:
    Every object is bound by the Law of Gravity and Newton's 3 Laws, of course. So when an object falls, it is being acted upon by gravity. Gravity is a force (therefore requiring an action), and according to Newton's 3rd law the object is required to supply a counter-force. So, theoretically (for me, at least) when any object is dropped, it is always changing directions, and therefore always moving (no matter how small).

    I hope you follow what I'm saying? Am I too far off the mark? I would love to be proven wrong in this, because that at the very least means I've learned something.

    I appreciate your answers.

    Regards,
    Tyler
     
    Last edited: Dec 4, 2005
  2. jcsd
  3. Dec 4, 2005 #2

    LeonhardEuler

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    Newton's third law says that whenever object A exerts a force on object B, then object B must exert an equal and opposite force on object A. For a falling object, this means that, since the earth is exerting a force downward on the object, the object must be pulling up on the earth with an upward force. The earth hardly accelerates at all in response to this force, though, since the earth is so massive. This is all that Newton's third law says. It does not say that an object must change direction. An object can be accelerated by changing its speed while not changing its direction at all.
     
  4. Dec 4, 2005 #3
    How can you stop something that never started? Since motion is relative, its necessarily subjective and thus cannot be stated as an objective fact. S'all a matter of perspective :biggrin:
     
  5. Dec 4, 2005 #4
    Ok, I've just had a skewed view of Newton 3rd Law this whole time. So, just to confirm: When the earth exerts a force on an object, the object excerts a force of it's own in the form of "resistance" (not by bouncing back or changing directions).

    Is this correct?
    If so, is there ever a point when the object is overtaken by the earth's force, and stops moving? (That to me just seems a little unlogical, but it's worth asking)

    Regards,
    Tyler
     
  6. Dec 4, 2005 #5
    Keep in mind that when an object is falling to the earth it is still effected by the rotation of the earth, and its rotation around the sun, and the galaxy's center. All creating angular momentum.
     
  7. Dec 4, 2005 #6
    EDIT
    Would it be best, in this example at least, for the perspective of the object to be limited somewhat, and not necessarily viewed from the galaxies perspective? (e.g. if the earth were a static mass in space, not moving at all), because invariably everyone would agree that objects are continuously moving if that were the case.

    Regards,
    Tyler
     
    Last edited: Dec 4, 2005
  8. Dec 4, 2005 #7
    Ok, thats fine. Now everything orbiting the earth pulls on the object and the earth causing more angular momentum. That right there should indicate that the stationary frame we chose is anything but.
     
  9. Dec 4, 2005 #8

    Doc Al

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    I'd say no. As LeonhardEuler stated, Newton's 3rd law just says that forces always come in pairs. I have no idea what you mean by "resistance". In the case of gravity and a falling object: Newton's 3rd says that if the earth exerts a certain gravitational force on the object, then the object exerts a gravitational force on the earth. But the only forces relevant to the object's motion are the forces on the object. The force that the object exerts back on the earth does not affect the object's motion. (At least with respect to an inertial frame.)

    I have no idea what this means.

    Ignoring complications like the rotation of the earth and air resistance, a dropped object falls straight down until it smacks into the ground.
     
  10. Dec 4, 2005 #9
    Ok, that makes perfect since, but I think that we may have strayed from what I'm really looking for. Here's a breakdown...

    For this I'll use a practical example...
    Let's say you drop a golf ball from a given height. The golf ball bounces off the floor and comes up short of where it started because of gravity and other forces. The next time it does the same thing, coming up short. Multiple bounces will decrease the height of the golf ball until you can no longer tell it is bouncing anymore.

    So, my question in general is: Just because we can't tell if the golf ball is bouncing, does that mean that it isn't? Or does the golf ball continue to bounce indefinitely, and we just consider it as "stopped".

    Regards,
    Tyler
     
    Last edited: Dec 4, 2005
  11. Dec 5, 2005 #10
    You cannot comment on these issues only with the help of gravitational force. there are many forces which are stronger than it. the bouncing is due to normal force. but take this example if the ball does not bounce, then a pitlike region is formed on the earth where it pitched. so earth is not displaced anyway but its shape has changed. this is now due to the force applied by ball on earth. Som my main question is What made you feel that a free falling object does no apply a force on earth?
     
  12. Dec 5, 2005 #11

    Danger

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    You can't say, as Sardine pointed out (sorry, Sir; I had to do that), say that something is or is not moving without supplying a reference frame. It also depends to some extent upon your definition of 'moving'. The only way to keep something from moving at all is to put it at 0°K.
     
  13. Dec 5, 2005 #12

    Doc Al

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    The reason the ball "comes up short" with each succeeding bounce is that the collision of ball and floor is not perfectly elastic: A portion of the ball's macroscopic KE is transformed into microscopic thermal KE, sound energy, and deformation. At some point, you will not be able to distinguish the bounced ball from an identical one that was just placed on the floor without ever bouncing (except for the difference in temperature). At that point, for all practical purposes I think we can say the ball has come to "rest" (with respect to the floor), at least macroscopically.

    I would say that after a point it's meaningless to think of the ball as still "bouncing".
     
    Last edited: Dec 5, 2005
  14. Dec 6, 2005 #13
    Ok, so I asked a Physics teacher at my school about this, and he said pretty much the same things as you did, Doc.

    He said that the ball has a certain amount of potential energy when you hold it up in the air, which is transformed to kinetic energy when you drop it. after it hits the floor (based on the elasticity of both the ball and the floor) the ball will come up short of where it started because of a loss of energy. It will continue to lose energy until you can no longer notice that it is moving. In short, the balls movement transfers from a visible movement to a molecular movement.

    But the fact still remains: The ball will continuously move, however little it might actually be, until it runs out of energy (which theoretically could last forever since the ball has an immense amount of molecules).

    I realize that is similar to what you just said Doc, but does what I recapped sound accurate?

    Thanks for all your help guys!
    Tyler
     
  15. Dec 7, 2005 #14

    DrGreg

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    If the ball were a mathematically ideal particle, it would bounce an infinite number of times. But the time interval between bounces would get shorter and shorter. Add up all the intervals and you get an infinite series that converges to a finite answer. So, mathematically, the ball wouldstop moving after a finite time.

    Of course, real balls aren't mathematically ideal, so, in practice, Doc Al's answer is the right one.
     
  16. Dec 9, 2005 #15
    It depends whether it's inanimate or not??!!
     
  17. Dec 9, 2005 #16

    DaveC426913

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    It does stop. At some point, friction and inelastic rebounding will overwhelm the diminishing degree of bouncing.

    Even in the ideal thought experiment, it will stop.

    On some arbitrary bounce, the recoil of the ball after hitting the ground will be insufficient to propel it off the ground for another bounce. At this point, the ball just stays on the ground and resumes its shape. There is no "next" bounce to squash it again, and the cycle is broken.
     
  18. Dec 9, 2005 #17

    Doc Al

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    A few wacks with my trusty hammer will solve that problem.:rofl:
     
  19. Dec 9, 2005 #18

    Danger

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    You running for Carpentry Guru now, Doc?
     
  20. Dec 9, 2005 #19

    Doc Al

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    Your recap is OK, except for your thinking that the ball might just bounce forever or that it will somehow "run out of energy". As DrGreg and Dave point out, the bouncing (a collective, macroscopic effect) will stop--in most cases in a surprisingly short time. But the microscopic motion--the random thermal vibrations of the molecules--will continue.
     
  21. Dec 13, 2005 #20
    What about Zeno's Paradox of Motion?

    I haven't looked into the other three concerning space and time, but if you're looking for a more (philosophical?) sense, this might be it.

    http://mathforum.org/isaac/problems/zeno1.html

    Zeno states that when an object is trying to reach its destination, in speculation it never will. A runner is trying to run 100 meters, and by fifty he is halfway. He has fifty left. Then, by twenty five meters later, he is halfay there. However, twelve and a half meters later to that, he is only halfway from the point once again. Since any distance can be infinitely divided into two, Zeno predicted that you will never reach your destination.
     
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