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The force between two objects

  1. Aug 23, 2015 #1
    Hey Everyone,

    When I was reading about the apparent weight in an elevator I started to think about something. I think If two objects are in side contact moving at the same velocity, neither of them applies a force on the other, because it's as if they were at rest together. A force doesn't arise unless the further objects moves at a less velocity than the other. I guess this is true but I can't explain it to my myself in a proper way. I have the intuition for it but I'm not able to reach a neat explanation. Can it explained in terms of relative kinetic energy? I'm not sure.
  2. jcsd
  3. Aug 23, 2015 #2

    Vanadium 50

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    If I put a bowling ball on my toe, they are at rest together. But there is most certainly an applied force exerted.
  4. Aug 23, 2015 #3
    If these are the conditions: the elevator is falling within a gravitational field. If the elevator is falling at terminal velocity and the objects are not moving in the elevator, then the objects are falling at terminal velocity as well.

    Then you could say: the energy of both objects is kinetic energy, zero potential energy. If one object slows down and is no longer traveling at terminal velocity, then it has transfered some kinetic energy to potential energy.

    I think that works.
  5. Aug 23, 2015 #4
    Yes, I meant both of them are in contact such that neither of them is exerting its weight on the other.
  6. Aug 23, 2015 #5
    I never meant that these objects are in a moving elevator. I just wanted to create an analogy by imagining that two objects were in side- contact, and I tried to imagine the force that one would act on the other at different conditions.
  7. Aug 23, 2015 #6


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    It's about the sums of all other forces on each object divided by their masses. If those are the same, they can remain at relative rest without a force between them. Ignoring rotation.
  8. Aug 23, 2015 #7


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    Do you mean in free fall or some other speed? To go at any speed other than free-fall, there are non-gravity forces at play on both objects. In general, one object might be transferring some force to the other. You say they are side-by-side, so it sounds like you are stating that they are not applying force on each other. Then it is logically wrong to question if there is force between them, when you stated otherwise as an assumption.
    That is wrong. Just because they are at the same velocity doesn't mean that one can't be applying a force to keep the other at that velocity.

    PS. The case of free-fall is special. You know that gravity alone will make two objects (in a vacuum) accelerate and fall at the same speed. So there is no force between them. If they were at a non-free-fall speed, then you know that there are other forces involved and you don't know if those forces are separately applied to the objects or if one object is applying some force on the other object.
    Last edited: Aug 23, 2015
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