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Some confusion about balanced forces

  1. Feb 11, 2013 #1

    My question is:

    1)When an object falls to the ground, shouldn't the ground exert a force greater than the weight of the object to stop its motion altogether? Because if the forces experienced by the object the moment it strikes the ground are equal and opposite in direction, then that should only stop the object from accelerating further and it should continue moving with the velocity it had the instant before the forces cancelled. (I know it sounds silly for the ground because it's a solid, rigid body and can not allow the object to continue its motion through it. Maybe it'd be better if you replace the ground with a trampoline and explain it for that? )

    Thank you for your time. :)
  2. jcsd
  3. Feb 11, 2013 #2

    Doc Al

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    Staff: Mentor

    Your thinking is quite correct. In order to stop the object, the force from the ground must be greater that the object's weight. (At least for some time.)

    Why do you think that is a problem?
  4. Feb 11, 2013 #3
    Oh, I'm sorry I didn't make myself clear enough.

    The problem is, I don't understand where that greater force comes from.

    According to my understanding, The forces on the object are gravity (downward force) and the normal force of the ground (upward) and I think they are equal. But as you just confirmed, the normal force must be greater than that with which gravity is pulling the object. I'm just unable to pin down where that extra normal force is coming from.

    Thank you for your prompt reply, Doc Al!
  5. Feb 11, 2013 #4

    Doc Al

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    Staff: Mentor

    They are only equal under certain circumstances. Such as when the object is no longer accelerating.
    That "extra" force comes from the collision of the moving object with the ground. The material structure of the ground allows it to resist the movement of the object by exerting an upward force on it.
  6. Feb 11, 2013 #5
    Hm. I see.

    Thank you for your help! I appreciate it a lot. :)
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