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Why things bounce

  1. Nov 2, 2007 #1
    This is probably a stupid question, but why is it that things bounce back at you when they hit the floor or a wall? When the kinetic energy is transformed into potential energy as the ball compresses, is there a force that transforms the potential energy back into kinetic energy? From my extremely limited understanding, potential energy requires a force to be turned into kinetic energy -- i.e., gravitational potential energy is turned into kinetic energy by gravity. What is the force that acts on a bouncing object?
  2. jcsd
  3. Nov 2, 2007 #2


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    The force is called the "normal" force. According to Newtons second law every force has a counter force. So when the ball exerts a downward force on the ground the ground exerts an upwards force (called the normal force) on the ball. But since the ground is so heavy it barely moves, the ball on the other hand flies back up.
  4. Nov 2, 2007 #3
  5. Nov 2, 2007 #4


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    The force is electromagnetism between the molecules of the object... The magnetic force I believe is insignificant in magnitude compared to the electric force... so essentially it is electric forces...

    ie: the potential energy that the kinetic energy is converted to is: electric potential energy.
  6. Nov 2, 2007 #5
    Okay, I don't know any physics, but isn't it important that the ball be compressible, while the ground is not? The ball is not only compressible, but it returns to it's original shape quickly. Isn't that why it bounces? Clay won't bounce much...

    A rubber ball will bounce on a steel surface, and steel balls bounce on a rubber surface.

    I guess the rubber ball is pushing the earth a little when it returns to it's original shape... but it's not enough to matter.

    Pool balls also bounce, but isn't that because they are in an environment (the pool table) with a small amount of friction?
  7. Nov 2, 2007 #6


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    Actually, elasticity isn't directly related to deformation. Steel is extremely elastic, so a steel ball will bounce very well on any hard surface (as long as the force is below the plastic deformation limit).
  8. Nov 3, 2007 #7


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    Pool balls are also elastic, and bounce very well on a hard surface that isn't deformed by the impact of the pool balls. I've seen the results of this at a friends house.

    There's always deformation in a collision, it's only a question of how much energy is lost through the collision process to heat, or deformation at the time contact ceases.
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