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Homework Help: Easy question about bouncing

  1. Nov 23, 2012 #1
    Why does an object bounce when speaking about momentum and Newton's third law
    I would like to say it is because the upward force (the force the object falling enacts on the ground and the ground acts on the object) is greater than the force of gravity, which would be impossible in an object that is dropped not thrown. My second thought is that the momentum is greater in the falling object but would that cause it to bounce and if so why? I am not asking about the elasticity or the molecular structure just bouncing and how it relates to the third law.
  2. jcsd
  3. Nov 23, 2012 #2
    Welcome to PF.

    Who says that the force the ground exerts on the object can't be greater than the force of gravity? The contact time can be very short.
  4. Nov 23, 2012 #3
    The force the ground exerts on the object is the same as the force the object exerts on the ground third law of motion the only force downward is gravity so all of it is exerted on the ground.
  5. Nov 23, 2012 #4
    Thanks for the welcome hopefully i dont have too many questions
  6. Nov 24, 2012 #5


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    When the ball is at its lowest point (somewhat flattened onto the ground by the impact), the lower part of the ball is under compression. So the forces acting on the mass centre of the ball are gravity down and a compression force up. The forces at the ground are the same compression down from the ball and an equal reaction from the ground. Since this compressive force exceeds gravity the ball accelerates upwards.
  7. Nov 24, 2012 #6
    I did not even think of compression forces thank you for your aid.
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