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Q & A: bouncing balls and Newton's 3d law

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Most recent answer: 08/10/2015
Can you show me that bouncing of ball as an example of every action has a reaction?
- Parashar (age 12)
Guwahati, Assam, India

A ball bouncing on the ground or off a wall makes a very poor illustration of momentum conservation (Newton's 3d law). The reason is that the momentum lost by the ball goes to the earth, which is so huge that it hardly changes velocity at all.

If, on the other hand, two balls bounce off each other they make excellent illustrations of momentum conservation. You can see that the change of the vector velocity of one ball times its mass is opposite to the change in vector velocity of the other ball times its mass.

Mike W.

p.s. We don't really like to use the "action-reaction" description of forces. Nature has no idea which is the "action" and which is the "reaction".

(published on 08/10/2015)

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