Engineering Question #1572

mark irvine, a 45 year old male from sacramento asks on September 11, 2003,

If you have a square 1 pound block with a front flat surface strike a ball, would it be the same as if you had the same weight block, but cut out in the middle to allow the ball to pass through most of the block until it strikes the rear wall of the cut out? In other words, does it matter where the mass is upon impact with the ball, whether it's behind the ball when it strikes or ahead of the ball?

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The answer

John Jones answered on September 26, 2003

No, it doesn't matter. The two principles we use to calculate the velocity with which the ball bounces back are the conservation of energy and the conservation of linear momentum. In applying these two principles, the only relevant facts are the masses of the two objects, their velocities, and their elasticity. [Editor: so you would have to be sure there was no difference in elasticity ("bendiness") of the thinner back wall in the second case where the ball hits "inside" at the back of the block, compared to when it hits at the front surface.]

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