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Originally posted by Kurt
What if the cars were made from Diamond?

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Originally posted by mwg
who cares

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Originally posted by sand-eel

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Originally posted by CorsAsh

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Originally posted by Limecat
Walls move if you hit them...

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Theoretically, in every impact there is moment of stillness as energy is passed from one subject to another.

To elaborate, if a train going at 100mph hits a person standing on the tracks, the person can not physically instantly achieve 100mph, and must accelerate from zero. Therefore, there is a miniscule moment when the train stops as it hits the person. It gets very messy, very quickly after that though

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The person accelerates very fast, the train doesn't stop

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The point is thus:

The tennis ball does not instantaneously decelerate and change direction. The deceleration occurs over a non-zero time and a non-zero distance (the distance being somerthing less than the diameter of the tennis ball as it deforms on impact).

During this brief (but non-zero) contact, the tennis ball transfers an amount of energy to the train, that energy is first put into deforming the steel surface of the head of the train. The steel surface rebounds, giving most of its energy back to the tennis ball, but some of the energy is transmitted down the length of the locomotive in the form of a shock wave (i.e. sound wave, i.e. WHAP!). This wave has the effect of very slightly (VERY slightly) decelerating the train. This deceleration also occurs over a non-zero time and a non-zero distance. Note that it occurs atom by atom too, as each steel atom shoves the one next to it.

Meanwhile, the tennis ball has the energy given back to it by the train (and some more by the train's movement). It rebounds, (over a non-zero time and non-zero distance), accelerating away from the train.

The key (if it isn't obvious enoguh by now) is the interaction (i.e. energy transfer) over a non-zero time and non-zero distance*.



*(Interestingly, this strikes at the very heart of our current lack of understanding of the universe: why GR and QM are irreconcilable. How can a theoretically zero-dimension GR particle transfer its energy to another zero-dimension particle over a zero time? That results in an energy transfer rate of infinity! But I digress....)

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At the instant the ball is stationary relative to the train, the train is stationary relative to the ball.

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