... auditorium.1
Note: In any problem involving estimation, your answer should include an estimate of the uncertainty in your estimation. (I.e., express the result in the form \fbox{~$A \pm \delta A$\space } where  A  is your answer and  $\delta A$  is your uncertainty.) 

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... friction.2
This is a common approximation, but should not be taken for granted. For a ping-pong ball the effects of air resistance would be dramatic! 

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... collision3
In a perfectly elastic collision, the ball rebounds with exactly the same velocity it had coming in, except in the opposite direction. Genuinely perfect elastic collisions do occur in nature, but only on the atomic scale; actual steel balls do not really have perfectly elastic collisions, but they are surprisingly close! 

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