Re: Physics Test Message #22 Posted by Dave Shaffer (Arizona) on 19 Jan 2013, 9:30 p.m., in response to message #1 by Howard Owen
I used to teach this stuff - this is an exceedingly poorly-stated (but interesting) problem. As I see it, there is no obvious solution. You need to make assumptions not stated in the formulation of the problem.
Various of the answers above address my 3 possibilities below. Take your choice!
In fact, I rather doubt that it is a real physics problem (if so, and if I were Department Chair, I would sit the original poser of the question down and ask him to refine his problem statement!), and that it is only a set-up for the "dead bodies" answer.
Otherwaise, there are three ways to attack it, all of which involve assumptions (which is why it is poorly stated).
1) You can chose to ignore Y effects - in which case only the X speed changes, by a kinetic energy change of 1 joule per bounce
2) You can chose to ignore X effects - in which case the X speed does not diminish, and the Y speed/height of bounce changes by 1 joule of kinetic energy per bounce
or
3) You can chose that the 1 joule loss is apportioned (but you must still state your assumptions as to how much!) to both X and Y speeds.
(there is a 4th mode, noted above, that the ball would start to rotate and some energy would go into spinning up the ball - but since we don't have a size for the ball, you can not really address this - i.e. you can not calculate the moment of inertia!)
If I had to pick, I would go for Walter's equal-angle bounce version above - it is my mode 3. However, I see no logical reason, other than by stating your case, that the bounce take-off angle should match the incoming angle - this requires an assumption as to the apportionment of energy loss to the X and Y components. (Light from a mirror bounces this way, but real-world balls could do almost anything!)
The question poser gets the failing grade!!!!
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