hmmm i still dun quite understand the reasoning behind why the train can't stop, can u give a clearer explaination. And also why the tennis ball will explode if it happens. thx!DaveC426913 said:The logic goes thus:
The tennis ball clearly must stop in order to change direction, even if only instantaneously.
If the tennis ball is stopped even for a moment, and is in contact with the locomotive, then one could conclude that the locomotive is stopped even for a moment.
...if one didn't know that tennis balls and locomotives can deform.
It could happen with the conservation of momentum, however ...tommyers said:If I were to throw a tennis ball at an oncoming train would the ball stop the train momentarily while the ball changes direction? Due to the conservation of energy!
Conservation of momentum dictates that the train must keep moving.Delzac said:hmmm i still dun quite understand the reasoning behind why the train can't stop, can u give a clearer explaination.
No, it would merely slow it down by a very tiny bit.tommyers said:Hi,
If I were to throw a tennis ball at an oncoming train would the ball stop the train momentarily while the ball changes direction?
That is not a complete argument - in fact, I see no possible way that you can arrive at this from energy conservation. Provide a complete argument, and we can point out the error.Due to the conservation of energy!
The parenthetical is really only true from a limited number of reference frames. It's very easy to see that there are inertial frames of reference where the ball travels an arbitrary distance. (Not to mention that, since it deforms the tennis ball's diameter isn't well-defined.)DaveC426913 said: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).
I believe this actually confuses the issue becasue it's not the same thing at all as the original problem.pervect said:Let's start with an easier question. If you threw a tennis ball at a train that was standing still, would the train move backwards?
Use common sense. Suppose a tennis ball hit an automobile - would the car go flying backwards, or would it stay put?
Not really. In fact, the point is that it is the same problem!DaveC426913 said:I believe this actually confuses the issue becasue it's not the same thing at all as the original problem.
It could be argued that, yes, if you hit an automobile with a tennis ball, the car does move backwards a tiny bit (no, not flying backwards like you claim but "a bit"). This lead the OP away from correct answer to the question.
Yes, in principal it is, but for someone struggling with how a ball does NOT stop a train, it can be confusing to show them how a ball CAN move a train.pervect said:Not really. In fact, the point is that it is the same problem!