A train approaches an observer at the speed of 100 km/h. The observer tosses a tennis ball at the speed of 100 km/h towards the train. What will be the speed and direction of the tennis ball after it has bounced off of the collision with the train. All idealizations apply (bouncing consumes no kinetic energy; front of train is flat and vertical; friction is ignored; train/ball weight ratio is practically infinite; etc.)

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Al Nikolov(01:15:57) :Then, the total kinetic energy will be conserved.

Instead of writing the maths, I’d refer http://en.wikipedia.org/wiki/Momentum#Elastic_collisions

So, the ball will bounce at 300 km/h backwards. This was so easy…

Interesting tough, what if there’s another tennis ball pirsuing the train at 100 km/h since being tossed by some other guy after the train so precisely so they are touching themselves slightly?

Al Nikolov(01:20:12) :PS: I know the answer. But it scares me.

rlankine(08:37:12) :Korrektly answered by Al Nikolov – and it was not even Friday yet. Mind ya, here’s an extra question for those who still want something to ponder over the weekend:

Suppose the train has a slanted windshield. With otherwise the same parameters as in the original puzzle, the ball hits the windshield and bounces exactly vertically (as observed by a static observer on the ground). [1] In what angle is the windshield slanted? [2] What will be the upward speed of the ball after having bounced off of the collision?