[yor_on (OP)]

This one is fun.

" Imagine standing on a train platform, and throwing a ball at 30 km/h toward a train approaching at 50 km/h. The driver of the train sees the ball approaching at 80 km/h and then departing at 80 km/h after the ball bounces elastically off the front of the train. Because of the train's motion, however, that departure is at 130 km/h relative to the train platform; the ball has added twice the train's velocity to its own.  "
=

Is it right, or wrong?

[Halc]

Is it right, or wrong?

That's right.  If the ball is large enough to measurably slow the train, it will rebound a bit slower, but it is a small ball I presume.

[Bored chemist]

I think it's sort of related to this
https://en.wikipedia.org/wiki/Bouncing_ball#Multiple_stacked_balls

[yor_on (OP)]

Full points BC :)