 So the ball is moving at 10 meters per second. Alice is stationary, so the difference in velocity between the ball and her hands is 10 meters per second. So that's how fast Alice will feel the ball collide with her hands. So now let's talk about things from Bob's point of view, who is standing on the platform watching the train go by. Now Bob sees the train going by really fast. It's a bullet train traveling at 90 meters per second. So in Bob's coordinates, how fast does Bob see the ball moving and how fast does Bob see Alice moving?