Question

Playing in the street, a child accidentally tosses a ball at 21 m/s toward the front of a car moving toward him at 18 m/s. What is the ball's speed after it rebounds elastically from the car?

Answer 1

In an elastic collision (1/2) m v^2 does not change.
21 m/s

Answer 2

in the frame of the car, the ball comes in with 39 m/s speed, and bounces back at 39 m/s in the other direction. In the observer frame we need to add the 18 m/s relative speed of the car, and get 57 m/s

Answer 3

57m/s