physics!
posted by Eliana .
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?

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

physics! 
Ram
57m/s
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