Why Is the Coefficient of Restitution Less Than 1 in a Tennis Racket Collision?

[charlie1990]

1. calculate the coefficient of restitution when a player hits a ball traveling into a racket at 10m/s and the racket is stationary before the ball hits it. after the ball hits it, the ball has a velocity of 5m/s and the racket is moving in the opposite direction at 1 m/s (Velocity before a - velocity before b)/(Velocity after a - velocity after b)
I know this is what's used to get the answer normally, but I am getting the wrong answer (above 1).

I think it has something to do with racket going opposite direction but not sure what to do?

 

[Doc Al]

(Velocity before a - velocity before b)/(Velocity after a - velocity after b)

You have that a bit backwards (or upside down). You want the relative speed after the collision over the relative speed before the collision.

 

[charlie1990]

You have that a bit backwards (or upside down). You want the relative speed after the collision over the relative speed before the collision.

cheers, poor mistake

 

[haruspex]

cheers, poor mistake

... and the sign reverses.