Originally Posted by bhupaes
Toly is right. The relative speed between the racquet and the ball will determine what happens in the collision. You can look at it from the ball's frame of reference, or the racquet's frame of reference, or a stationary frame of reference for purposes of calculation, and transform the results from one frame to another using simple calculations.
You could do that, but the relative speed between the racket and ball would be constantly changing because racket head acceleration is not a constant, due to the variable force being exerted by the hitter with respect to time. You couldn't calculate it using algebra, you'd need to use vector calculus, a differential equation, or some other sophisticated technique that would more accurately model the situation.