[Vass]

Originally Posted by Merton

I am just talking about high school physics in Greece, i guess it is similar elsewhere. The fact that a particle cannot accelerate without the influence of an external force looks pretty elementary. After the racquet contact, the only force on the ball is its own weight.

i'll be a smart-ass for a sec:
Balls with top spin have a Magnus force acting on them, which is why they drop faster. Perhaps this force might also accellerate the ball horizontally, although i'm not too sure.