Originally Posted by toly
Direction of the sidespin is exclusively defined by direction of horizontal tangential component of the racquet velocity, because the ball is absolutely symmetrical figure about his center. Inside or outside ball have nothing to do with these types of spins.
In practice it is very important. Ball is not a hard sphere and racket is not a hard rod. The contact is distributed over the whole impact area and can never be tangential.
It is interesting that a truly rigid racket/string and a truly rigid ball can never produce topspin. No, tangential contact will not do, so don't try to rotate your head and hands now and imagine how is possible. In theory, a tangential force has no component into the ball.
Failure to understand that there is a zone of impact is responsible for much confusion. There is no true tangential or grazing shot. A tangential movement of the strings actually deforms the ball and the strings and automatically imparts forward momentum. If you have studied the 2-spring model of topspin, you can see that it is the linear forces of the springs that actually impart the angular rotation. They will automatically also impart a forward momentum.
This is true even for table tennis, where the ball is harder, the rubber is thinner, and the coefficient of restitution is higher. Even in that case, there is no such thing as pure tangential grazing. In tennis, the soft ball and the deforming strings make the forward momentum even more pronounced.