Originally Posted by rkelley
But a tennis racquet is a plane that interacts with the ball. The point where that plane contacts the ball must always be normal to the surface of the ball. The reaction force from that contact has to go through the ball's center of mass. So using a plane to apply the force, how do I create the moment about the ball's center of mass?
Not sure if this is most relevant, but this was put up a couple of days ago:
Reaction force components of racquet on ball. For reasons discussed below, the normal force is typically directed in front of the ball's center of mass.
The perpendicular rebound force usually does not act directly through the center of mass, but rather, in front of it. This happens whether or not the strings move laterally. That is because the ball starts to spin off of the strings on its back side and into the strings in front. The ball thus exerts greater force on the strings on its leading side. Friction slows the bottom half of the ball, causing the upper half to stretch and deform forward, shifting weight as it does so. The greater force on the strings at the front of the ball causes the net normal force to shift forward in front of the CM by a distance D — usually about 2-9 mm, depending on the stringbed setup. This distance, D, is often used to designate the normal force offset as "D-offset."