Originally Posted by Dimcorner
I think technically speaking you do not need upward force to create spin. If you hit with more forward force on the top of the ball vs the bottom of the ball then the net effect would be that the ball rotates with the leading edge being the side that was hit with more force.
That is how top spin is modeled by TWU professor. The contact area is modeled as having two springs, one at the top and one at the bottom. If the top spring is compressed more than the bottom spring, top spin is produced. Vice versa produces slice.