As a tennis player, I'm interested in the effects of top spin in tennis. I've done some research, but find myself somewhat confused about the concepts. (my questions assume the ball has top-spin) Magnus force & separation wake: in what direction do they act? do they act straight downwards or in a downwards direction? I can't seem to find anywhere in which it explains the direction they act in. When the balls approach high speeds, why does the boundary layer become turbulent and what affect does it have on the ball? And how can the boundary layer possibly 'reverse' or have no velocity? When the symmetry of the pressure forces are no longer equal behind and in front of the ball (due to turbulence/ unequal separation wake - delaying/hastening of airflow), how does this cause it to move downwards? I have many other questions, but i won't flood you guys. Thanks in advance!