Can someone please answer my question on this paragraph from my physics book regarding basic aerodynamics:
"Now we consider a spinning baseball moving through the still air. As the ball spins, it tends to drag air around with it. If the streamlines are curved, there are resultant pressure forces on the packets in the centripetal direction. Thus, as with the [airplane] wing, the pressure immediately above the ball is significantly less than the pressure far above the ball. The streamlines just below the ball are almost straight, so the puressure just below the ball is almost the same as the pressure far below the ball."
The diagram shows the streamlines above the ball sharply curved, while the streamlines below the ball are almost straight. What I don't understand is why the streamlines below the ball have less of a curve than the streamlines above the ball. If the ball is symmetrically shaped, why would there be a difference?
Is this related to the spin of the ball? Does that mean that different spins produce different aerodynamic lifts? Can you reverse the spin direction so that the ball has an aerodynamic down force (in addition to gravity)? Can you deliver a minimal spin so there is a minimal aerodynamic lift?
Thanks!