How a spinning ball produces a force?

Question

I've read that a spinning ball, when it picks up velocity in air, encounters a free stream flow of air which acts in one direction, say left to right. Can someone explain from here how a ball, spinning in clockwise direction, causes a force to be constituted? I just want a clearer picture of this. I don't understand the explanation about the change in velocity, and then pressure, and then a net force up.

Since the ball spins clockwise, so the free stream flow over the top of the ball is assisted by the circular flow; the free stream flow below the ball is opposed by the circular flow.
The above is extracted from a source. So, i was thinking, since the velocity of air is lower below the ball and higher above the ball, (assuming its spinning in clockwise direction like how ur finger does when pointing to the screen) can i apply Bernoulli's Principle here? Where lower velocity means higher pressure and higher velocity means lower pressure, and so constitutes an upward force as well.

Answer 1

You're right; it's an application of Bernoulli's principle, and is often called the Magnus force. Be careful of references like "clockwise" since that could be rotation about any axis. Rotation about the velocity axis would have no effect. The spin must produce a force that is not along the velocity axis; i.e., a lateral or vertical force. When I first learned of this force I had trouble understanding why it was in the direction it is. I thought, the side that moves forward (relative to the center of the ball) sees the greatest relative airspeed, so that should be the direction it curves toward. But the opposite is true. The air's speed over the ball, not relative to the spinning surface, is what matters, because Bernoulli lift is an inertial phenomenon based on the air's mass being forced around a curve. BTW, a ping-pong ball will give an excellent demonstration of curving (including an up-curve) in a restricted space.