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Suppose a ball is spinning through mid air. We know through the Magnus effect the way the ball will go. On the side where the air flow is slowed down due to the spin of the ball, the pressure increases because of Bernoulli's theorem (which, unless I'm wrong, states that pressure and speed/velocity are inversely proportional). So does a high pressure zone on one side of the ball cause the ball to move to a lower pressure zone?

Is this how it works for all types of things? And if so, why does it do that? Does the high pressure zone produce a pushing force on the opposite direction? I know wind is caused when air moves from a high pressure to low pressure zone.

PNS
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Viradeus
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1 Answers1

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There is a force opposing the ball velocity, the air drag. It is proportional to the square of the velocity for usual sports balls. If it is spinning, the relative air velocity is different for 2 sides of the ball. The reasoning is better detailed here.

So, the components of that force in the direction transverse to the velocity are not balanced, and there is a net force pushing the ball to the side where the relative air velocity is smaller.

Because the drag force (and the pressure on the ball surface) is proportional to the square of the relative velocity, the result is numerically similar to what happens to fluid flow in pipes, where Bernoulli equation applies. But it is another phenomenon. For example: there is a drag coefficient that also depends on the velocity (more exactly Re number). So the proportionality $P$ and $v^2$ is not exact.

Claudio Saspinski
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