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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.

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There is a force opposing the ball velocity, the air drag. It is proportional to the speed. If it is spinning, the relative air velocity is different for 2 sides of the ball.

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.

A top spin goes down and a back spin goes up.

We could think that the effect should be the opposite, because in a fluid, increasing the flow velocity causes a decrease in pressure. And in a top spin ball the relative velocity is bigger at the top. But it is not what happens, showing that the drag force is much more important.

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  • $\begingroup$ "...net force pushing the ball to the side where the relative air velocity is smaller". This means that the ball is moving towards an area with a pressure that is smaller? $\endgroup$ – Viradeus May 25 at 14:23
  • $\begingroup$ No. It's just the opposite. $\endgroup$ – Claudio Saspinski May 25 at 23:43

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