# Bernoulli's principle on a curve ball

I've seen a few excellent answers here on the Magnus force, which explains why balls with a spin will curve. However, my intuition is still telling me that the Bernoulli's principle would push it the opposite way and I need help understanding why my reasoning is flawed.

Imagine that you kick a soccer ball on the left side so that it's spinning clockwise (viewing the ball from above) and the ball will curve to the right. Since the left side of the ball is spinning against the air, wouldn't this mean faster relative speeds and thus a lower pressure than the right side? And wouldn't this lower pressure on the left side cause it to curve left instead of right?

What produces lift is circulation, which causes the airflow to be deflected in one direction, causing an equal reaction in the other direction.

If you want to think in terms of Bernoulli on your soccer ball, the air on the left side is being slowed, while that on the right side is being accelerated by the spin of the ball.

• From the article: "There is a widely-held misconception that it is the velocity relative to the skin of the wing that produces lift." This is exactly what I was incorrectly thinking. Thanks! Mar 2, 2014 at 22:57
• Relative or not, either way you slice it the air will be accelerated more on the advancing side than the retreating side. So the question is still not answered. The only way I can make it work out is because flow separation occurs sooner on the advancing side giving the low pressure area on the retreating side more area. But I don't know if that's correct.
– Tom
Jun 20, 2017 at 20:55

Your intuition is correct. If you only consider relative velocities, Bernoulli's principle and equations will show the ball moves in the opposite direction to how it actually moves. The reason for this is that what is happening with a spinning ball is a little more complex. The roughness of the ball takes a membrane of air with it as it spins and it is the interaction between the ball and this membrane of air that Bernoulli's principle and equations apply.

• i don't think his intuition is correct. The side which spins against the air is much slower, and thus has more pressure on that side. May 25, 2020 at 0:10

one side of the ball is rougher while the other side is softer .the rougher side brings the stream lines above the ball closer decreasing the cross sectional area.then the velocity at that side becomes greater decreasing the pressure at side. this doesnt happen on softer side causing a lesser velocity and greater pressure on that sidetherefore ball travels in an inclined path