Q: Why does putting spin on a ball change how it moves through the air?

Physicist: This question is especially perplexing after a first year physics course, where every question starts with “ignoring air resistance…”. There are a couple of different way to approach the answer, but I like this one best.

Way back in the day a dude named D’Alembert came up with the unsurprisingly named “D’Alembert’s paradox” which essentially says that in air and water an object traveling at subsonic speeds experiences neither drag nor lift. That’s obviously not the case which is why this is “D’Alembert’s paradox” and not “D’Alembert’s well known fact”.
The resolution is to take into account vorticity and the rotation of air flow around the object in question.
Rotation of air allows the air to flow faster over the top or the bottom of the ball. But a funny thing happens when air moves quickly; the transverse air pressure (pressure to the sides) drops. This effect is called the Bernoulli effect or, in certain cases, the Venturi effect.

If you combine the flow of air rotating around a ball, with the flow of air past a ball, then the result is a faster flow past one side and a slower flow past the other side. In this example there is less pressure on the top than the bottom.

So how can you induce air to rotate around a ball in the air? Base balls, golf balls, and especially tennis balls are designed to have rough surfaces that grab the air a little bit. So if the ball spins as it moves it’ll drag the air around it a little, which leads to air rotating around the ball, which makes the air on one side move faster than the air on the other, which leads to a lower pressure on one side compared to the other, which creates a net force, which pushes the ball around.
So if you want the ball to curve downward you make it spin as though it were rolling away from you. If you want it to curve up (well… Stay in the air longer, no Human can make a ball actually curve upward in flight) you’d want the ball to spin as though it were rolling toward you.  And of course, you can also make the ball veer left or right using the corresponding spin.

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3 Responses to Q: Why does putting spin on a ball change how it moves through the air?

  1. Bill Graham says:

    I thought I understood what makes a baseball curve by using the Bernoulli effect. But I realize I’m confused. In your diagram above, the air is moving faster over the top surface of the ball. In your diagram, it says there is less pressure on the top. That makes sense so far. But, less pressure on the top would tend to make the ball stay in the air longer. That’s the reverse of what is said in the last paragraph.

    From experience, I know a baseball (any ball) will curve to the left when it is moving forward and spinning counterclockwise as viewed from above. Again, that seems to be the opposite of what Bernoulli’s effect would cause. I’m still confused or else your explanation is incorrect.

  2. The Physicist The Physicist says:

    I dig!
    The Bernoulli effect is caused by the velocity of the air passing by the ball itself, as opposed to by a particular piece of the surface.
    The threads and bumps and imperfections on the surface of the ball grab the air a little and drag it around in the direction that the ball is rotating. So, in your example, when the ball is turning counter-clockwise it pulls the air around its left side a little faster. So overall the air is passing the ball on the left a little faster.
    However, due to the spin, a point on the right side of the ball feels the air passing faster. But, the actual passage of air (regardless of cause, or how it feels to a point on the surface) is what’s important.
    That trips me up too. Always have to take an extra minute.

  3. Anas Ashif.p says:

    Very useful

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