Why does a ball swing in air.

[unbiased]

When a ball, is thrown while spinning, it swings in the air. The direction of swing is such that it dips or swerves toward the direction on which side the relative velocity of air wrt. the surface of the ball is lesser, that is towards the side on which the direction of the spin is the same as the direction of wind.

Now isn,t it exactly opposite to how the thrust on aeroplane wings is created. I mean the pressure should be lesser toward the side which has greater relative velocity between air and the surface, but in the case of the ball it is more.

Why is it so? I have forgotten my physics. Will somebody help?

 

[Calin]

Bernoulli principle I think it is called - it says that the sum of static pressure, dynamic pressure and pressure of position stays the same in a fluid in non-turbulent flow.
So, if you have the same fluid (like air) moving at different speeds on different surfaces of an object (like a wing, or a ball), the side on which the speed is greatest has a lower dynamic pressure (dynamic pressure is the pressure that attracts something tangent to the fluid's movement).
Anyway, there are more than 10 years since I studied this, so a better explanation might be in order.

If you shoot a ball "with effect" (rotating in the air), its trajectory will curve to the side where the passing speed is higher (the side that rotates forward) - you can see this in soccer.
And if I remember correctly, there was once a ship that instead of sails had motorized funnels, their rotation combined with wind speed made her move (again different pressure on different areas of the cylinders)

 

[CycloWizard]

Playing soccer is the only thing I've been doing for longer than studying fluid mechanics. Hopefully I can give a decent explanation. 😛

The easiest way to look at this is to consider two cases:
1. Ball with rotation but zero translation (stationary spinning).
2. Ball with translation but zero rotation (moves straight).

Considering the first case, the velocity of the air at the ball's surface will be tangent to the ball and equal to the angular velocity of the ball's surface. Accordingly, the pressure at the ball's surface is lower than the surrounding air.

Considering the second case, the pressure will be much higher in front of the ball than behind - similar to the draft effect of a racecar.

Now, superimpose the solutions of 1 and 2. In essence, the 'back' of the ball is in a lower pressure zone, so it can't exert enough force to cancel out the rotation that is caused by the 'front' of the ball pressing against the air. Thus, if you kick a ball and spin it counterclockwise, it will bend to your left, since the 'front' of the ball's tangential velocity is to your left.

This is readily apparent in soccer when you observe that the amount the ball curves depends on where you kick it (radial position) and how hard you kick it (velocity). For example, in my game last weekend, I hit a shot from about 25 yards from the goal. I hit almost tangent to the ball as hard as I could. The ball headed for the goal originally, but missed wide by about 15 yards. This is also known as a 'shank'. 😛 If I had hit it more softly, it would not have curved nearly as much and may have gone in. If I had hit it as hard but closer to the center of the ball, it might have gone in. But, alas, my touch isn't what it used to be.

 

[Atomicus]

CycloWizard knows his fluid mechanics 😛

Another example would be table tennis.

 

[unbiased]

Hi cyclowizard, thanks for the explaination. What I gather from the answer in essence is that the tangential velocity in 'front' of the ball remains uncompensated by virtue of the pressure differential between the front and back surfaces of the ball.

Consider me a dumb-brick , but I would request a more graphical explaination if possible.
Thanks all the same.

 

[unbiased]

Eureka! Eureka!😀

If interested Google for 'Magnus Effect' .

 

[dkozloski]

The definitive text on the subject is "How To Throw A Curveball" by Don Sutton.