Why does a spinning ball follows a curve path during flight ?

You must have noticed this while watching a football match that a spinning ball follows a curve path during flight ! So why does this happen ? Lets find out…

Bernoulli’s principle :-

For an inviscid(non viscous) flow of a non-conducting fluid, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy.

The Bernoulli’s principle is named after Daniel Bernoulli.

So here’s why a spinning ball follows a curve flight –

Consider top view of a spinning ball moving with some velocity towards left side. The ball is given a spin in clockwise direction. What you observe here is that –  the top side of the ball the velocity of the fluid(air) is more than the velocity of air at the bottom side. Now, why ?
When the fluid comes contact with the upper side of the ball, the speed of air increases because the speed of ball adds to the speed of air(As the flow of fluid is in the same direction of spin).

When the fluid passes by the bottom side of ball, the speed of air decreases due to friction between both of them.

We see a velocity difference between both sides of the ball. Now according to Bernoulli’s principle, at the top side of the ball, due to high velocity a low pressure region gets created. This low pressure region pulls the ball with certain force – Magnus force. A spinning ball or cylinder curving away from it’s principal flight path is called Magnus effect. The Magnus effect is named after Gustav Magnus, the German physicist who investigated it.
In case of our example, the ball gets pulled upwards(because we are watching from top side). Actually, it is changing it’s path & following a curved one.

Where do we get to see it ?

1. In ball sports. Table tennis players – when hit a top spin, Magnus force pulls the ball downwards, which gives less time to the opponent to hit it back again. Also during backspin, the ball stays in the air for longer period causing delay in it’s flight time.

2. Football – Kicking the ball on either of the side of ball, gives both – velocity & spin to it which curves the path during its trajectory. Sometimes this is enough to confuse the goalkeeper.

3. Flettner rotors – they are high speed rotating cylinders. By controlling the direction and speed of rotation, strong lift or downforce can be generated.
This effect is used in a special type of ship stabilizer consisting of a rotating cylinder mounted beneath the waterline and emerging laterally.

4. Magnus force also affects spinning missiles.

An amazing video demonstration on Magnus effect with a help of basketball ! Watch the difference between the ball just dropped & ball given a spin.

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