A body in liquid experiences buoyant force, which is known fluid, it also experiences another force known as dynamic lift.
The figure shows a ball moving horizontally in air without
spinning. The velocity of air at points 1 and 2 is the same. So,
by Bernoulli’s equation, the pressures at 1 and 2 would also be the same. Hence the dynamic lift on the ball is zero.
Now consider a ball, which is moving horizontally with velocity
v’ and also spinning about the horizontal axis passing throughperipheral velocity v. When the surface of the ball is rough,
it drags some air along with it. This results in increase in
velocity of streamline flow of air at point 1 to v’ + v and
decrease at point 2 to v’ - v. Hence pressure of air at point
1 is less than the pressure at point 2, which gives dynamic lift
to the ball in the upwards direction. Again, if the ball was
spun about a vertical axis lying in the plane of the figure
making it rotate in the horizontal plane, it may deviate towards
the off or leg stump. This is the reason for swing of the ball in fast bowling.
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