In soccer games the trajectory of a football
very often has a lateral deflection, or
swerve,
which makes it difficult for the goalkeeper to guess
where the ball will enter his goal.
The above video shows that swerve occurs
when whirl is given to the ball.
The lateral deflection of a spinning object was first studied
by the German physicist
Gustav Magnus
somewhere in the first part of the nineteenth century.
The idea is that the ball should be spinning about an axis
perpendicular to the flow of air across it.
This causes that on one side the surface of the ball moves faster,
on the other side slower,
than the speed of the ball with respect to the air.
Now, according to Bernoulli's principle, the pressure on the ball
is lower on the side where the air flow has the larger speed.
Hence, the pressure on one side of the ball is lower
than on the other side of the ball.
That results in an extra force and, consequently,
in a nonzero component of the lateral acceleration of the ball.
The ball swerves towards the side where the pressure is lower.
From NASA we have
borrowed
the figure below in which it shows the air flow around the spinning ball
and the direction of the resulting force.
