Howzat* for an explanation of spin?

Feliciano Lopez prepares to serveThese days, sports involve a lot of science and not just
when it comes to getting the athletes into tip-top condition.

Players use
sophisticated spin and swing techniques in any game that involves a ball, even
if participants and spectators don’t always understand the physics of what’s
going on. 

Take tennis, for instance. When you try to
‘ace’ your important
by serving as hard as you can, most people know that you need to apply topspin
to the ball. If you simply hit it hard and straight, the ball will most likely fly
out. By hitting the ball obliquely and adding spin, aerodynamic forces will
cause the ball to curve downwards in flight and stay in. Isaac Newton commented
on the trajectory of a tennis ball as far back as 1672.

You may have seen soccer superstars David Beckham or
Gareth Bale hit a free-kick up and over the wall and into the back of the net.
It takes many hours of practice to become such a dead-ball expert. But what’s
the explanation for their success? It’s our old friend spin again and the way a
spinning ball interacts with the fluid (air) it’s moving through.

So what’s going on? Physicists call it the Bernoulli Effect
after the 18th century Swiss physicist Daniel Bernoulli. Air
pressure on one side of a spinning ball is higher than it is on the other side.
Inevitably air from the high-pressure side pushes the ball towards the low-pressure
area, which causes the ball to curve in flight. (Obviously the angle at which
the ball is hit or kicked, as well as the force of the strike, also has an
effect on the trajectory, as does gravity.)

This phenomenon is vital to many sports. In baseball, it enables
pitchers to throw curveballs. In table tennis it is essential for good serving
technique. In golf, you’ll most likely encounter it if you slice your tee shot.

IStock_000001771717XSmallCricket is an esoteric game and bowling technique can be a
bit of a mystery. Spin bowlers rotate the cricket ball to get it to behave in
quite complex ways, especially if the ball is also interacting with a
cross-wind. What makes good spin bowling so hard to play is that the trajectory
changes during the course of the delivery as different forces come into play
when the ball is rising and then falling. This kind of delivery is particularly
difficult to hit because humans find it hard to track objects that don't move
in a linear trajectory.

Such is the fascination with bowling techniques that NASA
has investigated the art of swing bowling
using a wind tunnel. In contrast
to the slower spin bowlers, fast bowlers or ‘seamers’ employ a technique that
causes the ball to swing in flight due to the air turbulence on one side of the
ball generated by the stitching of the cricket ball’s seam, which sets up a
pressure differential between the two side of the ball.

Laminar air
separates from the surface of the ball earlier than turbulent flow air, so that
the separation point moves toward the front of the ball on the laminar
side. (Bowlers keep polishing the ball between deliveries to maintain one side
of the ball as smooth and shiny as possible.) On the turbulent flow side it
remains towards the back, which induces a greater lift force this side of the
ball. The calculated net lift force is not enough to account for the amount of
swing observed: Additional force is provided by the pressure-gradient force.

As the ball gets more use, the seam effect diminishes and
other factors come into play, such as the roughness of the ball when it becomes
marked during play. Artificially roughing up one side of the ball led to the
‘ball tampering’ controversies of the past.

*Howzat is the traditional cry of the bowler appealing for a
wicket in the game of cricket.


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