## The Science of Spin

With Wimbledon coming up, we thought it would be fitting to think about spinning tennis balls.

The basic spin shots in tennis are topspin, backspin, and slice or side-spin on the serve.
The spinning of tennis balls is defined by Bernoulli’s principle which states that when the velocity of a fluid increases the pressure decreases. Air behaves as a fluid and so when a tennis ball is spinning the air flows faster on one side of the ball and slower on the other. This creates a pressure difference on the two sides of the ball which in turn creates a force on the ball towards the area of low pressure causing the ball to move through a curve.

When a ball rotates, the air in contact with the ball’s surface rotates with the ball. The hairy, fuzzy nature of a tennis ball means it has the ability to drag a lot of air relative to a smooth ball, and therefore spin is enhanced.

A topspin shot is made by sliding the racquet strings up and over the ball. The friction between the racquet strings and the ball makes the ball spin forward, towards the opponent. The shot dips down after impact and also bounces at a lower angle to the ground than a shot hit with no topspin. This is the normal direction of spin when a ball bounces due to friction from contact with the ground, but additional spin is applied by the strings. This additional forward spin makes the ball come off the ground at speed.

A backspin shot is hit by sliding the racquet strings underneath the ball as it is struck. This causes the ball to spin towards the player who just hit it. This stroke requires about half the racket head speed of a topspin shot because the player is not required to change the direction of spin. When the ball bounces it comes off the ground at a slower speed to a topspin shot.

In the case of topspin, the top of the ball spins into the oncoming air and the front of the ball moves downwards dragging air down with it. More air gets pulled under the ball than goes above it. Since more air has to pass under the ball it has to move faster. This means there needs to be a higher velocity on the lower side of the ball, and subsequently a lower velocity on the top of the ball.
On the top side of the ball this lower velocity creates a higher pressure, and at the bottom the higher velocity creates a lower pressure as in Bernoulli’s Law. With high pressure on top and low pressure on the bottom, there is an imbalance in the forces on the ball which curves it downward from its straight line path. In backspin, the same principles are in action, except in this case the bottom of the ball has the lower velocity so the pressure is higher. The same principle also applies to side-spin.

To see all these types of spin being put to good use, simply turn tune in to the Championships in the next few days.

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