Spin has transformed modern-day tennis. Here’s the physics behind it
- Written by The Conversation
Watch any match at this year’s Australian Open and you’ll see balls curving in the air or bouncing higher or lower than expected. Players such as Novak Djokovic, Iga Swiatek and Coco Gauff are particularly masterful at the art.
The secret? It’s all about spin.
The ability to control a tennis ball’s spin has transformed the modern game, making it faster and more spectacular than ever. But how exactly do players make the ball move through the air or bounce off the court like that?
The complex physics of spin
The physics underlying the effect is complex. But the easiest way to explain it is to consider the Magnus effect.
When a spinning ball moves through the air, it creates a force that makes it curve away from its straight-line path. Scientists call this the Magnus force, named after German physicist Heinrich Gustav Magnus who first described it in 1852.
The force is generated by the spinning ball dragging air around it in an unusual way. When a tennis player puts topspin on the ball (for example, by brushing the racquet upwards over the back of the ball during the shot), the ball spins forwards end-over-end after contact. As it spins, the friction between the ball and the air starts the air moving in the direction of spin.
Because air spins around the ball, oncoming air hitting the top of the ball collides with oncoming air due to the spin, and slows down. But air hitting the underside meets air moving in the same direction and travels relatively faster.
According to a law first introduced by the Swiss mathematician, Daniel Bernoulli, pressure is greater on top of the ball, where air flows slowly, than under the ball where the air moves quickly. This pressure difference pushes the ball downwards.
It’s the same principle that helps aeroplanes fly – except in reverse so the ball drops rather than flies.
Swiss mathematician Daniel Bernoulli first described the law that explains how a difference in air pressure helps aeroplanes fly.
Ozant/Shutterstock
Topspin: a lethal weapon
Topspin is one of the modern tennis player’s most powerful weapons. When top exponents such as recently retired star Rafael Nadal deploy it, the ball can spin at more than 50 rotations per second!
But why is topspin so lethal?
If a player hits the ball at a high speed, it should normally sail over the baseline because there’s insufficient time for gravity to drag it down into the court.
When hit with topspin, the ball will dip down quickly into the court. This allows players to hit the ball with a lot of horizontal velocity without the ball sailing over the baseline.
And because the top-spinning ball spins in the same direction as the ball travels, the ball continues quickly even after bouncing. So, the ball travels quickly both before and after its collision with the court, allowing less time for the opponent to get to it.
A final benefit is that the ball also tends to bounce higher than normal, which can make it harder to hit for an opposing player.
This is partly because the ball strikes the ground at a steeper angle as it curves downward toward the ground and partly because a ball spinning in the same direction as its travel will skid less as it bounces, saving energy and allowing the ball to bounce higher.
Backspin: the gentle winner
Alternatively, a player might “slice” the ball to put backspin on it by brushing under the ball with their racquet.
This creates an upward Magnus force, making the ball float and stay in the air longer than expected. The ball can then be kept low over the net and yet still travel far into the opponent’s side of the court.
When it lands, it stays low and can even skid slightly. So the ball slows down and stays low, making it hard to hit. In response, opponents often have to hit a slower return shot because the ball doesn’t bounce high enough to hit it with a lot of topspin.
The most dramatic use of backspin comes in the form of the “drop shot”, where players disguise a gentle, heavily underspun shot that barely clears the net, then “dies” on the bounce. This leaves little time for the opponent to rush to the net to make the shot before the ball bounces for its second time and the point is lost.
The Spanish star, Carlos Alcaraz, is a master of this type of shot, as you can see in the below video.
Harnessing the physics of spin
This year when you watch the Australian Open, pay attention to how players use spin to control the court: that “heavy” forehand that pushes an opponent back, the slice serve that draws them wide, the delicate drop shot that brings them forward.
And you might watch for players who put sidespin on the ball, the effects of which I’m sure you can now guess.
They’re all examples of players harnessing the physics of spin to their advantage.
