The physics of Ash Barty’s backhand slice

This week, Australian Open commentator Jim Courier compared Ash Barty’s backhand slice to that of Roger Federer. Though Barty played down the high praise, Courier has a point: while the world’s number one women’s tennis player possesses a whole range of dominating and technically sound shots, there’s something special – and deadly – about her backhand slice.

“Ash’s slice technique is arguably the best backhand slice technique in the world – period,” said Courier during Channel Nine’s broadcast of Barty’s quarterfinal win over Jessica Pegula.

“Men’s, women’s tennis, it doesn’t matter. It’s spectacular. Roger Federer – if you’re back in Switzerland, yeah that’s right I said it, buddy. [Hers] might be better than yours.”

In a post-game press conference, Barty said she’s learnt that her slice is a “weapon” for her.

“I love to use my slice, I love to get creative with it, to use it offensively and defensively… Being able to use it with variety and have different options has been a massive part of my game through this last couple of years of my career.”

It has been. The shot has become her trademark, taking her to two major titles and lifting her to world number one. With killer pace, the slice is often used by Barty as defense to return powerful shots, to set up attacks with her own booming forehand, or as a rhythm breaker.

But what makes it so lethal?

Like the entire game of tennis, it all comes down to physics. Newton’s three laws of motion, for example, can be seen in action in every game – the second law (force = mass x acceleration) governs how a tennis racket hits a ball, with the force of the racket determining the resulting acceleration, and Newton’s third law tells us that the racket and ball exert equal and opposite force on each other.

Elasticity comes into play, too. Since the racket’s strings are highly elastic, they exert a restoring force on the ball and help catapult it off again across the net, while the much more inelastic ball loses energy when it bounces on the court due to friction (though how much it loses depends on the type of playing surface).

But what about a specific shot like the backhand slice?

Machar Reid, Head of Innovation at Tennis Australia and possessor of a PhD in biomechanics, says that there are four key ingredients to hitting this shot.

The first is actually getting to the ball, to ensure that the impact point – where the ball hits the racket – is comfortable. Then, the player needs to be balanced, regardless of which stance is adopted (open, square or closed).

Third, Reid says, is “the forward swing trajectory of the racket – this needs to be downward, about 15 degrees to the horizontal or thereabouts”.

And the final critical ingredient is the angle of the racket face on impact: “It needs to ever so slightly angle back,” Reid explains.

So, how does Barty’s shot shape up?

If you watch in slow-motion as Barty gets ready to play a backhand slice, you can see her taking her racket back high behind her head. Her left hand is on the throat of the racket and her right hand is in a continental grip, with her right arm bent at an angle greater than 90 degrees.

While most other players bring their rackets just up to neck level, Barty’s is much higher, allowing her to generate more speed when she brings it back down towards the ball. The bend in her elbow straightens out until it’s fully extended, and she hits down and through the ball between hip and shoulder height with a slightly angled racket face. This gives the shot its lethality: spin.

Spin is essentially a result of the downward motion of the racket as it comes in contact with the ball. Since the two objects are not moving in the same direction, there is friction between the highly tensioned strings of the racket and the ball as they impact. This causes the strings to “grab” the ball (helped along by its fuzzy exterior) and drag it in the same direction the racket is moving.

So when the ball rebounds off the racket, not only has it been accelerated in a particular direction, but it’s also spinning. A backhand slice gives the ball backspin, meaning it’s moving counterclockwise as it soars over the net.

In shots that are aiming to give a ball backspin, the swing path of the racket needs to be from high to low – hence Barty raising her racket high behind her head in preparation. The opposite is true for topspin. For both types of spin, the racket face needs to be nearly vertical.

But the physics of the shot doesn’t end there.

As the ball travels across the court, it interacts with the air – and fluid mechanics comes into play. The air acts like a fluid as the spinning ball passes through it. The ball has a layer of air particles that “cling” to it as it rotates, and these smash into the oncoming air as the ball travels. For a ball with backspin, this reduces the velocity of the particles on the bottom of the ball, while the opposite is true for the top of the ball.

This creates an upward force that pushes the ball upwards, called the Magnus effect. (For a ball with topspin, the opposite is true: it’s pushed down.)

Of course, gravity is still acting on the ball, pulling a ball with backspin back down to a fairly flat trajectory. This creates the illusion that Barty’s backhand slice seems to cause balls to “float” and stay in the air for longer than physically possible.

Then there’s the landing. A ball with backspin bounces lower than expected, forcing the opponent to generate more power to hit it back up over the net.

“This is largely on account of the angle of incidence of the shot or ball coming in, as distinct from the spin itself, which theoretically makes the ball sit up,” explains Reid.

But sometimes, the ball from a backhand slice will float too much or bounce a little higher with not enough speed, and gives the opponent more time to set up their own shot.

To prevent this and keep the ball on a flatter trajectory, Barty uses her body position. As she prepares to take the shot, she turns her body so her right side faces her opponent. Then, as she hits the ball and her racket swings out, she uses her left arm as a counterbalance to prevent her body swinging fully around too. This stops her racket face from opening up and changing the spin on the ball.

For players who are used to finding power in rotation, this can be difficult to master, but Barty has perfected it.

According to Reid, this is why Barty has one of the best backhand slices in the business.

The shot, he says, is not hard in a relative sense: “Notwithstanding you’re not in control of the oncoming on slice shots, the stroke… is infinitely less complex from a coordination point of view than something like the serve.

“Ash has incredible hands and a great eye, but she’s also practiced it a lot! Most players haven’t – or still don’t.”

Turns out that where physics is involved, practice does make perfect.