There's a lot happening here that you can see, but there's also a lot of invisible forces at play. That's true whether you're watching Naomi Osaka at the US Open or a match at your local horse. So I'm not a tennis player, but the Grand Slams have always been a must-see.
And as a scientist watching the pros play, I had this thought. How do they hit the ball with so much force at extreme angles with incredible accuracy, all while keeping it in the court? What it comes down to? Physics. Let's take a look.
In tennis, players leverage spin when they make contact with the ball. Specifically, topspin and backspin. Spin is exactly what it sounds like, an object rotating, whether it's when you throw a beach ball up in the air or hit a ball on the tennis court. So why does spin affect the way a ball moves? It's a phenomenon known as the Magnus Effect.
Let's take a spin back in time to the 1800s. and meet physicist H.G. Magnus.
He noticed that rotating cylinders moved sideways when they were perpendicular to the airflow. Some tried to take advantage of that to move boats. That didn't catch on.
For a more modern take, let's flip over to NASA, where researchers have been studying this. These pictures were taken in a NASA wind tunnel to demonstrate how balls with spin interact with air. Why was NASA playing around with tennis and soccer balls?
NASA isn't interested in aerodynamic software. sports balls. NASA is interested in aerodynamics, of course, because it applies to airplanes and spacecraft.
And sports balls are a good way to get kids interested, which is how this project got started. Aerodynamics is the study of objects moving through air, and Robbie Mehta has been studying that since the 1970s. To see spin in action, let's go back to the tennis court, specifically to a top spin serve or a kick serve, as they call it.
Let's zoom in. See how the racket is angled down? When it makes contact with the ball, the balls propel toward the other side of the court.
That motion, combined with the friction between the ball's felt and the racket's strings, gives the ball a gentle upward tug, making it spin up and forward. And what we're really interested in is where does the smoke leave the surface of the ball? That breaking point plus the combination of smoke currents and the turbulent wakes behind the ball tell the story of topspin and backspin. So if the wake is deflected downwards, then there's an upward force. If the wake is deflected upwards, then there's a downward force.
Simple, huh? Maybe for a NASA aerospace engineer. I stalked to biology for a reason. Let's go over that force generation step by step.
Take a look at this forehand with topspin. Now look at this NASA image. What scientists are looking at is this point right here.
It's the breaking point where the air layer comes off. Notice that the separation in NASA's topspinning ball is not symmetrical. On the top side, the air comes off earlier than on the bottom.
And it is that asymmetry that is key to how the magnet force is generated. And it's the same whether it's a tennis ball, soccer ball, baseball, what have you. That force comes from a difference in pressure between the ball's top.
and bottom. For a top spinning ball, the pressure is greater at the top, resulting in a downward force. That force gives you the ability to hit the ball harder and still make it in the court. You're leveraging the magnus force and gravity to pull the ball down. Now let's look at what happens during a backhand with backspin or slice.
Spoiler alert, it's the opposite. That pressure difference results in a slight lift. Lift is that upward force that Robbie referenced earlier. Let's look at a close-up of a tennis ball with backspin to understand where it gets its lift. See how the separation of the smoke, or the wake, is deflected down?
That got me thinking about Isaac Newton and his laws of motion. He watched tennis balls and noted how their flight was affected by spin in the 1670s, long before Magnus was even born. I guess he was too busy discovering gravity and devising our overarching laws of motion to claim this phenomenon too. Still, there's no escaping Isaac. Yeah, so Newton's law actually...
invoked when I said that if the wick is deflected one way, there's an equal and opposite reaction in the other direction. So that's where Newton's law comes in. And it's not just the flight of tennis balls where this concept is critical. These principles apply to everyday life. Lift, for example, is critical for airplanes, drones, and kites.
You can even test this principle at home with a beach ball. Now that I understand how physics gives tennis pros their edge, I'm even more excited for the U.S. Open.
While you're watching, keep an eye out for all that spin. And remember that you can experience science anywhere you go, just like Magnus and Newton did. Remember, we're all scientists. Don't forget to hit subscribe so you don't miss any of our updates.
And if you're interested in science and sports, go over to WSJ.com. I've left some links down below to get you started. Hasta luego.
Nos vemos pronto. Gracias.