∫ Things I learned from teaching undergraduates: Tennis Ball Topspin Edition

The Youtube clip above shows Roger Federer hitting a topspin forehand in slow motion. The ball is moving too fast after he hits it for you to be able to see the direction of its spin, but you can tell pretty clearly from the motion of his racket that he's hitting a little bit beneath the ball and then sliding the racket upwards along the backside of the ball at the same time as he follows through. The result is the the ball will be spinning with what tennis players call "Topspin."  It's a rotation where the ball the top of the ball is moving in the same direction as the ball is flying, and the bottom of the ball is moving the opposite way.

Topspin is a huge deal. Part of the reason Roger Federer is so good is that he hits with 2,500 RPM of topspin, which is more than either Pete Sampras or Andre Agassi (only Rafael Nadal hits with more, although it's been argued that he actually hits with too much!)

Now, like anyone who's ever taken a tennis lesson, I know tennis reasons the pros hit most of their shots with topspin. Coach after coach after coach has told me over the years that there are basically two principal reasons: first, a ball with topspin will "kick" upwards when it bounces (I think it's pretty easy to see why) and that can make it harder for your opponent to hit, because they wind up having to hit it up around their ears. But then there's this second reason: Topspin is supposed to make the ball "drop" sooner, meaning you can hit it higher over the net and still get it to land inside the court. Being able to hit higher  in turn means you have less chance of hitting it into the net, and it means that you can hit it a little harder. Seems like a good deal all around.


But one thing I definitely did not know until a couple of days ago is why topspin causes the ball to move like this. After all, why would spinning make a ball want to fall downward faster? It's not like it makes it any heavier.

Read more »