# Topspin and backspin (tennis)

1. Aug 10, 2013

### ViolentCorpse

Hello everyone,

Using what little knowledge I have of physics, I guess that when a tennis ball is given a topspin, the velocity of the ball relative to the air is higher on the top than on the bottom of the ball. If Bernoulli's principle is applicable here, then the pressure should be higher on the bottom, giving the ball lift.

Now my guess is obviously very wrong, since the contrary always happens in fact (a top-spun ball dips and back-spun ball stays in the air longer), but I can't figure out the correct explanation for this effect.

Any help would be greatly appreciated!

2. Aug 10, 2013

### rcgldr

The faster relative speed should correspond to lower pressure, but the boundary layer is so thin (small) that it shouldn't have much effect. The theory behind Magnus effect is that the air flow near the ball tends to separate (detach) further behind the ball on the side spinning in the direction of relative air flow than it does on the side spinning into the relative air flow, resulting in diversion of the wake that trails the spinning ball, with lift being generated in the opposite direction of the wake diversion. Wiki article:

http://en.wikipedia.org/wiki/Magnus_effect

3. Aug 11, 2013

### ViolentCorpse

Magnus effect is way out of my depth, hehe.