The fuzz, more properly called felt, is usually made of wool treated with various additives to make it last longer and resist staining. http://tennis.about.com/od/tennisballfaq/f/faqballs019.htm The tennis ball cover is rough but not like sandpaper. Looking closely at the felt you can see that air flows through those raised fabric filaments - better known as fuzz. So the surface of a tennis ball is a "porous surface." It's more like dealing with the wind blowing through leaves on a tree and just as complicated. The fuzz filaments act like tiny cylinders each having their own drag component. In addition to the skin friction drag from the cover itself, drag is created from the airflow over these fuzz fibers interacting with all the other fibers behind it. Rabi Mehta dubbed this phenomenon "fuzz drag." Experiments Mehta conducted also showed that as the ball speed increases the fuzz filaments lay down on the ball and the fuzz drag declines. Here are two pictures from the wind tunnel test. You can see in the picture on the right that the fuzz is closer to the ball's surface. The air passing over the ball on the left is about 45 mph, while on the ball on the right the air is 135 mph. http://www.tennisserver.com/set/set_01_10.html Researcher Rabi Mehta, working with students, discovered that the "fuzz" on a tennis ball created more drag than was previously believed, since each individual fuzz filament contributes to drag. In addition, he found that fuzz drag is particularly interesting since the fuzz filaments can change orientation during play and gradually wear off. http://www.nasaexplores.com/show2_articlea.php?id=02-083

How would you weigh a plane without a scale?

It allows the racket to control the direction of the ball better. Friction I suppose.

The most expensive ingredient in the tennis ball is the felt, which is adjusted in various ways to produce different properties. Steaming the felt raises its nap to make it fluffier, which increases wind resistance and control (through increased resistance when striking the players' rackets) while reducing bounce and speed. A compact nap causes the ball to skip on hitting the court or a racket

Because they used to be ping pong balls but now they're all grown up!