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They want to look as if they are adultsSurface area is increased. I would guess this does several things - it allows the racket to get a better grip for spin although this can be done with both squash balls and snooker balls - it will slow the ball down because of increased air turbulence - it does change other aerodynamic properties; the seams stabilize the ball Other things are possible but it is beyond me
Tripping the boundary layer to reduce drag on spheres is widely used in sports. For example, it is the reason why golf balls are dimpled. The dimples act like a very effective trip wire, and the reduction in drag due to the delayed separation allows the ball to travel further for the same effort. A driver shot in golf can easily make a golf ball carry 250 yards, but the same shot using a smooth ball will only carry about 100 yards. Similarly, a tennis ball has a textured surface with a convoluted seam, much like a baseball. Figure 6 shows how effective different degrees of roughness can be in reducing the drag on a sphere.Figure 6. Drag coefficient as a function of Reynolds number for spheres with different degrees of roughness. From Munson, Young & Okiishi, Fundamentals of Fluid Mechanics, John Wiley & Sons, 1998.
Wow !!..Thank You....thank you all for your fantastic speculations, opinions theories and quoted texts. I really do appreciate the time taken to answer and have enjoyed reading the comments and posts. Excellent texts above George...cheers.You have all handled my hairy balls very well !!........................(sorry....couldn't help myself) 
You will not be able to play tennis correctly if you use old tennis balls. The balls are made up of a pressurized rubber core covered in felt. When this felt becomes worn & smooth, it will not react properly when it is struck by the racquet. The ball will tend to skid when it hits the court.
You have all handled my hairy balls very well !!
Quote from: neilep on 01/08/2007 03:00:11You have all handled my hairy balls very well !!I knew there was a good reason for delaying my response to this thread........................
(How sad am I that I find amusement in my own threads ?)