Why do the dimples on golf balls, the stitches on base balls,
and the fabric on tennis balls all make them go farther? In other
words, why does turbulence cause something to obtain less
friction rather than more?
Brad
It has a lot to do with the Reynold's number of small elements
of the surface.
Add some dimples or hairs or other odd additions and the
surface's effective Reynold's number is increased and as a result
lift is increased (especially if the 'ball' is spinning - watch
what cricket bowlers do to their balls (one side is more polished
than the other - ie the smooth side has a lower Reynold's number
than the other).
If you want a more precise explanation refer to fluid mechanics
texts in particular those that deal with turbulent boundary
layers and wakes about spheres.
Essentially, the roughened surface results in the early
development of a turbulent boundary layer about the surface of
the ball , substantially reducing the wake and thereby reducing
the effect drag (by perhaps 50%)--- it flies farther for the same
effort.
I think (but am not sure) that the reason is something like as
follows.
If the ball is roughened then when it is moving it causes nearby
air to rush away, therefore causing a pressure reduction nearby.
Now when a golf ball flies through the air, due to its spin the
relative speed between the upper surface and the air is greater
than the relative speed between the lower surface and the air.
This means that the pressure of air above the ball is lower than
the pressure of the air below the ball, and this pressure
gradient causes lift. This effect is obviously going to be
exaggerated the more effect the moving ball has on nearby air,
therefore it is better for it to be slightly roughened.
With cricket, the bowlers don't polish the ball to give it extra
lift or make it fly farther - rather to make it swing to one
side. Same principle though. In cricket, the question of why
reverse swing works is still open - though there are many
theories. Also the question of why swing bowling is more
effective in overcast conditions - something which is empirically
true - has not been answered.