Re: Why does a curveball curve?

A curveball curves because a pitcher who is far more talented than I am
is able to throw it in such a fashion that the ball spins.  

There is a web page that will tell you exactly why this causes it to curve,
using a lot of very scary Greek letters and things.  It is at:
http://math.gmu.edu/~cle5/baseball.html

There you will find the long answer.

There is a more "lay person" page at
http://muttley.ucdavis.edu/Book/Sports/instructor/baseball.html

That is the short answer.

Now here is my answer:

The spinning ball creates drag.  An air flow is created on one side of
the ball that is in the same direction as the air that is rushing past the
ball in its flight.  On the opposite side of the ball, the air flow is
reversed, and runs against the air rushing past.  This has the effect of
slowing down the overall air flow on that side of the ball, while on the
other side, where the flow is in the same direction, it has the effect of
speeding it up.  The Bernoulli Effect comes into play here:  the greater
the velocity of air, the less pressure it will exert.  So the side of the
ball where the velocity of the air has been reduced will have a greater
pressure exerted on it...and hence, the ball will be pushed to the other
side.

It's the same effect that makes the shower curtain reach out for you when
you're in the shower.