How does a curveball curve?
Imagine that the ball is moving through the air. If the ball has a top spin, spinning toward the direction it’s moving, that speeds up the flow of air below the ball and slows down the flow above the ball. This is a result of friction between the air and the ball.
Daniel Bernoulli, a famous scientist who lived in the 1700s, said that when a fluid is flowing rapidly, its sideways pressure is reduced. This means there is less pressure on the bottom of the ball than there is on the top. As it spins, it has a downward force that adds to the downward force due to gravity.
But the Bernoulli effect is not the primary reason that a curveball curves.
The Magnus effect, named after physicist Gustav Magnus, is more important. Magnus studied the forces on objects moving through a fluid and concluded that the important part is the air being carried around by the spinning ball. This air is released on the back side of the ball, going in an upward direction. Since every action has an equal and opposite reaction, if the air is being pushed upward, then the ball would be curving down.
Just like with a rocket, when air is shooting off in the downward direction, it pushes the rocket in the opposite direction upward.
The rule to remember for curveballs: whichever way the front face of the ball is spinning will be the direction in which it curves.
The effect is more dramatic with ping pong balls and tennis balls because of their lower mass compared to baseballs.