Sport Science

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All Questions are from the publication The Sporting Life unless otherwise indicated.


This list was last updated September 18, 1997

Click me to find out the answer  Why do my muscles sometimes burn when I'm excercising?

Click me to find out the answer  Why do I feel sore the day after I exercise?
Click me to find out the answer  What happens to my heart when I exercise?
Click me to find out the answer What is "VO2 max" and how does it measure cardiovascular fitness?
Click me to find out the answer What's the best position for my hands when I swim freestyle?
Click me to find out the answer How high can you jump?
Click me to find out the answer Why do long jumpers "run" several steps in the air after they take off?
Click me to find out the answer How does ice help a sprained ankle or other injury?
Click me to find out the answer How important is my grip on the bat when I'm striking the ball?

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 Why does spinning a ball make it curve?

 The secret to understanding the curveball is the speed of the air moving past the ball's surface.


The ball in the photo is spinning clockwise, like a ball thrown with backspin. As the ball spins, its top surface is moving in the same direction in which the air is moving (from left to right in the picture). At the bottom of the ball, the ball's surface and the air are moving in opposite directions. So the velocity of the air relative to that of the ball's surface is larger on the bottom of the ball.

What difference does that make? Well, the higher velocity difference puts more stress on the air flowing around the bottom of the ball. That stress makes air flowing around the ball "break away" from the ball's surface sooner. Conversely, the air at the top of the spinning ball, subject to less stress due to the lower velocity difference, can "hang onto" the ball's surface longer before breaking away.

As a result, the air flowing over the top of the ball leaves it in a direction pointed a little bit downward rather than straight back. As Newton discovered almost three hundred years ago, for every action there is an equal and opposite reaction. So, as the spinning ball throws the air down, the air pushes the ball up in response. A ball thrown with backspin will therefore get a little bit of lift.

Now imagine that the photo was taken looking down from above on a ball spinning clockwise (rather than being viewed from the side). The ball would curve toward the right, toward third base, from the pitcher's point of view.

A major league curveball can veer as much as 17 inches from a straight line by the time it crosses the plate. Over the course of a pitch, the deflection from a straight line increases with distance from the pitcher. So curveballs do most of their curving in the last quarter of their trip. Considering that it takes less time for the ball to travel those last 15 feet (about 16 of a second) than it takes for the batter to swing the bat (about 15 of a second), hitters must begin their swings before the ball has started to show much curve. No wonder curveballs are so hard to hit.

One important difference between a fastball, a curveball, a slider, and a screwball is the direction in which the ball spins. (Other important factors are the speed of the pitch and rate of spin.) Generally speaking, a ball thrown with a spin will curve in the same direction that the front of the ball (home plate side, when pitched) turns. If the ball is spinning from top to bottom (topspin), it will tend to nosedive into the dirt. If it's spinning from left to right, the pitch will break toward third base. The faster the rate of spin, the more the ball's path curves.

Click me to find out the answer How does Michael Jordan manage to hang in the air for so long when he goes up for a slam dunk?

   ©1997 The Exploratorium