Understanding how a paddle affects a ball’s motion can help players win games. Whacking the ball transfers energy from the paddle to the ball. Harder hits transfer more energy, moving the ball to the other side of the net faster.
But just hitting the ball hard is not the best strategy, says Chang Kee Jung. He studies sports science at Stony Brook University in New York. Instead, Jung says, players should hit the ball in ways that make it rotate, or turn, in the air. This is called adding spin. It makes the ball move in ways that are hard for opponents to predict.
For example, if players brush the top of the ball with their paddle as they hit it, the ball rotates forward in the air. That motion is called topspin (see Topspin Up Close). The spin causes the force of air to be greater above the ball than below it. The air pushes the ball down, making it move in an arc instead of a straight line.
As the ball hits the table, it transfers energy to it. Some of that energy is transferred back to the ball. That causes the ball to bounce. If the ball has spin, friction between the ball and the table pushes it in a new direction.
In professional table tennis games, a ball can spin up to 50 times in a second. Players must watch their opponents closely to predict where the ball will bounce. They have just a fraction of a second to hit it back!