Q:
One of my young, inquisitive students asked me what made some slice serves curve more than others. I know it is relative to ball spin, but I had a hard time explaining why this occurs. Could you explain this technical aspect of the service?
A: The spin of the served ball does create a curved path. Let's use the example of comparing a flat serve hit with no spin (which, by the way, is almost impossible) and a slice serve hit by a right-handed player.
A flat serve will behave like a knuckleball pitch in baseball. The ball moves erratically as it travels toward the opponent's service box, but basically follows a straight line. When a slice serve is hit by a right-hander, the racquet moves from left to right as it brushes the ball. This brushing action causes a "3-o'clock" or a sidespin, and the tennis ball will rotate on a vertical axis as it travels forward. The typical flight path is similar to that of a curve ball from a right-handed baseball pitcher, causing the ball to "break" from right to left. The more spin on the ball, the more it will curve.
The curving action is caused by pressure zones created around a ball. As a spinning ball travels toward the opponent's court, it meets a flow of air, and the spin forces a thin boundary layer of air to be carried with the ball. A ball with sidespin will have a slightly lower pressure change on the side where the boundary layer travels in the same direction as the oncoming air. Since the boundary layer is traveling in the same direction as the oncoming air, the speed of air is increased, causing slightly lower pressure. On the other side of the ball, the boundary layer is going in the opposite direction as the oncoming air, which creates turbulence on the side of the ball. This turbulence means that one side of the ball will have a higher pressure zone than the other side, forcing the ball to move from right to left as it travels forward. Interestingly enough, the more spin created, the higher the pressure created on one side of the ball and the greater the change in the ball's flight path.
The same situation occurs with topspin and underspin. A ball hit with topspin encounters turbulence above the ball. This high-pressure zone causes the ball to be pushed downward, creating the arc in its flight path. Likewise, a ball struck with underspin tends to stay in the air longer because the air turbulence is located beneath the ball, which prevents it from having the leaping effect of topspin.
These flight patterns also will have an effect on the resultant bounce. A ball with topspin has a sharp descent angle, whereas a ball with underspin usually will have a lower angle of approach to the court.
Basically, the more spin placed on a ball, the more it will curve in the direction of the spin. This explanation works for all spins placed on a tennis ball, baseball or any other spherical object flying through the air at a high speed.
