The Physics of Ball Spin in Tennis, Topspin and Backspin

-         A topspin shot is hit by sliding the racquet up and over the ball as it is struck.  By dragging the racquet over the ball, the friction between the racquet’s strings and the ball is used to make the ball spin forward, towards the opponent.  The shot dips down after impact and also bounces at an angle lower to the ground than a shot hit with no topspin.  As a ball travels towards a player after bouncing, it has natural topspin that is caused by the friction of the tennis court.  When hitting a topspin shot, the player is reversing the spin of the ball, which requires more energy.

 

-         A backspin shot is hit in the opposite manner, by sliding the racquet underneath the ball as it is struck.  This causes the ball to spin towards the player who just hit it as it travels away.  Physicist Howard Brody has pointed out that generating slice, or backspin, requires only about half the racket head speed compared to hitting topspin, because the player is not required to change the direction in which the ball is spinning. The oncoming ball bounces off the court with topspin, spinning from top to bottom as it comes toward the player. When a player returns the ball with a slice shot the direction in which the ball spins around the axis of rotation is maintained. The direction of the shot changes, but the ball continues to spin from top to bottom, from the player's perspective as it moves away from the player.

 

-         Sergi Bruguera, is know as a player who utilizes a lot of topspin.  His topspin forehand shot was recorded at an incredible rate that averaged over 3300rpm.  In men's pro tennis it normally takes about 1 and 1/3 seconds for the ball to travel from one player’s racket to the other in a baseline exchange. In the case of Sergi Bruguera's forehand, this means the ball is turning over itself 70 to 80 times as it travels to his opponent!

 

-         Examine the 3 different clips of the tennis balls in a wind tunnel at a simulated 120 mph.  The first clip is with no spin, the wake of the streamlines goes straight down the middle behind the ball. Now look at the wake of the ball with topspin. The wake is pointed upwards. In the case of backspin, the wake is pointed downwards. The fact that the wake is pointed downwards signifies that the ball is actually being forced up. For topspin the opposite is true - the wake pointing up signifies that the ball is being forced down.

 

-         When a ball rotates, the air that is in contact with the ball’s surface rotates with the ball. The air a little farther from the ball gets drawn along too but not as fast.  In the case of a tennis ball, there is a relatively large amount of air drawn in because of the fuzziness of the ball and its ability to drag air.  This makes topspin and backspin more pronounced in tennis than it would be in a sport with a smooth ball.

 

-         Think of a tennis ball being struck.  If the ball wasn't rotating as it flew through the air then both the top and bottom sides of the ball would meet the air rushing over it at the same speed. 

In the case of topspin, the top of the ball spins into the oncoming air and the front of the ball is moving downwards. Because the boundary layer draws the air down along with the front of the ball, more air gets pulled under the ball than goes above it. Now, since more air has to pass under the ball it gets squashed together and has to move faster. This means there needs to be a higher velocity on the lower side of the ball, and subsequently a lower velocity on the top of the ball.

On the top side of the ball this lower velocity creates a higher pressure and at the bottom the higher velocity creates a lower pressure. (This is an application of Bernoulli's Law.) With high pressure on top and low pressure on the bottom there is an imbalance in the forces on the ball which curves it downward from its straight line path.  In backspin, the same principles are in action, except in this case the bottom of the ball has the lower velocity so the pressure is higher; thereby causing an imbalance in force that pushes the ball up.

To see videos on how tennis balls create a wind tunnel as they fly through the air, click on the picture below.

-  The following three graphs demonstrate the point made earlier about how topspin and backspin affect the trajectory of the ball after bouncing.  As can be seen in the graph, a ball hit with topspin bounces at a lower angle relative to the ground than a ball hit with no spin.  Conversely, a ball hit with backspin, bounces with an angle higher than a ball hit with no spin.  Both of these strokes are important parts of the tennis game and have many different uses in different match situations.