In sports topspin is a property of a shot where the ball rotates as if rolling in the same direction as it is moving. Topspin on a shot imparts a downward force that causes the ball to drop, due to its interaction with the air (see Magnus effect). In racquet sports, it can be generated by hitting the ball with an up-and-forward swing, with the racquet facing below the direction it is moving. A topspin shot is the opposite of the slice; topspin itself is the opposite of backspin.
One way of explaining the Magnus effect is that - because of the rotation and the fact that air acts as a viscous or "sticky" substance on the surface of the ball, a stream of air in the wake of the ball is being ejected upwards. As a reaction to this, the ball is pushed downwards.
Often Bernoulli's principle is used to explain the topspin effect, as the difference in speed between ball surface and air is greater on the top of the ball. For example, if the air flowing past the bottom of the ball is moving faster than the air flowing past the top then Bernoulli's principle implies that the pressure on the surfaces of the ball will be lower below than above. In other words, since there is more air friction occurring on the top surface of the ball compared to the bottom, this differential causes a greater pressure to be applied on the top of the ball, resulting in the ball being pushed down.
The topspin shot is very effective on hard surfaces. The ball spins forward and descends toward the ground quicker, so it can be hit with more force compared to another type of shot.
On most court surfaces, topspin also makes the ball bounce higher. As a result, it is often used on clay or "soft" court surfaces which have a naturally higher bounce, in order to make the ball harder for the opponent to hit. An opponent with a one-handed backhand is especially vulnerable to a topspin shot because it is difficult to hit a high ball with a one-handed backhand.
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