THIRD SECTION: TOPSPIN AND BACKSPIN ANALYSIS
To sum up, it is not only the court that “determines” the speed of the game, but also the style adopted by the players. Remember the transition phase mentioned during the explanation of the physics of bounces? It’s time now to look at how a topspin shot, which reaches the ground at a wider angle and with a heavier rotation, faces a much shorter transition phase and is therefore much less slowed down by the friction of the surface.
To explain what happens during the impact of a shot with considerable topspin, it becomes crucial to examine the rotational components of the speed of the ball: we are talking specifically about the angular velocity (ω1 in the first figure, or how quickly the ball spins on itself) and tangential speed, that is, the effect that the rotation of the ball gives to the global motion of the ball itself. Although each point of the circumference of the ball has its own angular velocity, which points in a different direction, to simplify the analysis we consider the velocities of the top and the bottom of the ball. In the case of a heavy-rotating stroke, the tangential velocity of the top of the ball is greater than the overall outbound horizontal velocity at the moment of impact with the surface; somehow, it is as the ball was spinning more “on itself” than moving forward. During the impact, the higher tangential speed is reduced (because the ball decreases its rotational motion) until it equals the overall outbound horizontal one; at this point, the ball resumes spinning and starts up again after having suffered less considerably the effect of the friction of the surface as compared with a flatter shot. Indeed, in this case the friction of a surface such as clay acts in favour of the topspin shot, helping the ball to retain much of its horizontal outbound velocity.
In synthesis, clay slows down flat shots more than it does with topspin-infused ones. It’s like topspin has found a secret way to bypass the friction of clay and generate retain more speed than a flat shot of the same power.
DIGRESSION: BACKSPIN SHOTS
Let’s analyze what happens with a backspin shot that is executed with a backward rotation. In this case, the transition phase of the rebound is much longer because the frictional force exerted by the surface acts for a longer time on the ball, so that it reverses the direction of its rotation and bounces forward towards the opponent who is waiting to hit it. Compared to flat and topspin shots, the two main differences on a physical level concern the overall speed, which decreases to a greater extent (the ball slows down more), and the post-bounce angle, which is much higher than that of incidence. We don’t notice it because a slice shot bounces very low anyway, and it seems to us that the angle of the bounce is equally small; this is true in an absolute sense, but this angle is still greater than that of incidence.
The rebound angle, differences between topspin and backspin (source: tennisindustrymag.com)
On which surface is the backspin most effective? Physically, the answer must be hard and grass, because the friction is lower and therefore both the effect on the decrease in speed and on the increase in the rebound angle are lower. The ball bounces lower and faster, forcing the opponent to bend in order to hit. On clay, friction has a huge overall effect on the backspin so that the ball bounces higher and slower; the opponent therefore has more time to hit it, but the difficulty resides in imparting power and pace to a shot that no longer has any, or very little at the most – in a nutshell, there is no inertia to lean on.
