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Physics of the Tennis Kick Serve (Rod Cross)

Source file: Physics_of_the_tennis_kick_serve.pdf.txt

Domain: biomechanics / serve-physics

Tier: reports/tier-1-high-value-gaps

Used in artifact: artifacts/reports/tier-1-high-value-gaps.md

Abstract

The kick serve spin axis is tilted, not vertical. Mostly sidespin, not topspin. Tilting the racket head forward is equivalent to a ball bouncing off the court at an angle — generates topspin even when the racket rises only a few degrees. Same outgoing spin achievable with about half the effort of a groundstroke.

Key points

  • Kick serve spin axis is tilted — sidespin component dominates topspin.
  • Racquet head tilt generates topspin even when racket is rising only a few degrees.
  • Outgoing spin achievable with about half the effort of a groundstroke (no incoming backspin).
  • Sideways racquet component generates sidespin; vertical component generates topspin.
  • Magnus force acts at right angles to the tilted spin axis.

Raw dump (first ~1000 words)

Physics of the Tennis Kick Serve about:reader?url=http://twu.tennis-warehouse.com/learning_center/kicks... twu.tennis-warehouse.com Physics of the Tennis Kick Serve 38-48 ph�t ABSTRACT The kick serve in tennis is difficult to master since it is difficult to generate enough topspin for the ball to kick up sharply. Furthermore, the ball needs to be served at around 100 mph (depending on the court surface -- grass and clay being very different surfaces), and it needs to land well short of the service line in order to bounce to around shoulder height. The main problem is that the racquet is near the top of its trajectory when it strikes the ball, so it is impossible to swing up at the ball at the same steep angle as that used in a topspin groundstroke. A high ball toss will help since a falling ball is equivalent to a rising racquet in terms of topspin generation. In addition, it helps to strike the ball with the racquet head tilted forward slightly. Additional topspin is generated simply by the fact that the racquet is rotating forward when it strikes the ball. The physics of each of these effects is described in this article, and is illustrated with slow motion video film showing both the serve action and the fact that the resulting spin is mostly sidespin in a typical kick serve. 1. INTRODUCTION 1 trong 29 2:48 CH, 07/03/2021 Physics of the Tennis Kick Serve about:reader?url=http://twu.tennis-warehouse.com/learning_center/kicks... A kick serve is one that bounces up around shoulder height as it crosses the baseline. It also swings into or away from the receiver due to sidespin imparted to the ball. Not everyone can serve a good kick serve. The male pros all have a good kick serve, and so do some of the female pros. The secret of success is being able to serve with topspin (in addition to sidespin) and being able to serve fast. In general, the faster the ball lands in the court the higher it will bounce, but a fast serve on its own does not necessarily bounce to around shoulder height. A fast, flat first serve usually bounces to around waist height. In order to bounce to shoulder height, the ball must land at high speed and at a relatively steep angle. Normally, the faster the serve, the lower the angle of incidence on the court since a ball served at high speed needs to pass low over the net in order to land in the serve box. But if the ball is served with topspin, the ball will dive onto the opponent's court at a relatively steep angle, even when serving at relatively high speed. An interesting question is how the serve speed, the angle of incidence of the ball onto the court and the amount of topspin can all be optimized to generate the highest possible trajectory as the ball crosses the baseline. In this article, I present some measurements and calculations to indicate how the bounce height can be increased. Measurements of kick serves were needed in order to figure out the physics of the problem, which turned out to be more complicated than expected. One of the mysteries concerning the kick serve is how the server manages to generate topspin in the first place. In order to achieve a good kick serve, the racquet head needs to rise up the back of the ball, as it does in a topspin groundstroke. To return a 2 trong 29 2:48 CH, 07/03/2021 Physics of the Tennis Kick Serve about:reader?url=http://twu.tennis-warehouse.com/learning_center/kicks... groundstroke with topspin, players normally swing the racquet upwards at an angle of about 30 degrees to the horizontal. That way, an upwards friction force is exerted on the back of the ball, causing the ball to rotate with topspin if the racquet is swung fast enough. The incoming ball bounces off the court with topspin, so the spin direction needs to be reversed in order to return the ball with topspin. In a serve the racquet head is almost at the top of its trajectory when it strikes the ball, so the racquet is rising at an angle of only a few degrees just before it strikes the ball. Therein lies the mystery. If the racquet needs to rise at 30 degrees to hit a good topspin forehand, how can anyone serve a ball with a significant amount of topspin when the racquet head is rising at only a few degrees? One saving grace is that the ball is not spinning backwards when it is struck, so the server does not need to apply as much spin in a kick serve as in a groundstroke. In effect, the same outgoing spin can be achieved with only about half the effort. Another significant factor is that the racquet is usually swung faster when serving a ball than when hitting a groundstroke. The outgoing spin is proportional to the speed of the racquet head, and is also proportional to the approach angle of the racquet head. The first two factors help to increase the amount of topspin in a kick serve, but there are several other factors that also add to the spin, as described in Section 2. When serving a kick serve, right-handed players toss the ball over their left shoulder, arch their back, bend at the knees and then jump up off the court. The end result is that the racquet head strikes the ball in a direction that is partly sideways across the back of the ball and partly vertical up the back of the ball, as 3 trong 29 2:48 CH, 07/03/2021 Physics of the Tennis Kick Serve about:reader?url=http://twu.tennis-warehouse.com/learning_center/kicks... shown in Fig. 1. The sideways component generates sidespin and the vertical component generates topspin. In Fig. 1, the ball will have more sidespin than topspin since the horizontal speed of the racquet head is greater than the vertical speed. Sidespin causes the ball to curve from right to left through the air, as viewed by a right-handed server, or from left to right for