The "Pace" of a tennis court, whether it is "fast, medium or slow", refers to the reduction in horizontal speed of the ball following a bounce off the surface. It is most critical during the service when the reduction in speed caused by the bounce could determine whether or not returning service is within the reaction time of the receiver. During service the player strikes the ball, usually at maximum speed, from a height of about 2.8 m into the service court some 18 m away where it bounces before reaching the receiver. The trajectory of the ball may be tracked and the dynamic properties of the bounce determined using radar based "speed guns". These guns operate by emitting radar waves and detecting those which are reflected off moving objects back to the gun. The Doppler frequency shift of the reflected waves is then used to determine the velocity of the object. Some of the guns, originally developed to monitor road vehicle speeds, have sufficient precision (0.2 km/hr) and sampling rate (up to 30 per second) to enable the accurate measurement of tennis ball speeds. Measurements of ball trajectories have shown that many professional tennis players serve at speeds of more than 200 km/hr off the racquet. The speed of the ball then decreases during its trajectory due to aerodynamic drag and may reach the court surface at horizontal speeds of about 130 km/hr, and exit the bounce contact at speeds between 90 and 110 km/hr, depending on the Pace of the surface. The total time of flight between server and receiver may be as short as 0.7 seconds so that any diminution in horizontal speed by the bounce may be critical to the receiver. Measurements of typical services played onto various tennis court surfaces indicate that Pace varies from below 70 % (a horizontal speed reduction of 30 %) for a slow clay court to above 80 % for a grass surface. The Pace of the surface is dependent on a number of factors, primarily, the coefficient of friction between the ball and surface, the coefficient of restitution of the ball on the surface and the angle at which the ball strikes the surface. The first two are controlled by the mechanical properties of the ball and surface so it is possible to engineer surfaces of different playing characteristics. The third factor is dependent on the server and, generally taller players can achieve smaller contact angles and hence higher Pace values.
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