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151.
152.
Tennis courts are normally classified as fast or slow depending on whether the coefficient of sliding friction (COF) between
the ball and the surface is respectively small or large. This classification is based on the fact that the change in horizontal
ball speed is directly proportional to the COF if the ball is incident at a small angle to the horizontal. At angles of incidence
greater than about 16° it is commonly assumed that the ball will roll during the bounce, in which case one can show that the
ratio of the horizontal speed after the bounce to that before the bounce will be 0.645 regardless of the angle of incidence
or the speed of the court. Measurements are presented showing that (a) at high angles of incidence, tennis balls grip or ‘bite’
the court but they do not roll during the bounce, (b) the bounce:speed ratio can be as low as 0.4 on some courts and (c) the
normal reaction force acts through a point ahead of the centre of mass. An interesting consequence is that, if court A is
faster than court B at low angles of incidence, then A is not necessarily faster than B at high angles of incidence. An exception
is a clay court which remains slow at all angles of incidence. The measurements also show that the coefficient of restitution
for a tennis ball can be as high as 0.9 for an oblique bounce on a slow court, meaning that the ball bounces like a superball
in the vertical direction and that slow courts are fast in the vertical direction. 相似文献
153.
Measurements are presented of the friction force acting on a tennis ball incident obliquely on the strings of a tennis racket.
This information, when combined with measurements of ball speed and spin, reveals details of the bounce process that have
not previously been observed and also provides the first measurements of the coefficient of sliding friction between a tennis
ball and the strings of a tennis racket. At angles of incidence less than about 40° to the string plane, the ball slides across
the strings during the whole bounce period. More commonly, the ball is incident at larger angles in which case the ball slides
across the string plane for a short distance before gripping the strings. While the bottom of the ball remains at rest on
the strings, the remainder of the ball continues to rotate for a short period, after which the ball suddenly releases its
grip and the bottom of the ball slides backwards on the string plane. The bounce angle depends mainly on the angle of incidence
and the rotation speed of the incident ball. Differences in bounce angle and spin off head-clamped and hand-held rackets are
also described. 相似文献
154.