女生小视频

IN TENNIS, hard courts are generally described as 鈥渟low鈥� since they make
balls bounce at a higher angle, and hence at a lower speed. But it鈥檚 a different
story for the players themselves, and physicist Rod Cross from the University of
Sydney has been measuring the friction between various surfaces to find out why.
Cross found that tiny grains of sand on the hard court surface can act either as
ball bearings or sandpaper. The sand gets trapped in the tennis ball fabric and
scrapes along the ground, slowing the ball down. But when a shoe slides on the
sand, it sweeps excess grains out of the way, leaving a single layer of rollers
that helps the player move around the court faster (The Physics Teacher,
vol 39, p 330).