This study showed that an increase in the coefficient of friction between the ball and the foot caused the ball to acquire more spin.

这项研究表明，脚和球之间的摩擦系数的增加导致了球获得更多的角动能。

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For a ball rolling across the floor, that outside force could be the friction between the ball and the floor, or it could be the toddler that kicks the ball in another direction.

对于一个在地面上滚动的球而言，外力可能是球与地面之间的摩擦力或者幼童相反方向踢球产生的力。

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Friction causes a rolling ball to stop finally.

摩擦力使滚动的球最终停了下来。

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The CPR numbers give a hint as to whether a ball will bounce higher or lower on a specific surface, and how much friction there is to either slow the ball or allow it to skid on.

CNN: Tennis interactive: Why clay is so different

When a spinning ball hits a low friction surface such as grass, it will continue to spin.

BBC: SPORT | Tennis | Rules and Equipment | On the rebound

But on a high friction surface like clay, the ball will grip more and change the trajectory of the ball quite significantly.

BBC: SPORT | Tennis | Rules and Equipment | On the rebound