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。
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.
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。
This study showed that an increase in the coefficient of friction between the ball and the foot caused the ball to acquire more spin.
滚动托料球，减少板料划伤，并可减少摩擦阻力。
Rolling material support ball is set, to minimize fish tail with sheet bar and to reduce frictional resistance.
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