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。
2
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.