质量为 m 的小球在竖直平面内的圆形轨道内侧运动,经过最高点而不脱离轨道的最小速度是 v . 若小球以 2v 速度通过最高点,则它对轨道的压力为 ( )
A.
. mg
B.
. 2mg
C.
. 3mg
D.
. 4mg
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