如图所示,细杆的一端与小球相连,可绕过 O 点的水平轴自由转动,细杆长 0.5m ,小球质量为 3.0kg ,现给小球一初速度使它做圆周运动,若小球通过轨道最低点 a 处的速度为 v a = 4m/s ,通过轨道最高点 b 处的速度为 v b = 2m/s ,取 g = 10m/s 2 ,则小球通过最低点和最高点时对细杆作用力的情况是( )
A.
a处为压力,方向竖直,大小为126N
B.
a处为拉力,方向竖直向上,大小为126N
C.
b处为拉力,方向竖直向上,大小为6N
D.
b处为压力,方向竖直,大小为6N
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