单选题
质量为m，长为2l的均质细杆初始位于水平位置，如图所示。A端脱落后，杆绕轴B转动，当杆转到铅垂位置时，AB杆B处的约束力大小为______。
A．F Bx =0；F By =0
B．F Bx =0，F By =
C．F Bx =l，F By =mg
D．F Bx =0，F By =
A．F Bx =0；F By =0
B．F Bx =0，F By =
C．F Bx =l，F By =mg
D．F Bx =0，F By =
【正确答案】
D
【答案解析】[解析] 根据动能定理，当杆转动到铅垂位置时，杆的
