What worried the child most was ____ to visit his mother in the hospital. A. his not allowing B. his not being allowed C. his being not allowed D. having not been allowed 答案:B 解析: 略
如图所示,质量为M的圆环用轻绳吊在天花板上,环上有两个质量均为m的小环自大环顶部开始分别向两边滑下,当两个小环下落至与大环圆心等高时,小环所受摩擦力为f,则此时绳对大环的拉力为( ) A.(M+m)g B.(M+2m)g C.Mg+f D.Mg+2f