问题 问答题

如图所示,位于水平桌面上的物块P,由跨过定滑轮的轻绳与物块Q相连,滑轮到P、Q的两段绳都是水平的.已知Q与P之间以及P与桌面之间的动摩擦因数都是μ,两物块的质量都是m,轻绳与滑轮之间的摩擦不计,在水平向右的拉力F作用下,P向右做匀速运动,则F的大小为(  )

A.4μmg

B.3μmg

C.2μmg

D.μmg

答案

对Q物块,设跨过定滑轮的轻绳拉力为T

木块Q与P间的滑动摩擦力f=μmg ①

根据共点力平衡条件T=f  ②

对木块P受力分析,P受拉力F,Q对P向左的摩擦力f,

地面对P体向左的摩擦力f′,根据共点力平衡条件,

有F=f+f′+T ③

地面对P物体向左的摩擦力

f′=μ(2m)g ④

由①~④式可以解得

F=4μmg

故选A.

阅读理解

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