如图所示,在建筑装修中,工人用质量为m的磨石对斜壁进行打磨,当对磨石加竖直向上的推力F时,磨石恰好能沿斜壁向上匀速运动,已知磨石与斜壁之间的动摩擦因数为μ,则磨石受到的摩擦力大小为( )A. Fsin θ B. (F+mg)cos θC. μ(F-mg)sin θ D. μ(F-mg)cos θ 答案:【答案】C【解析】磨石受重力、推力、斜壁的弹力及摩擦力而处于平衡状态,由图可知,F一定大于重力;先将重力及向上的推力合力后,将二者的合力向垂直于斜面方向及沿斜面方向分解可得:平行斜面方向:f=(F-mg)cosθ;垂直斜面方向:FN=(F-mg)sinθ;则f=μ(F-mg)sinθ,故选C.( ) 2. He really late last night. So today he feels sleepy. A. stayed healthy B. stayed up C. stayed away D. stayed with