為輕便拉動(dòng)重物P,將其放在滾輪O上,如圖所示。考慮接觸處A,B的滾動(dòng)阻礙,則作用在滾輪上滾動(dòng)阻力偶的轉(zhuǎn)向是()
A.MfA為順時(shí)針轉(zhuǎn)向,MfB為逆時(shí)針轉(zhuǎn)向; B.MfA為逆時(shí)針轉(zhuǎn)向,MfB為順時(shí)針轉(zhuǎn)向; C.MfA,MfB均為逆時(shí)針轉(zhuǎn)向; D.MfA,MfB均為順時(shí)針轉(zhuǎn)向
如圖所示,已知F′=60kN,F(xiàn)=20kN,物體與地面間的靜摩擦因數(shù)fs=0.5,動(dòng)摩擦因數(shù)f=0.4,則物體所受摩擦力的大小為()
A.25kN B.20kN C.17.3kN D.0