A.7.5m B.12m C.2.25m D.9.75m
A.x=t2-t B.y=2t C.y-2x2=0 D.y+2x2=0
已知力P=40kN,S=20kN,物體與地面間的靜摩擦系數(shù)f=0.5,動(dòng)摩擦系數(shù)f′=0.4,則物體所受摩擦力的大小為:()
A.15kN B.12kN C.17.3kN D.0