(2) F'dt=mv,-mva (F=F-umg) F=70-10t (4≤t≤7) my =[(70-10t-jmg)dt+mva ,=(5-t)dt+4=2.5(m/s) EN 30 v6=(5-t)dt+4=4(m/s) 47t/sd )2( 47 7 4 ′ −= mvmvtF ∫ = − ≤ ttF ≤ )74( 1070 4 7 4 mv7 d)1070( +−−= mvtmgt ∫ μ )m/s(5.24d)5( 7 4 7 =+−= ∫ ttv )m/s( 44d)5( 6 4 6 =+−= ∫ ttv 30 0 47 t/s F/N ( ′ = − μmgFF )