(1)有用功是:W有=GXh=600NX10m=6000J;
(2)总功是:W总=F X(n Xh)=250NX(3X10m)=7500J;
(3)人拉绳的功率是:p=FXV=250NX(3X10/50)m/s=150W;
(4)该滑轮组机械效率:n1=W有/W总=6000N/7500N=80%.
(5)若工人仍用此滑轮组将重800N的重物匀速提升,则该滑轮组机械效率为
G动=G总-G物=250NX3-600N=150N;
n2=G/(nXF)=800N/[3X(800N+150N)/3]=84.2%.