用一个n=3的滑轮组,所提重物G为660N,绳自由端拉力F=300N,此时重物以0.3m/s的速度向上匀速直线运动,不计绳重和摩擦力

(1)
W有用 = Gh = 660 * h = 660h
W总 = FS = 300 * 3h = 900h
机械效率 = W有用 /W总 = 660h/900h = 0.73 = 73%
(2)
W额外 = G动h = 900h - 660h = 240h
即动滑轮重 G动 = 240N
F = 1/n * (G物+G动) = 800
1/3 * (G物+240) = 800
G物=2160N
W有用 = Gh = 2160 * h = 2160h
W总 = FS = 800 * 3h = 2400h
机械效率 = W有用 /W总 = 2160h/2400h = 0.9 =90%