物体上升的速度:10m/20s = 0.5 m/s.
起重物所做的功:w1 = 900N X 10m = 9000 Nm = 9 kNm.
拉力所做的功:w2 = 400N X 30m = 12000 Nm = 12 kNm.
滑轮组的机械功率:k = 9000 Nm / 12000 Nm = 0.75.