(2)滑輪組機械效率:η=W其中W total = ghfs = G3f = 9.0×103n 3×4.0×103n = 75%；
(3)W total = W withη= 9.0×104j 75% = 1.2×105j，克服動滑輪所做的功:W total = W with-0.3W with = 1.2×15j
可以由W =G = H，G = W = H = 3×103j 10m = 300n得到。
答案:(1)舉起重物所做的有用功為9.0×104J。J.
(2)滑輪組的機械效率為75%。
(3)動滑輪的重量為300 n .