（1）奶奶所做的功：$ W_{\text{總}(cāng)} = Fs = 240\ \text{N} \times 6\ \text{m} = 1440\ \text{J} $
（2）由題意知，繩子股數(shù) $ n = 3 ，$物體上升高度 $ h = \frac{s}{n} = \frac{6\ \text{m}}{3} = 2\ \text{m} ；$爺爺?shù)闹亓?$ G = m_{\text{人}}g = 60\ \text{kg} \times 10\ \text{N/kg} = 600\ \text{N} ；$不計(jì)繩重和摩擦，$ F = \frac{G + G_{\text{動(dòng)}}}{n} ，$則 $ G_{\text{動(dòng)}} = nF - G = 3 \times 240\ \text{N} - 600\ \text{N} = 120\ \text{N} ，$動(dòng)滑輪（含座椅）的質(zhì)量 $ m_{\text{動(dòng)}} = \frac{G_{\text{動(dòng)}}}{g} = \frac{120\ \text{N}}{10\ \text{N/kg}} = 12\ \text{kg} $
（3）有用功 $ W_{\text{有}} = Gh = 600\ \text{N} \times 2\ \text{m} = 1200\ \text{J} ，$機(jī)械效率 $ \eta = \frac{W_{\text{有}}}{W_{\text{總}(cāng)}} \times 100\% = \frac{1200\ \text{J}}{1440\ \text{J}} \times 100\% \approx 83.3\% $
