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<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">某小区正在进行改造施工。工人用如图所示的滑轮组将</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">750N</span></span><span style="line-height:150%"><span style="font-family:新宋体">的沙子匀速提升了</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">10m</span></span><span style="line-height:150%"><span style="font-family:新宋体">，用时</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">100s</span></span><span style="line-height:150%"><span style="font-family:新宋体">。若工人所用的拉力为</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">400N</span></span><span style="line-height:150%"><span style="font-family:新宋体">，则拉力的功率为<u>　</u></span></span><u><span style="line-height:150%"><span style="font-family:"Times New Roman"">120</span></span></u><u>　</u><span style="line-height:150%"><span style="font-family:"Times New Roman"">W</span></span><span style="line-height:150%"><span style="font-family:新宋体">，滑轮组的机械效率为<u>　</u></span></span><u><span style="line-height:150%"><span style="font-family:"Times New Roman"">62.5%</span></span></u><u>　</u><span style="line-height:150%"><span style="font-family:新宋体">。使用该滑轮组既能<u>　省力　</u>又能改变力的方向。使用滑轮组<u>　不能　</u>（选填“能”或“不能”）省功。</span></span></span></span></span><br />
                                                                                                                                 <img height="106" src="/uploads/meth/20201228/ab445690ce05c2c7e9cce4a0c2ff2da2.png" width="83" />

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<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">解：由图可知，重物由</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">3</span></span><span style="line-height:150%"><span style="font-family:新宋体">段绳子承担，已知沙子上升高度</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">h</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">10m</span></span><span style="line-height:150%"><span style="font-family:新宋体">，则绳子自由端移动距离</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">s</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">3h</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">3</span></span><span style="line-height:150%"><span style="font-family:新宋体">×</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">10m</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">30m</span></span><span style="line-height:150%"><span style="font-family:新宋体">，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">总功</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">W</span></span><sub><span style="font-size:12.0pt"><span style="line-height:150%"><span style="font-family:新宋体">总</span></span></span></sub><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">Fs</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">400N</span></span><span style="line-height:150%"><span style="font-family:新宋体">×</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">30m</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">12000J</span></span><span style="line-height:150%"><span style="font-family:新宋体">，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">拉力的功率</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">P</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span></span></span></span><img height="41" src="/uploads/meth/20201228/db2f86e8e9e196a8213c4433a501e14e.png" width="28" /><span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span></span></span></span><img height="35" src="/uploads/meth/20201228/0ae82d29d529c4d7f13af392a37c0f0d.png" width="53" /><span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">120W</span></span><span style="line-height:150%"><span style="font-family:新宋体">；</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">有用功</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">W</span></span><sub><span style="font-size:12.0pt"><span style="line-height:150%"><span style="font-family:新宋体">有用</span></span></span></sub><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">Gh</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">750N</span></span><span style="line-height:150%"><span style="font-family:新宋体">×</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">10m</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">7500J</span></span><span style="line-height:150%"><span style="font-family:新宋体">，</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">滑轮组的机械效率</span></span><span style="line-height:150%"><span style="font-family:"Cambria Math"">η</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span></span></span></span><img height="47" src="/uploads/meth/20201228/243ae6f179b45a851576041362e87eb3.png" width="41" /><span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">×</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">100%</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span></span></span></span><img height="35" src="/uploads/meth/20201228/47770cc59b0baf94ec4d0d9762a29148.png" width="53" /><span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">×</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">100%</span></span><span style="line-height:150%"><span style="font-family:新宋体">＝</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">62.5%</span></span><span style="line-height:150%"><span style="font-family:新宋体">；</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">使用该滑轮组既能省力又可以改变力的方向，使用任何机械都不省功，故使用滑轮组不能省功。</span></span></span></span></span><br />
<span style="font-size:10.5pt"><span style="line-height:150%"><span style="font-family:Calibri"><span style="line-height:150%"><span style="font-family:新宋体">故答案为：</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">120</span></span><span style="line-height:150%"><span style="font-family:新宋体">；</span></span><span style="line-height:150%"><span style="font-family:"Times New Roman"">62.5%</span></span><span style="line-height:150%"><span style="font-family:新宋体">；省力</span></span>  <span style="line-height:150%"><span style="font-family:新宋体">不能。</span></span></span></span></span>

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