题目

This ruler isn’t mine. I think it is ＿＿.A. he B. him C. his D. her 答案：【答案】C【解析】his=his ruler。质量为64kg、底面积为0.04m2的物体A静止在水平地面上．用如图所示的滑轮组在40s内将物体A匀速竖直提升10m，所用拉力F为400N．（g=10N/kg）求：（1）提升前，物体A静止在水平地面上时对地面的压强；（2）拉力F的功率；（3）滑轮组的机械效率．