题目

如图，位于水平桌面上的物块P，由跨过定滑轮的轻绳与物块相连，从滑轮到P和到Q的两段绳都是水平的，已知Q与P之间以及桌面之间的动摩擦因数都μ，两物块的质量都是m，滑轮轴上的摩擦不计，若用一水平向右的力F拉P使它做匀速运动，则F的大小为（    ）A．4μmg B．3μmgC．2μmg D．μmg 答案：ADear Jenny, I’ve got the letter you wrote to me December 5th last Friday. I’m very happy to hear that you’re doing well. It’s been about two months since you left. I’m very glad hear that your new job is interesting, but I’m sorry to hear that you don’t like Washington. I think you’ll like better once you make more friends there. Good luck to you! Thank you very much your invitation, I’m very sorry that I can’t go to your Christmas party on December 24th my grandfather will come here that day to celebrate Christmas our family. Please forgive (原谅) me! I hope you can come to New York to spend New Year holiday with us. Yours, Lucy