题目

限时7分钟 I can well remember that I was once asked to deliver a speech titled “A Real Test in My Life” before the whole class at the age of 9! You can imagine how  __1__ (terrible) shy I was the moment I thought of that with so many eyes  __2__ (fix)  upon me.  I had no   __3__ (choose)  but to prepare for it,  though.  First of all,  I was to draft the speech,  which was just a piece of cake for me, a good writer. But the hard part   __4__ (lie) in my oral presentation from my memory—for to read from the paper was not allowed. The real moment began  __5__   如图一装配线上的一个“机械手”，其固定点O为转轴，现它水平抓住一只重40N的工件G，并在0.5 s内匀速将这一工件提升了20 cm，而动力机械的功率为20 w．求：（1）动力机械M至少要施加的推力；（2）机械手做的有用功及机械效率．