题目

在农村常有家庭挖井取水。小明同学用如图(甲)所示的滑轮组提土，他用20s的时间把重为140N的土从井中匀速提升4m，拉力做的功W随时间t的变化图像如图(乙)所示。不计绳重和摩擦，求：(1)有用功；(2)滑轮组的机械效率；(3)拉力的大小；             答案： (1)W有=Gh=120N×5m=600J；(2分) (2)由乙图可得：W总=800J，η=75%；(2分)(3)F=80N；(2分)21、 ―How are you getting along with your new job and your classmates? ― ______.       A. I can’t tell you                                               B. So far, so good       C. I’m glad you asked me about it                        D. I’m not getting along with Jane