题目

— I don’t know _______ next. — Let’s ask our teacher for help.       A. what to do            B. what should I do        C. how to do             D. how I should do 答案：A如图所示，滑轮组的机械效率为80%，设重1600N物体G在10s内匀速上升了4m，求：（1）10s内拉力做的功；（2）拉力F的大小；（3）拉力的功率．