题目

小丽用一个动滑轮匀速提起10N的物体．所用的拉力为6N，使物体在2s内升高0.5m，则有用功是________J，动滑轮的机械效率是_________，拉力的功率是_______W . 答案：5；83.3％；3Jack invited me ________ to the movie.A. go B. going C. went D. to go