题目

I can’t fix my mind on my study, with the children ______ so noisily outside my window. A. playing              B. to play              C. played             D. being played 答案：A如图所示的滑轮组，不计绳重及摩擦，重物G=100N，当拉力F=40N时，重物刚好匀速上升，则动滑轮重2020N，当重物上升0.5m时，拉力F端上升1.51.5m，若重物增加30N，这时拉力F'为5050N．