题目

小明用如图所示的杠杆在B处提升重物，O为支点，将重20N物体提升0.5m，此杠杆属于（省力/费力）杠杆，小明对杠杆做的有用功为J，若将拉力位置移到C处，仍将该物体提升0.5m，与原来相比机械效率（变小/不变/变大） 答案：【1】费力【2】10【3】不变Not until ______ stayed together for a couple of weeks _______ we had a lot in cmmon.        A. we had; did I find     B. had we; I found C. we had; I found D. had we; did I find