题目

---He failed his exam again. ---But what did you _______? Had he ever been working hard?      A. think    B. consider     C. regard D. expect   答案：D工人师傅用如图13所示的滑轮组提升重物，若他用100N的拉力匀速提升重240N的物体，在2s时间内，物体上升了2 m，求：    ⑴ 物体上升的速度多大？⑵ 绳的自由端移动的距离是多少？⑶ 拉力做功多少?     ⑷ 拉力做功的功率多大？⑸ 此滑轮组的机械效率多大？