题目

如图所示，一个物体受到1N、2N、3N、4N四个力作用而处于平衡．现保持1N、3N、4N三个力的方向和大小不变，而将2N的力绕O点旋转60°，此时作用在物体上的合力大小为( )A. 2 N B. N C. 13 N D. N 答案：【答案】A【解析】根据“一个物体受到1N、2N、3N、4N四个力作用而处于平衡”可知，本题考查力的平衡条件，根据平行四边形定则，运用多力平衡的推论求解剩下力的合力.由题意可知四力的合力为零，则可知1N、3N、4N三个力的合力2N；与2N大小相等方向相反；则2N的力绕O点旋转60°，其他三力的合力不变请听下面一段独白，回答第1至3题。 1. What is the speaker mainly talking about? [     ]A. A historic tree. B. A well-known park. C. A famous commander. 2. What do we know about the Washington Elm? [     ]A. It was less than one hundred years old. B. Washington couldn't have commanded under it. C. It was much older than people had thought. 3. What can we learn from the talk? [     ]A. Some historical stories are untrue. B. Washington commanded the army on his knees. C. The Washington Elm was destroyed before the celebration.