题目

I didn't know. A . where does Tom buy the gift. B . where Tom bought the gift. C . where did Tom buy the gift. D . where Tom will buy the gift. 答案：B如图所示，重500N的人站在重2500N的船上，用滑轮组把船匀速拉向岸边，人和船相对静止．船受到水的阻力为900N．不计绳重和滑轮轴摩擦，人拉绳的力为（　　）A．300NB．450NC．1100ND．2700N