题目

Botany, the study of plants, plays a strange role in the history of human knowledge. For many thousands of years it was one field about which humans had little knowledge. It is impossible to know today just what our Stone Age Ancestors knew about plants, but from what we can observe of preindustrial societies that still exists, a detailed learning of plants and their properties（特征）must be very ancient. This is reasonable. Plants are the basis of the food pyramid for all living things, even for other plants. They have always been greatly important to the good of peoples, not only for fo如图所示，轻杆长3L，在杆两端分别固定质量均为m的球A和B，光滑水平转轴穿过杆上距球A为L处的O点，外界给系统一定能量后，杆和球在竖直平面内转动，球B运动到最高点时，杆对球B恰好无作用力。忽略空气阻力。则球B在最高点时A. 球B的速度为B. 球A的速度大小为C. 水平转轴对杆的作用力为1.5mgD. 水平转轴对杆的作用力为2.5mg