题目

一This returned Chinese scholar has become one of the top experts in this field. —Yes,I know him very well , he      in Africa with animals for ten years. A. has worked        B、had worked    C.has been working     D.worked   答案：D如图所示，轻绳悬挂一质量为m=2.0kg的小球，现对小球再施加一个力F，使小球静止在绳子与竖直方向成60º的位置上，g取10m/s2。1.若F为水平方向，求F的大小；2.若要使F的取值最小，求F的大小和方向。