—Are we about to have dinner? —Yes, it in the dining room. A.is serving B.serves C.served D.is being served 答案:D如图所示,一个质量为m=2kg的小球在细绳牵引下在光滑水平的平板上以速率v=1.0m/s做匀速圆周运动,其半径r=30cm,现将牵引的绳子迅速放长20cm,使小球在更大半径的新轨道上做匀速圆周运动.求:
(1)实际这一过渡所经历的时间;
(2)在新轨道上做匀速圆周运动时,小球旋转的角速度;
(3)圆周的半径增大后外界对绳子的拉力为多大?