题目

绳子的一端拴一重物，用手握住另一端，使重物在光滑的水平面内做匀速圆周运动，下列判断正确的是（  ）A.每秒转数相同，绳短时易断  B.线速度大小一定，绳短时易断C.运动周期一定，绳短时易断  D.线速度大小一定，绳长时易断  答案：B解析:，由公式可看出转数相同，绳长时易断；线速度大小一定，绳短时易断，运动周期一定，绳长时易断  Welcome to the Historical Museum, everyone! This stone knife _______ in South Africa and experts can’t be exactly sure about its use. A. had been found       B. was found  C. has been found  D. would be found