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初次接触物理学的人们常常感到某些基本概念有点别扭。例如,关于匀速圆周运动的加速度,他们想不通:速度明明没有加快或减慢,怎么能说有加速度呢?又如,手提重物不动的人没有作功,或者手提重物在水平面上移动时提物的力不作功,他们也想不通:提着重物很累人,耗费精力,怎么能说没有作功呢? 当他们提出这类问题时,往往得到大致如下的回答:按照加速度的定义,不论速度的大小或方向有所改变,就有加速度;按照功的定义,必须有位移,并且只有沿着位移方向的力才作功;而定义是没有道理可说的。
People who are new to physics often feel that some basic concepts are a bit awkward. For example, with regard to the acceleration of uniform circular motion, they can not think: how can one say that there is acceleration when the speed is not increasing or slowing down? Another example is that a person with a fixed weight does not work, or a portable weight moves on a horizontal surface. When the power of extracting things does not work, they can’t think about it: they are tiring and exhausted with heavy things. How can they say that they haven’t done any work? When they asked this kind of question, they often got the following answer: According to the definition of acceleration Regardless of the magnitude or direction of the speed change, there is acceleration; according to the definition of work, there must be displacement, and only the force along the displacement direction does work; and the definition is unreasonable to say.