任何机械运动都具有相对性。对于同一机械运动,选择不同的参照物,观察到的物理情景大相径庭;表述该运动的各物理量,如速度、位移等也各不相同。例如一列火车以速度V匀速向东行驶,火车中的一物体从一定高度自由下降,从不同的参照物上能观察到以下各种不同轨迹: Any mechanical movement is relative. For the same mechanical movement, different reference objects are selected, and the observed physical scenes are very different; the physical quantities that describe the movement, such as speed and displacement, are also different. For example, if a train travels eastward at a uniform speed V, an object in the train is free to descend from a certain height. The following different trajectories can be observed from different reference objects: