判断一个物体的运动情况必须先选定参照物,通常参照物的选择可以是任意的,一般为了研究问题的方便而选择地面或地面上不动的物体作为参照物.但是有些相对运动的问题中,题目本身就要求以其中一个运动物体作为参照物,判断另一个物体的运动情况.此时若物体的运动情况稍微复杂就不容易判断其相对运动的结果了. To determine the motion of an object must first select the reference object, usually the choice of reference objects can be arbitrary, in general, in order to study the convenience of the problem and choose the ground or fixed objects on the ground as a reference object, but some relative motion problems , The subject itself requires that one of the moving objects be used as a reference to judge the movement of the other object, and it is not easy to judge the result of the relative movement if the movement of the object is slightly complicated.