You need that object to be a substance, to be something.

这个物体得是某种物质，某种实体。

麻省理工公开课 - 热力学与动力学课程节选

Whenever there is a force, it can be traced back to a tangible material cause, which is all the time a force of contact, with the exception of gravity.

如果有作用力,那它一定能找到一个明确的实体施力物体,也就是无论何时物体受的都是接触力,引力除外

耶鲁公开课 - 基础物理课程节选