The free-body diagram, it says you can pick any one body that you like and apply F = ma to it, provided you identify all the forces acting on that body.

所谓隔离法,就是说你可以任意 选取一个物体,分析出它所受的所有力后,单独对其运用 F = ma 进行计算

耶鲁公开课 - 基础物理课程节选