A free-body diagram is a sketch or drawing of the body,isola

A free-body diagram is a sketch or drawing of the body, isolated from the machine, on which the forces and moments are shown in action.
其实这句可以改成A free-body diagram （which is）isolated from the machine, is a sketch or drawing of the body, on which the forces and moments are shown in action.
on which the forces and moments are shown in action和isolated from the machine都是定语成分修饰前面的 diagram .只是英语会避免头重脚轻等结构因素,会分开来写,后置.
isolated from the machine就是后置定语
on which the forces and moments are shown in action是定语从句
主句就是A free-body diagram is a sketch or drawing of the body.