Every time you are finding out a different force that's acting on a body with different origin.

每次你都会发现不同来源的力,作用在物体上

耶鲁公开课 - 基础物理课程节选

I've got to find them by experimenting, by putting other bodies and seeing how they react and then finding out what's the force that acts on a body when it's placed in this or that situation.

我得通过实验找出这些力,我会增加其它物体,观察其反应,求出当它处在这种或者那种情况下时,作用在物体上的力

耶鲁公开课 - 基础物理课程节选