The physicist has to constantly find out what forces act on bodies.

物理学家必须,不断地寻找都有哪些力作用在物体上

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

I know this is the right answer, because if I now find the acceleration, I find it's mg divided by m and I get -g as the answer for all bodies.

我知道这个等式是正确的,因为现在如果我要求物体的加速度,用 mg 除以 m,我就得到 -g,这个结果对所有物体都成立

耶鲁公开课 - 基础物理课程节选