Gravity is one force that acts on a body without the source of the force actually touching it.

引力是一种受力物体收到了力的作用,但却没有接触施力物体的力

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