For example, how do I know that when I pull the spring the first time for the mass, it exerted the same force when I pulled the spring the second time for the elephant?

例如 怎样才能知道,第一次连着物体拉弹簧时,产生的力和,第二次连着大象拉弹簧时产生的力是相等的呢

耶鲁公开课 - 基础物理课程节选

We did the comparison with the elephant or something; a is the second derivative of x and for this problem, when F is due to a spring, we know the force is that by studying the spring.

我们也已经把它与大象或其它东西作过比较,a 是 x 的二阶导数,在这个问题中,F 是由弹簧产生的,我们在讨论弹簧问题时已经知道了力的大小

耶鲁公开课 - 基础物理课程节选