He would try to explain it this way when students asked for a top grade just for studying hard in chemistry class: What if a baseball player came to spring training and worked harder than all the others, but still could not play well.

VOA: special.2009.03.12

If you connect the body to a spring, which is neither compressed nor extended, there's no force acting on it.

如果将一个弹簧连在这个物体上,弹簧既没有被压缩也没有被拉伸,也不会有力作用在它上面

耶鲁公开课 - 基础物理课程节选

Even in the case of this spring, when you pull it, if you pull it to the right by some force.

甚至是在这样的弹簧问题中,当你拉它的时候,如果你用力向右拉

耶鲁公开课 - 基础物理课程节选

If every time I get the same acceleration, I'm convinced this is a reliable spring that is somehow producing the same force under the same condition.

如果每次都能得到相同的加速度,这个弹簧的可靠性就能令人信服,即它在相同的条件下能够产生相等的力

耶鲁公开课 - 基础物理课程节选

If you pull the spring, there is a force acting on it.

如果拉这个弹簧,就有力作用在物体上了

耶鲁公开课 - 基础物理课程节选

But if I'm accelerating, the force exerted by the spring and therefore, the needle the weighing machine reads is m.

但是如果我在加速运动,那么弹簧的弹力,也就是称重机上指针的读数就是 m

耶鲁公开课 - 基础物理课程节选

It also tells you that if you compress the spring, compress it means x is now-- x is measured from this position, where the spring is neither compressed nor expanded.

它还能告诉你,如果压缩弹簧,压缩就意味着现在 x...,x 是从这个位置开始算的,在这个位置弹簧既没有压缩也没有伸长

耶鲁公开课 - 基础物理课程节选

It says, if you pull it to the right, so that x is positive, the spring will exert a force which is in the negative direction; that's why you have a minus sign.

就是说,如果你向右拉,x 取正值,弹簧产生的力方向为负,这就是这里用负号的原因

耶鲁公开课 - 基础物理课程节选