That means the force you're applying is cancelled by another force, which has got to be the force of friction.

意味着你施加在物体上的力被另一个力抵消了,这个力就是摩擦力

耶鲁公开课 - 基础物理课程节选

If you assume that, you are not playing by the rule, we don't know what force it exerts.

如果你假定了,那你就犯规了,我们不知道它施加了多大的力

耶鲁公开课 - 基础物理课程节选

Here is some object, it's 5 kilograms and I apply 10 Newtons.

这有一个物体,5 千克,我对它施加 10 牛的力

耶鲁公开课 - 基础物理课程节选

You cannot say to me, "Take a force, due to a spring, and see what force it applies and divide by the acceleration and get the mass, " because we haven't defined force, either.

你不能说,"给弹簧施加一个力,看看它能提供多大的力,然后除以加速度,得到质量",因为我们还没给力下一个定义

耶鲁公开课 - 基础物理课程节选