The only force acting on this guy is this little f.

这个物体只受一个力的作用,也就是 f

耶鲁公开课 - 基础物理课程节选

Your problem is to only look at the forces of contact on you, and that is just this f.

你只需要注意那些直接作用在这个物体上的力,那就只有 f 了

耶鲁公开课 - 基础物理课程节选