Another way into this problem: let's look at the comments that Dr. Johnson made about Milton's Lycidas in the eighteenth century.

看这个问题的另一个角度：让我们看看,约翰逊博士在18世纪队弥尔顿的《利西达斯》所做的评论。

耶鲁公开课 - 弥尔顿课程节选

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