Let me first ask you this, is this a reversible or in irreversible process? Right, let me add one more piece of data here which I said in words but which is actually important to write down before doing the problem.

让我先问你们，这是可逆过程,还是不可逆过程？好,让我加上一点简要的,但是很重要的前提,【p1>p2】这是可逆的吗？

麻省理工公开课 - 热力学与动力学课程节选

Conversely, if you get an answer and it doesn't seem to make sense, then you've got to go back and ask, am I violating some of the assumptions, and here you will find the assumption that the particle had that acceleration a is true as long it's freely falling under gravity but not when you hit the ground.

反过来说,如果你得到一个结果,发现似乎是错的,那么你就应该回过头来问问自己,我是不是违反某些前提了,这个模型中,你就发现,只要质点在重力作用下自由下落,质点具有加速度a的前提是正确的,但是落地后就不成立了

耶鲁公开课 - 基础物理课程节选