Remember, you've got to keep track of the sign of acceleration, so if you're picking up speed towards the ground, a is negative, so it will be g plus a, but a is a negative number.

记住,一定要注意加速度的符号,所以当你加速下降的时候,a 是负的,还是 g 加上 a,但是 a 是负数

耶鲁公开课 - 基础物理课程节选

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