Then you guys know, you just told me, acceleration is -9.8, We call it "minus" because it's accelerating down and up was taken to be the positive direction.

然后你们都知道,你告诉我,加速度是-9.8,我们取负号是因为加速度方向朝下,而正方向为竖直向上

耶鲁公开课 - 基础物理课程节选

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