This is the starting speed, that's the rate at which it is gaining velocity, and that's how long it's been gaining it.

这是起始时的速度,这是速度的增长速率,这是加速的时间

耶鲁公开课 - 基础物理课程节选

It's gaining speed at some rate a.

速度的增长速率为a

耶鲁公开课 - 基础物理课程节选