can be represented by a graph, x versus t.

可以用一幅x-t图来表示
At any point on the graph

在图上的任意一点
you can take the derivative,

你可以进行求导
which will be tangent to the curve at each point,

得到曲线上每一点的切线斜率
and its numerical value will be

所得到的数值
what you can call the instantaneous velocity of that point

即为该点处的瞬时速度
and you can take the derivative over the derivative

然后你再求一次导
and call it the acceleration.

得出它的加速度
So, we are going to specialize to a very limited class of problems

在接下来的时间里我们会专门研究
in the rest of this class.

一类非常有限的问题
A limited class of problems is

在这类有限的问题里
one in which the acceleration is just a constant.

加速度均为常数