We can get the instantaneous rate of change of any function, not just
of position. If f is a function of x, then the instantaneous rate
of change at x=a is the limit of the average rate of change over
a short interval, as we make that interval smaller and smaller.
In other words, we want to look at
limx→aΔfΔx=limx→af(x)−f(a)x−a.
This is the slope of the line tangent to y=f(x) at the point (a,f(a)).
It can also be written as a limit
limh→0f(a+h)−f(a)h,
where h is just another name for x−a.
If this limit exists, we call it the derivative of f at x=a.