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To define what it means to say that a car is going at the speed of 2mph would again require the use of limits. If $\lim_{h\to 0}\frac{x(t_0+h)-x(t_0)}{h}=2$ then the speed at $t=t_0$ is $2$, where $x(t)$ is the displacement of the car from any given reference at time $t$. But I guess you already knew that. I don't what more can I say.I'm not really sure how this concept makes sense. We give it meaning by way of a limit but, to my mind, at any instant the car is motionless so how can it have a velocity? What, in essence, does it mean to say that at some point in time a car is going 2 mph?
At time t, the car is stationary...
Would you agree that the 'average rate of change' between $2$ and $4$ of the function $y=x^2$ is $(16-4)/2=6$?In fact, the 'issue' generalises to differentiation of non-linear functions. Take f(x)=x^2.
when x=2, the derivative is 2. Now if one is asked to explain what a derivative tells you, then you would probably say along the lines 'the rate of change of y with x'. But what does the rate of change mean at a single point? I hope this helps you to understand.
In fact, the 'issue' generalises to differentiation of non-linear functions. Take f(x)=x^2.
when x=2, the derivative is 2. Now if one is asked to explain what a derivative tells you, then you would probably say along the lines 'the rate of change of y with x'. But what does the rate of change mean at a single point? I hope this helps you to understand.