Question: What is the difference between average speed and instantaneous speed?

Claire Lee answered on 14 Jun 2013:

Good question!

Imagine you’re driving from your house to school. The trip is 10 miles long, and takes half an hour. That means that your average speed over the whole journey is 20 miles per hour.

But of course you’re not doing 20 miles per hour for the entire trip – sometimes you’re stopped at a traffic light, or you’re going slowly down your driveway, or you’re cruising at 60 on the highway. Your instantaneous speed is the speed at a particular moment – for example, at one moment on the highway, you’re going 60m/h, at another, you could be doing 0.

If you want to get more technical, think about how you measure speed. Any measurement of speed is actually a measurement of *average* speed – just the length of time changes. On the highway you pass a traffic cop doing speed traps. He has two lines across the road 3ft apart, and his machine measures how long it takes your wheels to cross the second line, after they’ve crossed the first. So again, this is an *average* speed – the time is now just a lot shorter.

Speed = distance / time taken. And basically, anything where your time at the bottom is anything except 0 is going to be an average speed. In fact, it’s impossible to make an actual, precise measurement of instantaneous speed.

(Imagine putting the two lines of a speed trap on the same point, so that the time between each measurement is 0. It’s impossible to get a measurement from that!) But you can get an idea of what your instantaneous speed would be by extrapolating from your average speed:

example: you’re going 20m/h. You slam on brakes, and 10 seconds later you’ve stopped (ie 0m/h). You could therefore assume that, as long as you were braking constantly, after 5 seconds you were travelling 10m/h.

So, basically: Average speed is your overall speed over a certain period of time. Instantaneous speed is your speed at a particular moment.