Lesson video

Hello! My name is Mr. Clasper, and today we are going to learn how to calculate speed from a distance-time graph.

Is it possible to work out the speed from the distance-time graph? Well, if we look at this carefully, we can see that it took two hours to travel a distance of 60 kilometres.

And if we remember that speed is calculated by dividing the distance by the time, then we should be able to measure our speed.

Our distance is 60 kilometres, and the time taken is two hours.

So if we calculate 60 divided by two, this means that we have a speed of 30 kilometres per hour.

We can also see this from the graph, as after one hour, we covered 30 kilometres.

Here are two questions for you to try.

Pause the video to complete this task, and click resume once you're finished.

And here are your solutions.

Remember, it's often useful to pick two clear points to find out your gradient, and once you have your gradient, this will give you the value for your speed.

Let's try this example.

We're going to calculate the speed for each of the sections of the journey, which are indicated by each line segment.

So my first line segment, I've travelled a total of 50 kilometres in a time of 30 minutes.

Now remember, 30 minutes is equivalent to 0.

5 hours.

So that means at this point, we're travelling at 100 kilometres per hour.

Another way to think of this is that if we travelled 50 kilometres in 30 minutes, and we travelled at the same speed for one hour, we would travel double the distance, which would give us 100 kilometres per hour.

In the second line segment, our speed is zero, as we haven't moved.

In the third line segment, we've covered 75 kilometres in a space of one hour, which gave us 75 kilometres per hour.

And in our last segment, we've covered a distance of 125 kilometres in 45 minutes.

And remember, 45 minutes is 3/4 of an hour, which is equivalent to 0.

75, which is where we get our calculation 125 divided by 0.

75 from.

And at this point, our speed or average speed is 166.

7 kilometres per hour.

Here's a question for you to try.

Pause the video to complete your task, and click resume once you're finished.

And here is your solution.

So for Meghan's journey, the first part she was travelling at five miles per hour.

Then she increased to 24 miles per hour before slowing back down or decelerating to four miles per hour.

And here is part B.

Pause the video to complete your task.

and click resume once you're finished.

And here is your solution to part B.

So when we look at the graph, we should be able to tell that the middle part of the graph is the fastest, as this part of the graph has the steepest gradient.

And here is part C.

Pause the video to complete your task.

Click resume once you're finished.

And here is your solution for part C.

So to work out the average speed of the whole journey, we're going to make a straight line from the origin of our graph up to the top of our graph at the point four, 28.

And if we work out the gradient of this line, we should find that we're going to calculate 28 divided by four, which gives us seven.

Therefore our average speed was seven miles per hour.

And this brings us to the end of our lesson.

By now, you should be able to calculate speed from a distance-time graph.

Why don't you show off your skills with our exit quiz? I'll hopefully see you soon.