Lesson video

Hello, my name is Mr. Clasper.

And today we're going to be calculating acceleration from a velocity time graph.

Before we begin, we should have a reminder on how to calculate acceleration.

Acceleration is calculated by dividing a change in speed by the change in time.

If we take this example, a lorry begins to move and reaches a speed of 40 metres per second in 16 seconds.

What is the acceleration of the lorry? Well, the acceleration is the change in speed divided by the change in time.

So, our change in speed, we've gone from zero to 40 metres per second, and our change in time has gone from zero seconds to 16.

So, therefore we can calculate 40 divided by 16, which gives us an acceleration of 2.

5 metres per second.

In this example, we assumed that the acceleration was constant.

Let's see if we can use this with a velocity time graph.

So again, to calculate acceleration, we need a change in speed, and we need to divide this by the change in time.

In our example, our change in speed is the difference between 35, our maximum speed on the graph, and five, which was our starting speed.

And we've divided this by six, as this has happened in the space of six seconds, or a difference between six seconds and zero seconds.

This gives us a calculation of 30 divided by six, or an acceleration of five metres per second.

Here's a question for you to try.

Pause the video to complete your task.

Click resume once you're finished.

And here is your solution.

So remember, you're going to calculate the gradient of the line, and this will give you your acceleration.

Remember also that your units need to be metres per second squared.

Here is a question for you to try.

Pause the video to complete your task.

Click resume once you're finished.

And here's your solution.

So, remember, to calculate your gradient, you need two clear points, So we can see, we have 0, 0, and if we look carefully, 20, 500 is another point we can choose.

And then from there we can calculate 500 divided by 20, which would give us our 25 metres per second.

Let's take a look at this example.

This graph shows the journey of a car for the first 13 seconds of its journey.

Work out the acceleration at each section.

Well, our acceleration for the first part would be calculated with 35 divided by six, which would give us an acceleration of 5.

83 metres per second.

For the second part of my journey, my speed doesn't change.

So I have an acceleration of zero.

An acceleration of zero is indicated with a horizontal line on a velocity time graph.

Our deceleration would come at a rate of seven metres per second.

So, when we decelerate, we still have this as a positive value.

So, we've calculated 35 divided by five, as we've covered 35 metres in five seconds.

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 for part A.

So the question was, what is the acceleration between zero and six seconds? If we work out the gradient of this line, we would calculate 60 divided by six, which gives us an acceleration of 10 metres per second.

And here is part B.

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

And here is your solution.

So this question asks for the deceleration.

So, remember, a deceleration still has a positive value.

So we're decelerating at a rate of 20 metres per second, in this example.

And here is part C.

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

And here is your solution to part C.

So the part of the graph which represents an acceleration of zero is the horizontal part of my graph.

And here is part D.

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

And here is your solution for part two D.

So, looking at the graph, we know that the acceleration is constant as the graph is composed of straight line segments.

Here's your last question.

Pause the video to complete your task.

Click resume once you're finished.

And here is your final solution.

So, remember, if the car is currently travelling at 10 metres per second, this is where my graph will begin.

So, it crosses at 0, 10, and it accelerates at three metres per second.

So, that means my line must have a gradient of three.

And that brings us to the end of our lesson.

So, you've learned how to calculate acceleration from a velocity time graph.

That's absolutely amazing.

Why not try our exit quiz just to show off your skills further? I'll hopefully see you soon.