Lesson video

Hello there, I'm Mr. Forbes, and welcome to this lesson from the Measuring and Calculating Motion unit.

In it, we're gonna be looking at calculations for motion graphs, and that involves using the graphs to find things like final displacement, distance travelled, speed, and acceleration.

By the end of this lesson, you're gonna have looked at a wide range of motion graphs and be able to take readings from those graphs to calculate things like distance, displacement, speed, velocity, and acceleration.

You'll also be able to convert from one graph to the other through a series of calculations.

Here are the key words that will help you understand the lesson.

First on the list, displacement-time graph.

And that's a type of graph that shows the displacement of an object over a period of time where displacement is how far it is from its starting position.

There's also a velocity-time graph, which shows the velocity of the object over the period of time.

Then there's velocity, and velocity is the rate of change of displacement, how fast the displacement is changing.

And finally, acceleration.

And the acceleration of an object is the rate of change of velocity, how much the velocity is changing each second.

And here's some explanations of those keywords that you can return to at any point in the lesson if you need some help.

In the first part of the lesson, we're going to be comparing the two types of motion graph, velocity-time graphs, and displacement-time graphs, and we'll see what type of information we can get from both.

In the second part of the lesson, we're going to be looking at one graph and trying to convert information we can obtain from it into the other type of graph.

So, we'll take a velocity-time graph and use it to make a displacement-time graph, or a displacement-time graph and use that to make a velocity-time graph.

And in the third part of the lesson, we're going to find information from both types of graphs and use that in a variety of calculations.

So when you're ready, we'll begin by looking at the two types of motion graph.

Displacement-time graphs and velocity-time graphs both look very similar and give us similar types of information, but they're not identical.

They're very different in important ways.

So I've got a displacement-time graph and a velocity-time graph here, and I'm gonna show you the movement of the same object on both graphs at the same time.

So, in the first stage of movement here, I've got a displacement increasing at regular amounts, and that indicates a constant velocity.

So if you look at the velocity-time graph, I've got a constant level of velocity there.

Then, I've got this secondary displacement graph where the displacement doesn't change, and that means the velocity is zero.

And as you can see on the velocity-time graph, I've made the line go down to zero, then I'm gonna increase the velocity again, but this time it's a small velocity and as you can see on the displacement-time graph, the line goes up again, but this time the gradient of the slope is lower.

So both graphs show information about movement but in different ways.

As you saw in those example graphs, the lines, even though they're representing the same movement, don't match each other on the graphs.

If I've got a displacement-time graph that's got a line like this, this indicates that there's a constant velocity.

This object is moving forward at a constant rate, and that, on the velocity-time graph, would be represented by a line like this.

The velocity is remaining constant at a steady amount, and it's producing a gradual increase in displacement, giving us that straight line.

Now, objects aren't always moving, and a displacement can stay at a constant value.

So, if I've got a displacement here where the object's not changing displacement over the period of time, well, that must indicate that the object's not moving.

It's got zero velocity.

So on a velocity-time graph, I'd have a velocity remaining constant at zero.

On those two earlier graphs, we had constant velocities, but velocity's not always constant.

Objects can accelerate, so we can have an object like this.

We've got an increase in velocity showing that the object is accelerating in a regular way, and that's gonna cause a change in displacement that's going to be greater for every second that passes.

So on a displacement-time graph, I wouldn't get a straight line at all.

I'd get a curve like this.

And as you can see in the first part of the movement, I've got a very shallow gradient.

And later on, the object's moving much faster and I got a much steeper gradient.

So increasing velocity produces curves on displacement-time graphs.

Objects can also decelerate, so that would mean that the velocity is decreasing over time.

So on a velocity-time graph that could be represented with something like this.

A decrease in velocity means the change in displacement every second is gonna get smaller and smaller, so I'd end up with a displacement-time graph, something like this.

As you can see at the start, the displacement's increasing quickly.

And towards the end of the graph later on in time, the displacement's flattening out.

The gradient is much shallower, so a decreasing velocity will give me a curve in that direction.

Okay, let's see if you understand the relationship between the shapes on displacement-time graphs and velocity-time graphs.

I've got three displacement-time graphs here, and I'd like you to match them up with their corresponding velocity-time graphs.

So A, B, and C are the displacement-time graphs and D, E, and F are the velocity-time graphs.

So what I'd I need to do is to just pair them up.

So pause the video, pair up the graphs, and then restart, please.

Welcome back.

Well, let's have a look at those pairings.

The first pair is A and F.

And in A you can see that the displacement is increasing all the time, but it's gradually slowing down so that maps to F where the velocity is decreasing.

The second pair is B and D, and in that I've got a constant velocity, so a constant increase in displacement.

And that leaves the final pair of C and E.

And in C and E you can see that the velocity must be increasing 'cause the displacement is increasing more and more rapidly as they go.

Well done if you got those pairings.

So far we've just looked at simple sections of a graph where we got just one phase of motion, but graphs can show multiple phases of motion.

So we can have a displacement-time graph like this where I've got a steadily increasing displacement and then displacement stops increasing over time.

So we've got two phases there, and that would match a velocity-time graph something like this.

So in the first part of the motion, we've got a constant velocity, and that represents the slope on the displacement-time graph, and a flat section of a velocity-time graph.

And then we've got the velocity falling to zero because the displacement no longer increases.

The graphs can show gradual changes in motion as well.

So, I'm going to try and plot a displacement-time graph that shows a constant velocity and then gradually slow into a stop instead of suddenly stopping, and a velocity-time graph that matches it.

So, here's the first section where we've got some constant velocity which results in that slope on the displacement-time graph.

And then I've got a gradual slowing here.

So that's represented by a curve on the displacement-time graph and a straight line on the velocity-time graph as the velocity gradually falls to zero.

And then when it reaches zero, I'll put that final section of the velocity-time graph in, and that gives me a displacement that doesn't change.

So I've got that final section on the displacement-time graph.

So I've got a complete picture of an object that's moving at a steady speed gradually slowing down to a stop.

Okay, it's time for you to try and interpret a graph that's got several phases of motion.

So, I'd like you to decide which of the lines A, B, C, or D on the displacement-time graph, which is on the right there, matches the motion shown on the velocity-time graph, which is on the left.

So pause the video, have a think about that, and then restart when you've made your selection.

Welcome back.

And the correct answer to that was answer B.

Well done if you selected that.

We've got a velocity that's constant, so that gives us a straight gradient upwards at first.

Then we've got a velocity that decreases, so on the displacement-time graph we're gonna get a curved section.

And then in the final bit we've got another constant velocity and so we're looking for another straight section of the graph, but this time at a shallower gradient because the velocity is less.

So well done if you selected B.

And now let's check if you can convert the graphs the other way.

So which line on the velocity-time graph matches the motion shown on the displacement-time graph? You can see there's a displacement-time graph there on the left, and I've got four different lines on the right.

I'd just like you to decide which of those four lines on the right velocity-time graph matches that displacement-time graph.

So pause the video, make your selection, and restart please.

Okay, welcome back.

And the correct answer to that was D.

Well done if you selected that.

We can see from the displacement-time graph in the first part of the motion, I've got a steadily increasing displacement, and that indicates a constant velocity.

Then I've got a curved section of the graph, and that indicates that the velocity is decreasing.

And then I've got a section of the graph where the displacement remains the same, so a zero velocity, and the only line that matches that was line D.

Well done if you selected that.

And now it's time for the first task of the lesson.

And what I'd like you to do is use everything you've learned so far to sketch a pair of graphs for me.

So I'd like you to sketch a velocity-time graph that matches the motion shown on the displacement-time graph on the left there.

And I'd like you to sketch a displacement-time graph that matches the motion shown on the velocity-time graph on the right there.

Now, there's no numbers on the graphs.

I just want the general shapes.

So pause the video, sketch your graphs, and then restart, please.

And welcome back.

The velocity-time graph you should have sketched for part one should look something like this.

We've got a constant forwards velocity at first, so a constant positive velocity, and that gives us that upward straight slope.

Then we've got a velocity of zero because you can see the displacement doesn't change, so the object's not moving, it's got a velocity of zero.

And then we gradually curve upwards, and that means a gradual increase in velocity.

And that gives us that upwards diagonal line on the velocity-time graph.

Well done if you sketched something that shape.

And here's the solution to the second part of that.

Your graph should look something like this.

At first, we've got a high constant velocity, so we get a straight line going upward like that on the displacement-time graph.

Then we have a decreasing velocity.

So that's a curved section on the displacement-time graph.

And finally, we've got a constant lower velocity so well done if your graph looks something like this.

So let's move on to the second part of the lesson now, and in it we're going to be converting from one motion graph to another.

So we'll be reading information from one type of graph, a displacement-time graph, and then using that to construct a velocity-time graph or the other way around.

So we're gonna be making those conversions from one graph type to another and we need to follow a fairly standard procedure to do that.

That involves reading data from the first graph, and then we use that data in the speed or the velocity equations.

And finally, from the results of those equations, we can plot the results on the second graph.

So let's start by looking at a few example graphs to see how we can do that.

So, so far we've just sketched the shapes of the graphs.

Now we'll try and plot an accurate displacement-time graph using velocity-time graph information.

And we're going to use the change in displacement for each part.

We're gonna calculate that using data from the velocity-time graph, and then we can plot it on our displacement-time graph.

So for example, if I've got a velocity-time graph like this, what I should do is calculate the change in displacement in each section of motion, so for the first two seconds and then from two to five seconds, and then from five to eight seconds, so let's do that.

So in this first section of the graph, I can calculate the change in displacement using the speed equation, and I get the change in displacement on the distance travelled is three metres for that section.

It's moving up 1.

5 metres per second for two seconds, and that gives us a change in displacement of three metres.

So that allows me to plot the value on the graph over here.

After two seconds, the object's moved three metres, so I can draw the red line there.

In the second section of motion here, well, you can see the velocity's zero, so there's not gonna be any change in displacement between two and five seconds, so I can transfer that to the displacement-time graph, and that will give me a line that looks a bit like this.

There's no change in displacement there.

And for the final section of the graph, again, I calculate the change in displacement using the speed equation there.

And the change in displacement is two metres per second times three seconds.

That gives me an increase in displacement of six metres.

And it's important to note that's an increase in displacement.

That's not the final displacement.

So I need my graph to go up by six more metres.

I've got a final displacement of nine metres on the graph there.

So I've successfully converted a velocity-time graph to a displacement-time graph.

Okay, let's see if you can match up a velocity-time graph to the displacement-time graph that should be produced.

So which line on the displacement-time graph, which is the graph on the right, shows the same motion as the velocity-time graph on the left there? So pause the video, make your selection, and restart please.

Okay, welcome back.

The answer you should have selected was line C.

So if you look carefully, the velocity is at one metre per second there, so after two seconds, the object should have moved two metres.

Then the velocity increases to two metres per second so the gradient would increase on my displacement-time graph and I end up with a total distance after four seconds of six metres, and that indicates that the red line is the correct one.

Well done if you selected that.

We've been using velocity-time graphs to produce displacement-time graphs, and now it's time to do the opposite to reverse that process.

So I've got a displacement-time graph here.

And what I'm going to do is try and make a velocity-time graph from it, and we use a very similar procedure.

We're going to calculate the velocity of each part of the motion and then plot it on the velocity-time graph.

So in the first section of the motion here, the first two seconds, you can see the displacement's increased to six metres.

So I can calculate the velocity during that section using the speed equation.

And I get a velocity of three metres per second.

It went up by six metres in two seconds.

That gives us three metres per second.

So, all I need to do is to go to my velocity graph and plot a line while it's a constant velocity of three metres per second, so it looks like that.

In the second section of the graph here, you can see the displacement doesn't increase, and that gives us a nice simple velocity.

It's zero metres per second.

It's not moving at all.

So in that section, I can draw in this part of the graph, it goes down to zero metres per second there.

In the third section of the graph, again, I can calculate the velocity by reading the change in displacement and the change in time.

The displacement's gone up by four metres, and the time is four seconds, from four seconds to eight seconds there.

And that gives me a velocity of one metre per second.

And I can plot that in this section of the graph here.

And that gives me the final part of the graph.

And I've successfully translated a displacement-time graph into a velocity-time graph.

Let's see if you can match up a displacement-time graph and a velocity-time graph.

So which line on the displacement-time graph shows the same motion as the velocity-time graph, which is on the right there? So pause the video, make your selection, and restart, please.

Welcome back.

You should have selected line A there.

So in the first part of the motion, the first two seconds, I've got a velocity of one metre per second so you would expect the displacement to be two metres after two seconds, which the blue line has.

Then you've got a velocity of zero for another two seconds so you should have a flat part of the blue dash line, and you do.

And then we've got another increase in velocity to three metres per second.

And finally, the final section of the line between six and eight seconds, we've got no increase in displacement so that was the blue dash line.

Well done if you've got that.

And now it's time for the second task of the lesson.

And what I'd like you to do is use the techniques I've shown you to draw some accurate graphs.

So I'd like you to draw an accurate displacement-time graph using the information shown on the velocity-time graph there in red.

And then I'd like you to draw an accurate velocity-time graph using the information from the displacement-time graph, which is shown in the green and blue there on the right.

So pause the video, draw your two graphs accurately, and then restart, please.

Welcome back, and for the first graph, you should have drawn something like this.

We've got zero velocity for the first second, so there's no change in displacement.

Then we're moving at one metres per second for two seconds so we should get up to two metres after that three-second mark.

Then we've got a velocity of three metres per second, so that steeper line there.

And finally, back down to a velocity of 0.

5 metres per second for the end two section.

Well done if you've got that.

And for the second part of the task, you were asked to draw a velocity-time graph, and you should have come up with a velocity-time graph that looks like this.

Well done if you got that.

And now it's time to move on to the final part of the lesson where we're going to be finding information from different motion graphs.

So we're gonna read values from the graphs and use them in different calculations to find things like distance travelled, velocity, and acceleration.

If we have a displacement-time graph where the motion is only in one direction where the direction of travel doesn't change, it's quite simple to find total distance travelled.

So if I've got a graph like this, and as you can see I've got changes in velocity there, but I've not got a change in direction.

I've got no decrease in displacement.

I can just read off the total distance travelled.

It's the same as the final displacement like this.

So the distance travelled there would be 10 metres.

This graph, the distance travelled would be 60 kilometres.

So as long as the direction of travel doesn't change, the displacement and the total distance travelled are the same.

In a lot of motion, we've got direction of travel changing.

Well, as long as the direction of travel is just along a straight line, so north to south or east to west or something like that, the distance travelled can still be found by just adding the separate phases of motion together.

So we've got a displacement-time graph here.

We've got displacement in kilometres and time in hours.

And as you can see, the displacement goes up and then down and then up again.

So we've got changes of direction here.

And so to find the total distance travelled, what I have to do is add up all of the changes in displacement, so I've got an increase in displacement of 60 kilometres there, and then I travel backwards 40 kilometres and then forwards again 30 kilometres.

And I can find the total distance travelled just by adding those three together.

So the distance travelled is those three values, and that gives a total distance travelled of 130 kilometres.

Let's see if you can find a total distance travelled for an object that's moving in two directions.

So I've got a graph showing the displacement and time for an elevator here.

As you can see, it goes upwards and downwards.

So what I'd like you to do is to find the total distance travelled from its journey where it starts at 50 metres to the ground, which is at a displacement of zero.

So pause the video, make your selection from the list on the left and then restart, please.

Welcome back.

You should have selected 130 metres.

In the first part of the motion, the elevator goes down 20 metres, and the second part of the motion, it goes back up 40 metres so we've already got 60 metres there.

And then down from its maximum height of 70 to the ground, so 70 metres down.

And if we add those three values together, we get a total distance of 130 metres.

Well done if you got that.

You can also find distance travelled from a velocity-time graph fairly straightforwardly as long as we've got a constant velocity.

So I've got a graph here, and let's say we've got an object moving at a constant velocity of six metres per second for a time of 40 seconds.

So we've got a graph like that, and we can calculate the distance travelled by that constant velocity as long as it doesn't change.

So we've got a velocity of six metres per second.

And what we need to do is just to use the value for that velocity and the value for the time taken in the distance travelled equation, so we can use the speed equation like this.

The distance travelled or s is equal to the velocity times the time, v times t.

We put those two values in, and that gives us a distance travelled of 240 metres in that case.

So the motion gives us a total displacement, our final displacement of 240 metres.

Okay, I'd like you to try and calculate a distance travelled now.

I've got a lorry.

It's got three phases of motion.

But I only want you to concentrate on the middle section between two and four hours.

And I want you to work out what distance the lorry travels between two and four hours, please.

So pause the video, make your selection from the list on the left, and then restart, please.

Okay, welcome back.

You should have selected 120 kilometres.

And we can calculate that distance here.

We've got the distance is equal to the velocity times the time, and it was 60 kilometres now all throughout that section of motion, which was two hours long, and that gives us 120 kilometres.

Well done if you've got the answer.

When there's a constant acceleration acting on an object, then we can find the acceleration from a velocity-time graph using the change in velocity and the change in time.

So I've got a velocity-time graph here, and the object is accelerating between two and four seconds.

You can see the velocity's increasing there.

So we can find acceleration using the acceleration equation that you may have seen before.

Acceleration is change in velocity divided by change in time.

And in symbols, you might have seen it like this, a equals v minus u over t where v is the final velocity, and u is the initial velocity and t is the time.

So if we look at this section of the graph where the acceleration is happening, we can read off some values and put those into the equation.

The acceleration is five metres per second, minus two metres per second, where v was five metres per second, u two metres per second.

And t, the change in time, well, that's four seconds to two seconds so we take away two seconds from that four seconds.

We carry out the calculation, and that gives us a answer for the acceleration of 1.

5 metres per second squared.

Okay, I'd like you to find out an acceleration for yourself now.

We've got our velocity-time graph here.

As you can see, there's acceleration going on between one and four seconds.

So I'd like you to find the acceleration between one second and four seconds for the journey shown.

So pause the video, calculate the acceleration, and then restart, please.

Welcome back.

You should have selected C for that, 1.

33 metres per second squared.

And if we read the values for this section of the graph, the part where it's accelerating as asked, we can substitute the values we read off the graph into the equation just like that.

And that gives us a final answer of 1.

33 metres per second squared.

Well done if you got that.

Acceleration isn't always a constant value, and we can see that by a changing gradient on a velocity-time graph.

So, if I've got an object that's got a changing acceleration I'll end up with a curve on a velocity-time graph like this.

That one there has got the lowest acceleration because the curve isn't changing much.

It's gradually increasing.

And if I've got a slightly higher acceleration, then I'm going to have a curve that becomes more steep, more rapidly.

So that dotted black line there would be medium acceleration.

And as you'd expect, if I've got a higher acceleration again, I'm going to get a curve that becomes steeper much more rapidly like that.

Let's see if you can identify the greatest deceleration here.

I've got an object that was moving at six metres per second, but which of those lines shows the object that is decelerating the greatest, the greatest deceleration? Pause the video, make your selection from the lines below, and then restart, please.

Welcome back.

You should have selected D, that becoming steepest more quickly.

The object is reaching a velocity of zero more rapidly, only takes four seconds, so that must have the greatest deceleration.

Well done if you selected that.

And now it's time for the final task of the lesson, and this task is explained over the next two slides.

So the displacement-time graph below shows the motion of a camera drone.

I'd like you to find the total distance travelled between naught seconds and eight seconds.

And then I'd like you to find the velocity between five seconds and eight seconds, please.

And the next two parts of the question, this is a velocity-time graph, and it shows the motion of a toy boat.

I'd like you to find the total distance travelled between naught seconds and three seconds, and find the acceleration between six seconds and eight seconds, please.

So pause the video, answer those four questions, and then restart.

Welcome back, and the solution to those questions.

Well, the total distance travelled is 25 metres, the sum of those three values there.

The velocity between five seconds and eight seconds, this section of the graph, we can use the speed equation to find that it's 3.

3 metres per second.

Well done if you've got those.

And the second part of the task, the total distance travelled between naught and three seconds.

Well, we can read the velocity of the graph, two metres per second, and the time was three seconds.

So multiplying those together gives us six metres.

And the acceleration for this final section of the graph here, again, we write out the equation.

We put in the values, and we get minus one metres per second squared.

Well done if you got that.

Okay, we've reached the end of the lesson now, and here's a summary of everything we've learned.

Motion graphs can show the same information in different ways.

I've got a displacement-time graph here and constant velocity is shown by a straight line, and changing velocity is shown by a curve.

And I've got a velocity-time graph, and constant velocity here is shown by a horizontal line, and constant acceleration is shown by a straight line.

We can calculate constant velocities using the speed equation, s equals v times t, and we can calculate accelerations using the acceleration.

Acceleration is v minus u over t.

Well done in reaching the end of the lesson.

I'll see you in the next one.