Hello there, and welcome to this lesson from the Moving by force unit.

Our lesson's called Changing Speed, and it's all about how forces can alter the speed of an object, making it speed up or slow down.

By the end of this lesson, you're going to be able to describe how the speed of an object changes when a force is acting on it.

You are also gonna know the difference between an average speed and an instantaneous speed.

So when you're ready, let's get going.

This is the set of the keywords we'll need throughout our lesson.

Resultant force is the effect of all the forces on an object.

A speed, as you'll see, we've got two types of speed in the lesson, instantaneous and average speed.

A dynamics trolley is just a little wheel trolley that we use during in experiments, and acceleration is a way to describe changes in speed.

And here's a set of the definitions for those keywords.

You can return to this slide at any point in the lesson if you're unsure about those words.

The lesson's in three parts.

The first part is a bit of a recap about resultant forces and how we can calculate them by looking at how the forces interact with each other.

The second part is how that resultant force then changes the speed of an object and why the object keeps changing speed as long as the force is acting on it.

And in the third part, we'll look again at some distance-time graphs and how they show the changes in speed.

So when we're ready, let's have a look at the first part, resultant forces.

In this lesson, we'll see how forces change the speed of an object.

But before we can do that, we need to understand what a resultant force is.

So I've got a diagram here of somebody pushing the shopping trolley, and we need to be able to represent the forces that that person is putting on the trolley, and we do that by using an arrow.

The force is shown in the direction of the arrow, so the big red arrow there is a push force the person's using to make that trolley move.

We should also know that forces are measured in newtons.

So, for example, this force might be 50 newtons pushing that trolley forwards.

Now, more than one force can act on an object at the same time.

And in fact, in most situations, there is more than one force.

So I've got a scenario here.

I've got a rocket flying through the air, and the first and most obvious force that you can think about on that rocket is the thrust force pushing the rocket forward, which I've represented by a red arrow.

But as the rocket moves through the air, it'll also experience air resistance, or drag, which acts in the opposite direction to the direction that the rocket is travelling.

And therefore, we've got a drag force pushing backwards shown by the green arrow there.

The combined effect of those two forces is called the resultant force.

And that resultant force can be found by comparing those two forces and taking one away from the other because they're acting in opposite directions.

So we get the thrust force and we subtract the drag force from it, that gives us an overall resultant force, which I'm gonna represent by a purple arrow.

And you can see that resultant is still forwards, that rocket is still being pushed forwards overall.

The resultant then is this.

To find resultant forces, we need to look at just two different scenarios.

The first is when the forces are acting in the same direction, something like this.

So I've got a blue car here, and I've got two forces that are gonna be acting in the same direction, forwards in this case.

To find resultant, we just need to add those two forces together so we get an overall larger force than each of the individual forces.

If the forces are acting in the opposite direction, we need to subtract them.

So we've got a scenario here, same blue car, but this time I've got a force going forwards and a smaller force going backwards.

And we need to subtract that smaller force from the larger force to get the resultant, and the resultant will act in the direction of the larger force overall.

So that'll give us a small resultant force forwards.

A bit like this.

Okay, I've got a first check here.

I wanna know if this statement's true or false, but I also want you to justify your answer.

So the size of the resultant force is the size of all the forces acting on the object, added together.

Is that true or false? So pause the video, and make a selection, and then restart.

Okay, so that was false.

Now I want you to justify that.

And so I want the reason for why that's false.

So pause again and select from one of these two things.

Some forces do not count towards the resultant force, or the direction of the forces need to be considered.

So pause, make a selection, then restart.

Welcome back.

The reason that's false is because the direction of the forces need to be considered.

So if they're going in opposite directions, you need to subtract them rather than add them together.

Now, in some situations, the forces can be the same size and act in opposite directions.

So I've got here someone whose jumped out a plane, opened their parachute, and they're falling to the ground.

And there's two forces acting on there that are important.

First force is the weight of the gravitational force of that person, and that's acting down, and I've given it a value of 800 newtons there.

But as they're falling through the air, in this situation, I've got an equal and opposite force acting upwards.

And that's the drag on the air resistance, and that's 800 newtons.

So what's the resultant force in this scenario? Well, the forces are acting in opposite directions, so they cancel each other out because they're the same size.

If I subtract one from the other, I get a resultant force of zero newtons.

So there's no resultant force there at all.

The size of resultant force is therefore zero.

Okay, we've refreshed your knowledge of resultant forces now, and I'd like you to try this first task.

I'd like you to find a resultant force acting in each of these four scenarios.

So I've got four blue cars, and I've got four sets of forces on them, and the size of those forces are shown by those arrows.

What I'd like you to do is to work out the resultant force in each of those cases and write it down for me.

So pause the video, work out resultant forces, and then restart once you've got the answers.

Okay, welcome back.

Let's have a look at those resultant forces.

So in the first example, we've got 15 newtons backwards and 30 newtons forwards, so the resultant force there is 30 newtons minus 15 newtons, giving us a resultant of 15 newtons, and the direction is forwards because that's the direction of the largest force.

So it should have been like this.

Well done if you've got that.

The second answer, we've got 30 newtons backwards force and a 45 newtons forwards force.

So we subtract the smaller number from the larger number, giving us a resultant of 15 newtons forwards again.

The third scenario, I've got 45 newtons backwards and 45 newtons forwards.

If I subtract those two, I get a resultant of zero.

So there's no resultant force in that one.

And the final scenario, I've got 45 newtons backwards and 15 forwards, subtracting those gives an answer of 30 newtons, and it's backwards because that's the direction of the largest force.

Well done if you got all four of those right.

Okay, we're gonna move on to the second part of the lesson now, and that's all about how forces cause changes in speed.

So let's get going.

You might remember from your previous studies that resultant forces can cause changes in the shape of an object, but they can also cause changes in the speed of an object.

You can have a resultant force making an object speed up, so the speed is increasing in that situation.

So you might be in a car, you press the accelerator, and your car speeds up.

Resultant forces can also cause things to slow down.

So again, if you're in that car, you might press the brake, and that will produce a force on the road, and that resultant force will help cause the speeds to decrease, so you'll slow down.

Both of those things are called acceleration in physics, and acceleration is a change in speed, speeding up or slowing down.

You might also hear the word deceleration used to describe a change in speed where the speed is getting lower.

But I'm gonna stick with acceleration for describing all changes in speed.

The direction of any acceleration will depend on the direction of the resultant force.

An object will always accelerate in the direction of the resultant force.

So we've got three possible scenarios.

We've got the first one, a resultant force that's in the direction of travel.

So if you're in a car and you're travelling along forwards and there's a resultant force that's forwards, what will happen is, you'll speed up, so the movement and the force from the same direction, and that causes you to speed up.

The second scenario, the resultant force is in the opposite direction to travel.

So you're moving along, and there's a backwards force acting on you.

So you've got the movement and the force in opposite directions, and that'll cause you to slow down.

And the third possibility is there's no resultant force at all.

And in that case, there's no change in speed forwards or backwards, so you are actually gonna stay moving at the same speed.

So just to show that in a bit more detail.

When the resultant force is in the same direction, you get an object that's speeding up.

So I've got a box here, and then let's pretend that box is moving to the right.

I'm gonna put a force on it, that's also acting to the right.

So a resultant force acting in that direction.

As I've said, what's gonna happen is that box is going to speed up, it's gonna increase its speed.

So we've got a little animation to show that.

If I push, that box gets faster and faster and faster, and it continues to get faster the longer I push it.

As long as there's a force acting on it, it's gonna speed up.

The animation loops a bit there, but you can see the idea with a little speedometer there.

He continues to push, and the object gets faster and faster and faster.

So that's what happens if you've got a force in the same direction.

I'm sorry, a resultant force in the same direction the object's moving.

A quick check to see if you understood that.

I'd like you to decide what will happen to that box.

I've got a box moving to the left, and I've got a resultant force acting to the left.

I've got four possibilities.

It'll move at constant speed, it'll slow down, it'll speed up to the left, or it'll start moving to the right.

So I'd like you to pause the video, make your selection, and restart.

Okay, welcome back.

So what you should have seen is that that box will speed up to the left, the box is moving left, and there's a resultant force to the left, so it's going to start going towards the left faster and faster and faster.

So well done if you've got that.

The next situation is if you've got a resultant force that's in the opposite direction to the direction that you're actually moving.

So I've got a situation, something like this.

I've got a box moving to the right here, and I've got a resultant force that's towards the left.

So the movement and the force are in opposite directions, and what will happen in that scenario is the object will slow down, the speed will reduce.

So I've got a little animation of that here as well.

So we've got a box that starts off moving very quickly, but the frictional force is in the direction opposite to the movement.

So gradually, the object slows down, and eventually, it'll come to a stop.

It doesn't quite reach that in the animation, but you can see from the speedometer there that the object is slowing down because the force is working in a different direction to which to the box is moving.

I've got another check here to see if you understood what you saw there.

I want to know what will happen to this box.

Let's examine the picture first.

I've got a box that's moving to the left, and that's the important thing here.

It's moving to the left before the start.

And then I've got a resultant force that's acting towards the right shown by the orange arrow there.

And I wanna know, will that box move at a constant speed, will it slow down, will it speed up to the left, or will it not move at all? I'd like you to pause the video, make your decision, and then restart.

Okay, in that scenario, you should have picked this option.

It will slow down.

The box is moving to the left, but the force is in the opposite direction, so the force is working against the movement, and that will reduce the speed.

So that box will gradually slow down until it's not moving at all.

I've got a third scenario now.

If the forces on the object are balanced and there's no resultant force at all, then the object doesn't accelerate.

And what that means is the object won't speed up and it won't slow down.

That must mean it's gonna be moving at a constant speed.

So we've got a box here, and there's no resultant force acting on that box.

And at the start, the box is already moving to the right.

So in the animation here, you can see that the object will move at a constant speed.

The two forces cancel each other out, giving no resultant force, and that means there's nothing to change the speed of the object.

You need a force to make the speeds change, and there's no resultant force here, so there's no change in speed.

The box will keep on going at that speed forever.

Okay, I'd like to check that you understood that.

So I've got three diagrams of forces acting on cars here and three descriptions of how that car will change its speed or not.

What I'd like you to do is to batch up those diagrams and those descriptions.

So we've got one at constant speed, one slowing down, and one speeding up.

And all you need to do is draw lines connecting those dots to connect those descriptions and diagrams. So pause the video, do that task, and then restart.

Okay, let's have a look at a solution to that.

The first one is constant speed.

And for constant speed, what you need are forces that cancel each other out, so there's no resultant force.

And that's the middle diagram here, 45 newtons in each direction.

The second scenario is slowing down.

And for that, we need a greater backwards force than forwards force, and that's this bottom diagram here.

We've got an overall resultant of 30 newtons backwards.

So that would be slowing down.

And in the third scenario, speeding up, that's gonna be the top diagram, and we've got a resultant force of 15 newtons forwards there, so that car would be speeding up.

Well done if you got those.

A resultant force acting on an object causes a change in speed, and as long as that resultant force is acting, the speed's gonna continue to change.

It'll keep getting faster or slower while that force acts.

If there's no resultant force, then you're not gonna get a change in speed, and there's no acceleration.

We're going to look at an explanation of that now, and we're gonna use something called a dynamics trolley.

We're gonna connect the dynamics trolley to a piece of string like this with some masses on the end.

A dynamics trolley is just a little wheel vehicle we use in physics experiments, and that's it over here.

We're gonna connect that to some masses over here.

And those masses are gonna be pulled downwards by a gravitational force, and that's going to try and make them move.

The string connecting the dynamic trolleys and the masses is gonna transfer that force and cause that force to apply on the dynamics trolley.

So we're gonna get a force pulling that dynamics trolley forwards.

Okay, so that resultant force is gonna start making the dynamics trolley move, and it's in this direction along the string.

So we've got this situation where we've got a constant force that's gonna act on the dynamics trolley once it's released.

So as soon as I let go of it, it's gonna start moving towards the left here.

I've got two pupils who are gonna make predictions about how it's going to move, and they're slightly different.

So I've got Andeep, and he makes a prediction, "The trolley will get faster and faster because the resultant force is still acting on it as it moves." I've got Sofia, who says, "The trolley will stay at the same speed because the resultant force stays the same." You should have a quick think about which of those you think is correct.

You might wanna pause the video before we move on and try and find an explanation.

We can watch a very brief video showing what happened in that experiment, and then we can analyse it afterwards.

So let's have a quick look.

Over the next few slides, we'll try and see what happened in that video in a bit more detail.

So I've made slow motion videos to analyse the motion of the trolley a bit more simply because it was a bit too fast to see.

In this first video clip on the left, I'm gonna look at the first one second of movement.

So I've got it looping, and I can measure the distance it moves, and it moves five seconds in that one.

Sorry, it moves five centimetres in that one second.

And in the second video, that's a two-second clip, and it can again measure how far it moved there.

And in that two seconds of movement, or two seconds of time, it moved 20 centimetres.

So if you continue with that experiment and measure the distance from the start, we get a set of data that looks a bit like this.

It starts at zero centimetres from the start.

After one second, it's five centimetres, after two seconds, 20 centimetres, three seconds, 45, and after four seconds, it's 80 centimetres from the start.

And we can calculate how far it's moved during each of those seconds by calculating the change in distance.

So at first, it hadn't moved at all, so zero.

And in the first second, it moved five centimetres, in the second second, it moved 15 centimetres, in the third second, it moved 25 centimetres, and in the fourth second, it moved 35 centimetres.

So what you should be able to see from that is the distance is increasing during each second.

That means that the speed must be increasing.

And so overall, you've got a conclusion that the trolley is constantly accelerating.

So while that force is acting through that string on the trolley, the trolley accelerates during each second.

It's getting faster and faster and faster.

Okay, let's check those two predictions the pupils made.

Andeep said, "The trolley will get faster and faster because the force is still acting on it as it moves." And Sofia said, "The trolley will stay at the same speed because the resultant force stays the same." I'd like you to decide which of the predictions was correct, if any of them were.

So put a tick in one of the boxes for A, one of the boxes for B, or one of the boxes for C.

Pause the video, and do that, and then restart.

Okay, welcome back.

You should have decided that Andeep's prediction was correct, the trolley was getting faster and faster, and it must be the force that's causing that change in speed.

So while the force continues to act, the speed continues to change.

Sofia's prediction was wrong, the trolley wasn't going at a constant speed, and both predictions can't be wrong 'cause one of them was correct.

So well done if you've got those three.

So what you've seen is resultant forces cause things to accelerate, to get faster or slower, and that depends on which direction the resultant force is.

So we've got a car here.

The car will continue to get faster and faster as long as the driving force, the force pushing it forward is larger than any resistive forces like friction or air resistance.

The pool ball here will get slower and slower as it rolls across the cloth because there's a frictional force acting on it in the direction opposite to movement, and there's no driving force pushing it forward now, so that slows down.

So I've got a check here to see if you understood about forces causing changes in speed.

I've got a stone being dropped from a very high bridge, so we don't have to worry about it reaching the ground, and it falls five metres in the first second after it's been dropped.

How far will it travel in the second second? Will it be less than five metres, about five metres, or just over five metres, or a lot more than five metres? So I'd like you to pause the video, think about that, choose an answer, and then restart.

Okay, welcome back.

You should have chosen a lot more than five metres.

The gravitational force is gonna continue to act on that stone, and that's gonna make it speed up 'cause there's gonna be a resultant force downwards.

So in that second second, it's gonna travel a lot more.

Okay, we're gonna continue thinking about objects that are falling.

I've got that stone dropped from the high bridge again, and I wanna know, why does it fall much further in the second second than in that first second? Is it because there are no forces acting on it? Is it because the size of the resultant force on it increases? Is it because the downwards resultant force causes it to accelerate? Or is it 'cause the stone is getting heavier as it falls? So pause the video, make your decision, and then restart when you're happy.

Okay, you should have selected the downwards resultant force causes it to accelerate.

That force is going to make the stone get faster and faster because it continues to act as the stone falls.

Well done if you've got that.

Okay, one more check of your understanding here.

A football is kicked and rolls across the grass.

What are two reasons it travels further in the first second than in the second second? So I'd like you to select two from these, there are no forces acting on the ball as it moves, there is no driving force acting on the ball after it's kicked, A frictional force acts on the ball, or the size of the resultant force acting on the ball increases.

So pause the video and choose two of those, and then restart when you're happy.

Right, the answers to that, there's no driving force after you've kicked the ball.

There's nothing pushing it forward anymore, so no driving force, and there's a frictional force.

As it rolls across the grass, that frictional force will act to slow the ball down.

So it'll go a smaller distance in that second second.

Well done if you've got those two.

Okay, we're onto the task now at the end of this learning cycle, and I'd like you to have a look at this scenario.

I've got a trolley rolling down a ramp onto some rough ground.

I'd like you to explain why the speed of the trolley increases as it goes down a ramp.

And then I'd like you to explain why the speed of the trolley decreases as it moves across the rough ground.

So pause the video, write out your explanations, and then restart, and we'll go through the solution.

Okay, your explanation should have been something like this, and they should be in terms of forces acting on the trolley.

The trolley must be speeding up because there's a resultant force acting on it, and that resultant force is gonna accelerate it, increasing its speed.

And similarly, when the trolley's on the rough ground, there's a resultant force acting backwards on it, and that's a resultant force because there's a frictional force acting on it, and there's nothing driving the trolley forward anymore, and so the speed of that trolley will decrease.

So well done if you've got explanations, something like these two.

Okay, we're gonna move on to the final learning cycle of the lesson now, and that's all about acceleration and distance-time graphs and how those graphs will show things speeding up and slowing down.

So a quick recap of distance-time graphs here.

I've got three different objects moving, and I've got a distance-time graph that shows all three of those objects.

And in previous studies, you've seen graphs like this before.

You should remember that the greater the speed, the higher the gradient, or the steeper that line is on the distance-time graph.

So I've got the fastest object there, the solid black line is moving fastest, and I've got the slowest, which is the dot-dash line, and that's got the shallowest gradient.

All three of those objects are moving at a constant speed, and you can tell that because those lines are straight, there's no change in speed for any of those three objects.

This object's getting faster, you can tell that because the gradient is increasing.

Any object that's getting faster is gonna have a steeper and steeper line on its distance-time graph.

So we've got an object here that's speeding up.

Now the reason we've got an object speeding up is because there's a resultant force acting on it, increasing its speed.

At the start, the object's moving slowly, and that's shown by that shallow gradient.

And then later on, the object is moving much faster because that force's continue to accelerate it, increasing the speed, and so we've got a much steeper gradient.

And if that force continues, we'll get a steeper, and steeper, and steeper gradient.

So the resultant force is constantly changing the speed of the object.

And you've already seen, resultant forces can also cause objects to slow down, and we've got a graph of that motion here.

At the start, this object is moving quickly, and you can tell that because it's got a steep gradient.

At the end, it's moving slowly.

Or, in fact, this one's slowing to a stop.

That resultant force is causing the object to change its speed until eventually it reaches a stop.

Right, let's check if you've understood that.

I've got three graphs here.

Which of those objects doesn't have a resultant force on it? Is it A, B, or C? So pause the video, make your selection, and then restart.

Okay, welcome back.

And the example you should have chosen was B.

That's got a constant speed.

You can see it's a straight line, must be a constant speed there, and that means there can't be a resultant force on it.

If there is a resultant force, it'll get faster or slower.

So that one's a constant speed.

This one shows an object speeding up, so there's a forwards resultant force on it.

And this one shows an object slowing down, so there's a backwards resultant force on it.

So well done if you chose B.

Okay, it's the final task now, and I've got a graph for you to try and describe.

So we've got a distance-time graph here, which is clearly a curve.

The distance-time graph shows the movement of a firework rocket launched upwards.

Explain how the graph shows the rocket has an upwards resultant force acting on it.

So I'd like you to think about that, pause the video, write down your answer, and then restart.

Okay, let's have a look at the solution to that question.

You should be able to see from the graph that the gradient is increasing.

We've got a shallow gradient and then a steep gradient later on.

What that means is we've got a low speed at the start and a higher speed later.

Now the firework is getting faster, it's speeding up, that means it's accelerating.

And as you've learned through this lesson, if an object is accelerating, there must be a resultant force on it.

And that resultant force, in this case, must be acting upwards because the rocket is getting faster.

So well done if you've got an answer something like that.

Right, we've reached the end of the lesson now, and here's a summary of what you should have learned.

A resultant force causes acceleration, and acceleration can be speeding up or it can be slowing down, so an increase in speed or a decrease in speed.

And that acceleration will continue as long as that resultant force continues to act.

You also saw that distance-time graph can show acceleration, and that will be a curved graph.

The steeper the line, the faster the movement of the object.

So we've got a speeding-up object there where the gradient is increasing, slowing down, or the gradient's decreasing, and no changing gradient means a constant speed.

Well, that's the end of the lesson.

So congratulations for reaching the end, and I'll see you in the next one.

Bye.