Hello there! My name is Mr. Forbes and welcome to this lesson from the Moving by Force Unit.

The lesson's called Newton's first law, and it's all about resultant forces and what they do to objects, how they speed them up, and how they slow them down and what happens when there's no resultant force at all.

By the end of this lesson, you're gonna be able to describe what a resultant force does to an object, whether it speeds it up or whether it slows it down and what happens when there's no resultant force on an object? Does it get faster, slower, stay at the same speed, or whatever? Okay, when you're ready to start, let's go.

So to understand movement, these are the key words that we're going to need.

Stationary means an object that's not moving.

So a stationary book is just a book left lying on the table, something like that.

Resultant forces the overall effect of forces acting on an object.

And the two forces that we're gonna consider during this lesson are frictional forces, the forces when objects slide past each other so we can use the sledge sliding across the ground.

and gravitational force, the attractive force between the earth and an object nearby.

This is a set of definitions for those keywords and come back to this point and read the definitions if you're confused about any of the keywords at any point during the lesson.

Okay, so the lessons in three parts.

The first part is about forces causing objects to speed up.

So we'll see some examples of forces increasing the speed of an object.

In the second part of the lesson, we'll look at forces causing things to slow down.

So we'll see how a resultant force such as friction can slow things.

And in the third part of the lesson, we'll look at something called Newton's first law of motion.

And that's a summary of what forces can do to an object, whether they'll speed it up, slow it down, or leave it moving at a steady speed.

So let's go on with the first part of that, forces and speeding up.

Hopefully you know what a sledge is.

It's basically just a board we sit on and it slides down a hill.

We usually use it on snow, but it will work on grass, on other surfaces as well.

The forces on the sledge that will make it start to slide, make it speed up.

So they change its movement.

And the two forces that we're gonna be looking at today are the gravitational force.

The force and gravity acts down the hill and it causes the sledge to speed up.

So that force is making you slide.

The second force acting on it, I think you'll remember is a frictional force, and that acts in the opposite direction to movement.

So in these cases, that's gonna be acting up the hill and that's a tendency to slow things down.

So those two forces and the difference between them are what's gonna affect the movement of the sledge.

All right, so we're gonna start our explanation about the forces by thinking about a stationary sledge.

This is a sledge resting on a flat surface.

I've got on the ground in green here because I can't use snow against my white background on the slide.

So if you imagine it, there's no horizontal forces acting on this sledge.

It's gonna stay stationary.

There's no force trying to push it and make it move.

So it'll just sit there doing nothing.

If I do put a force on it by pushing it from behind, it'll start to slide forwards.

So you should see that the force is gonna try and drive movement in the direction of the force.

So that push force is gonna move that sledge forwards.

Now here's my second example and I've got a sledge at the top of the hill and it's being held in position.

I don't wanna let it go yet.

So you can imagine you and a friend, one of you on the sledge and the other's holding the sledge from behind, ready to release it.

So it's held in position and there's no resultant force on it.

You've got a force downwards, that's the gravitational force trying to make it move forwards, trying to make it slide.

But then you've got a force in a rope or someone's hands that's pulling it backwards.

And in this case I've got a tension in it.

So this sledge has got those two forces on it and they're balanced.

So it's not going to start moving.

It's gonna stay stationary just like the earliest sledge when those balanced forces are on it.

So a stationary sledge.

Okay, so now what happens when you release the sledge? Well, that backwards force, the tension we saw before holding the sled backwards, that's disappeared.

And so at the instant we let go of it, there's gonna be some unbalanced force on it.

We've still got the gravitational force pulling it downhill, but we don't have any force pushing it backwards or pulling it backwards.

So we've got a resultant force acting on the sledge, and because of that, the sledge is going to start moving.

It starts to slide down the hill.

Okay, I'd like to check your understanding about objects that are rolling down hills.

And in this example I've just got the one force acting.

I've just got the force of gravity causing the stone to move.

So I've got a large round stone and it's rolling down a steep slope.

And there's a set of flags or markers on that slope.

The first one is O, that's the starting point.

Then it goes x, Y, and towards the bottom is Z.

So which statement about the speed of the stone are correct? Is it the speed of X is faster than the speed of O; the speed of Y is faster than the speed of X; the speed of Z is faster than the speed at Y? So I'd like you to decide which of those statements are true.

So pause the video, make that decision, and then restart and we'll go through them.

Okay, welcome back.

Let's have a look at the answers to that.

And in fact, all three of those statements are correct.

The speed at O, the starting point is gonna be very slow and then it's gonna speed up towards X 'cause that force is speeding up and then it continues to speed up so it's even faster at point Y and it continues to speed up again.

So it's fastest at point Z.

So all three of them are correct.

Well done if you've got those.

So here's our sledge again, and it's moving down the slope, but this time there's a frictional force acting as well as the gravitational force.

So we've got a gravitational force down.

I'm gonna give it a nice round number of a hundred newtons.

And I've got a frictional force acting backwards that's a bit smaller at 20 newtons.

So because of those two forces, what I have is a resultant force.

And if you remember, the resultant force is just the overall effect of the forces taking into account direction because the frictional force is acting opposite to the gravitational force.

We take it away from the gravitational force, given us that resultant force of 18 Newtons.

Now, because there's still a force acting on the sledge downwards, a resultant force, that sledge is gonna continue to speed up because there's a resultant force on it.

So it's gonna get faster and faster as it slides.

Now as the slide speeds faster and faster, the frictional force gradually increases.

So earlier we had a frictional force of 20 newtons but this slide's going faster.

So there's a larger frictional force, it's 50 newtons.

The gravitational force on it is still the same size as it was earlier, so it's still a hundred newtons.

So overall there is still a resultant force acting on that slide, and I think you can probably work that out.

That's a force of 50 newtons acting downwards.

And what that means is the resultant force is getting smaller, but there is still a resultant force acting on this sledge.

So it's still gonna be speeding up.

Let's see if you remember how to calculate resultant forces.

So I've got a sledge here and there are two forces acting on it.

There's a hundred Newtons acting down the hill and a hundred newtons acting up the hill.

And I'd like you to work out what the size of the resultant force on it is.

So pause the video, work that out, and then restart.

Okay, welcome back.

Well, the answer is nought Newtons.

You've got a force of a hundred down and a hundred up the hill so the resulting of those two forces, because they're acting in opposite directions, is that you subtract one from the other and that gives you an answer of zero newtons.

So there's no resulting force in the sledge.

Hold on if you've got that.

Okay, I've got a slightly more difficult question for you now I'd like you to decide what would happen to the speed of the sledge when the frictional force and the gravitational forces are equal like in a diagram I've shown there.

Will the sledge stop? Will it speed up? Will it slow down only to stay at the steady speed? I'd like you to pause the video, make your decision and restart.

Okay, welcome back.

Well, as I said, that was a little bit more difficult.

You should have realised straight away that there's no resultant force.

So there's no resultant force on it.

What that means is the sledge gonna remain at the same speed.

We've mentioned earlier that forces cause changes in speed and there's no resultant force here.

So what we would expect is there's no change in speed because the resultant force is needed to change that speed.

Well done if you've got the answer.

Right then, we've reached the first task.

And this task is all about your understanding of the forces acting on a sledge as it slides down a hill.

So I've got four diagrams here.

We've got a diagram of a sledge just as it's released, that's diagram A.

Then then moving slowly, that's diagram B.

Moving a bit faster, that's diagram C.

And then moving at a top speed and that is diagram D.

And what I'd like you to do is to just draw the arrows to show the forces acting on those that affect the movement.

I'd like you to include a resultant force arrow if there is a resultant force as well.

So I'd like you to pause the video, draw those force arrows and restart when you're ready.

Alright, let's go through the solution of those, starting with diagram A.

It's just been released so it's not moving.

There's no friction force.

So I only need to draw the gravitational force on as I've drawn there in purple.

And that gives a resultant force down the hill as well, which I've drawn in red.

So I've got just the two forces there.

In the second picture, I've got three forces because the sledge is moving.

I've still got the gravitational force, which you should have drawn the same size as you did originally.

And now there's a frictional force backwards up the hill.

And that's not.

Well, that's going to basically reduce the size of the resultant force.

So I've drawn my resultant force there in red, a little bit smaller than in the first diagram.

As I'm moving faster, the result force gets smaller again, 'cause the frictional force is larger.

So in diagram C there, a larger frictional force and a smaller resultant force.

The gravitational force is still the same size.

And in the last diagram, most difficult of all, the gravitational force and the frictional force have reached the same size now.

What that means is there's no resultant force.

Congratulations if you drew all those force arrows correctly.

Okay, we've looked at forces causing things to speed up as resultant forces making things get faster.

Now we're gonna look at forces causing things to slow down.

So so far we've seen a sledge sliding on a hill and we've looked at the forces causing it to speed up until it reaches a top speed, gravitational force and a frictional force.

But eventually it's gonna reach level ground and at that point there's no longer a gravitational force acting on it, pulling it down a hill, making it get faster.

So there is still a frictional force acting backwards because of the sledge rubbing against the ground.

And that force means that there's still a resultant force acting on the sledge.

So we've got a different scenario now.

We've got a sledge moving forward, but this force is only acting backwards on it.

So I'd like you to answer this question for me now, please.

I'd like you to work out what the resultant force on this sledge is.

I've got a movement toward towards the right, but I've got a frictional force towards the left of 50 newtons.

And I want to know the resultant.

I'd like you to pause the video, make a selection, and then restart.

Right, you should have selected 50 newtons backwards.

There's only one force acting on the sledge, and that's the frictional force, and that's backwards, opposite to the direction of the movement.

Well done if you've selected that.

So the frictional force acting on the sledge is going to gradually slow it down.

When the sledge is moving fast at high speed, you've got a large frictional force, in this example, a hundred newtons.

And that's gonna always act against the movement.

It's gonna act backwards.

So this sledge is gonna slow down because there's a resultant force backwards.

When the sledge is a bit slower, the frictional force is smaller, but it's still acting opposite to the movement.

So it's still gonna cause the sledge to slow down.

So over time the sledge is gonna slow and eventually it's gonna come to a stop.

Okay, let's see if you understand frictional forces on sledge.

I've got three diagrams here of sledges and they're all moving towards the right.

I'd like you to decide which of those sledge must be moving the fastest and they're all identical.

So pause the video and restart once you've made your selection.

Okay, welcome back.

Looking carefully at the diagrams, you should have selected C.

There's the largest frictional force there.

And as you learned earlier, the faster you're going, the bigger the frictional force is.

So that sledge must be going the fastest.

Sledge A is going the slowest, and B, somewhere in the middle.

Well done if you've got that.

So the friction force acting on the sledge moving across flat ground will gradually slow it to a stop.

I've got a sledge here moving at a slow speed and it's still a frictional force on it opposing movement because it's sliding across the ground.

So that sledge will get a little bit slower and the frictional force will get a little bit smaller, but it's still acting backwards, slowing the sledge.

So over time, thought that frictional force will gradually slow it down until eventually the sledge is stationary.

Then there's no frictional force and there's no gravitational force affecting the movement.

It just stays still.

So frictional forces cause things to slow down and that's why you see everyday objects slowing all the time.

You kick a football, it'll gradually slow down because of the frictional force.

Okay, I've got quite a challenging scenario here.

I've got a sledge moving across flat ground and there's a frictional force on it, and then I've got the sledge and it starts going uphill.

It slid along and starts sliding up that hill.

And I want to know why the sledge will come to a stop more quickly if it's moving uphill than it would if it's moving across flat ground.

So try and draw a force diagram and work out what the solution is.

Pause the video, do that, and then restart.

Right, welcome back.

So let's have a look at the forces act on it.

In the first scenario, we've got a resulting force just due to the friction acting on the sledge.

So it's gonna slow down, but it's only that resulting force acting on it because of friction.

But when it's going up the hill, we've got the frictional force, but we've also got a gravitational force.

Instead of speeding the sledge up this time, that gravitational force is acting down the hill causing the sledge to slow more.

We get a larger resulting force acting on it.

That's what's gonna make it slow down an awful lot quicker.

So we get a sledge coming to a stop on that hill in much shorter distance than it would if it was going across flat ground.

Well done if you've got an answer, anything like that.

Okay, let's use a task to summarise what you've learned in this section of the lesson.

I've got a car that's broken down so we don't have to worry about any driving force from the engine.

I've just got it pushed from behind by a person there.

So I'd like you to explain why it needs to be pushed from behind to keep moving forwards at a steady speed.

And then the more challenging part, explain why it would be harder to keep it moving at a steady speed if it's being pushed up that hill.

So I'd like you to pause the video, perhaps draw some force diagrams and write some explanations about those two questions, then restart.

Okay, welcome back and let's have a look at those scenarios.

In the first case, when the car's moving across the ground, it's gonna be a frictional force on it.

So if I want it to move at a steady speed, I need to put a force on it to actually counteract that friction so we get no resultant force.

So there's the two forces there, are pushing and frictional force and they need to be in opposite directions.

In the second scenario, it's a bit more complicated, but I need to push the car up the hill and I need to push with the size of force that counteracts or cancels out the gravitational force as well as the friction.

So I've got a much larger force now that I've gotta do, and so I need to push much, much harder.

It's incredibly difficult to push a car up a hill, whereas on flat ground it's not too difficult.

So there's the explanation.

Well done if you've got anything like that.

Okay, we've reached the third and final part of the lesson.

This is about Newton's first law of motion when Newton describes what forces do to optics and how it changes their speed or not.

So in the lesson so far, we've looked at the effect of the resultant force on an object.

So in some scenarios, the resultant force can cause the object to speed up if it's acting in the same direction as the object's moving.

So we've got a sledge sliding down the hill.

When the resultant force was in the opposite direction to the movement, you've got an object that's slowing down when there's no resultant force.

The object didn't speed up or slow down, it stayed at a steady speed.

So there are the three possibilities of how a force can affect the speed of an object.

Now those three scenarios were first fully described by Isaac Newton when he wrote his three laws of motion.

And his first law of motion was all about what happens when there's no resultant force acting on an object.

But it also gives us some information about what happens when there is a resultant force.

So let's have a look at what you said.

Newton's first law of motion when he wrote it sounded something like this.

Every object will remain at rest or in uniform motion in straight line unless compelled to change its state by the action of an external force.

Now what that means is when there's no resultant force acting on something, a stationary object will stay stationary.

It won't start moving because there's no resultant force.

If an object is already moving, it's gonna keep moving and it's gonna keep moving at the same speed and in the same direction as it was travelling.

Okay, let's check if you understood what Newton was saying there.

I've got an asteroid moving through space and there's no resultant force acting on it.

What does Newton's first law predict will happen to that asteroid? Will it A, suddenly stop; B, slow down over time; C, not change speed or D, speed up over time? Pause the video and make a selection and then restart.

Okay, what Newton said would happen was it will not change speed, it won't change direction either.

His law said that if there's no resultant force, an object that's moving will continue to move in a straight line at the same speed.

Well done if you got that.

Although Newton's first law is written about what happens when there's no resultant force, it also gives us information about what happens when there is a resultant force.

So for example, if you've got a stationary object and a resultant force starts acting on it, it will start moving.

So when you've got a stationary football and you kick that football, it will start moving.

If you've got a moving object and there's a resultant force on it, there will be a change in its speed or a change in its direction or both.

So when you've kicked that football and it starts rolling across the grass, the frictional force will start acting on it and that'll cause the speed to change.

It will slow it down.

So we can use Newton's first law to decide if the motion of an object will change.

So I've got an example here, got a pencil case sitting on a desk, it's got balanced forces on it.

And what Newton's first law says is if there's balanced force and it's just sitting there, it's stationary, then the object's gonna remain stationary.

So that pencil case has got no resultant force and it'll stay stationary.

And the second example, I've got a falling parachutist and this parachutist has got balanced forces acting on them.

While they are falling, they are moving downwards.

What Newton's law says is because the forces are balanced, there won't be any change in their motion.

So they won't speed up and they won't slow down.

So there's no result of force acting on them and they stay moving at a constant speed.

They don't stop because their forces are balanced, they just stay moving at that constant speed.

By my third scenario, I've got a car moving on the road and it's got unbalanced forces on it.

What's Newton's first law saying again is, that this has got a forward resulting force, therefore there's gonna be a change in the motion.

And the change in the motion is going to be that car speeds up.

Okay, let's see if you can apply Newton's first law to some motion diagrams. I've got four scenarios here and I'd like you to describe what happens next to the three vans and to the boat.

Looking at the force diagrams, (indistinct) some indication of whether the objects are moving or not there.

So pause the video, make your decision for each diagram and then restart.

Okay, let's look at the answers.

In the first one, I've got an object that's moving and there's a resultant force in the forwards direction.

The 38 newtons is larger than the 18 Newtons, so this is gonna gradually speed up.

In the second one, I've got a resultant force that's backwards.

The 300 newtons is larger than the 190 newtons so that object is gonna gradually slow down.

In the third scenario, I've got balanced forces so there's no resultant force, so the object's gonna keep moving at a constant speed.

And in the final one, I've got a stationary object with balanced forces acting on it so it's going to stay stationary.

Well done if you saw all those.

Okay, we've reached the final task for the lesson now, and we're gonna summarise the motion of optics that are speeding up, slowing down, or going at a constant speed and how the forces affect them.

So I've got a skater here speeding up as they're going down slope A, and then they're gonna go past point B at a steady speed and then they're gonna come to a stop at point B after going up that slope.

So I'd like you to draw three-fourths diagrams showing the resultant force on the skater at point A, B, and C.

And then I'd like you to explain why the speed changes at the three points.

Okay, so pause the video, carry out that task, and then restart and we'll go through the answers.

Okay, welcome back.

Let's have a look at the answers to those.

At point A, we've got a gravitational force pulling the skater down.

So you should have drawn an arrow like that along the slope.

At point C, again there's a gravitational force acting on them and that's downwards along the slope.

So a description now of how those forces change the speed.

At point A, the skater is speeding up.

They're getting faster because there's a resultant force acting on them and it's acting down the slope so they get faster due to gravity.

At point B, there's no resultant force on them.

So they're not getting faster or slower, they got a steady speed.

And at point C, there's a backwards force, so the skater's going to slow down 'cause the force of gravity is slowing them as they move up that hill and they just outreach the top.

So well done, if you've got those answers.

Okay, we've reached the end of lesson now.

So here's a quick summary of what you should have learned.

You should have learned that a resulting force from something like gravitational and friction forces will cause a change in movement and that change in movement we describe as acceleration.

Newton wrote his first law, which describes what happens when there's no resultant force acting on an object.

And it basically says there's no change in speed.

So if the object is stationary, it stays stationary and if the object's moving, it keeps moving at the same speed and in the same direction.

But it also tells us that if there is a resultant force, the opposites are true.

So if there is a resultant force and the object is stationary, it'll start to move.

And if there is a resultant force, it will stop moving in the straight line.

It will (indistinct) at the same speed.

It will go faster or slower or change direction.

And that's summarised by those diagrams. There we can see there are resultant forces on the van and that's causing it to change its speed.

Okay, well done on reaching the end of the lesson and I hope to see in the next one.