Lesson video

Hello, and welcome to this lesson on friction.

This is from the forces topic.

My name's Mr. Norris.

Friction is a force that's everywhere.

It's all around us, so it's useful to know a little bit about it, so let's get started.

Here's the outcome of this lesson.

By the end of the lesson, you should be able to describe the characteristics of friction forces, explain what causes friction, and also describe how friction can be reduced.

Here are some keywords which are going to be covered this lesson.

Friction, rough, irregularity, microscopic, and lubricant.

Now, each word will be explained properly at the appropriate part in the lesson.

The lesson is split into three sections.

The first section introduces friction forces and gives characteristics of friction forces.

The second section talks about what causes friction, and the third section talks about how friction can be reduced, so let's get started with the first section.

So friction is the force of one surface gripping, or rubbing on another surface.

So here's a situation, climbing, a climbing wall, that climber's right foot might slip off, that little kind of block of the climbing wall that their right foot is resting on, but there's a friction force.

There's grip between the bottom of their shoe and from that little block on the climbing wall.

And the grip provides a friction force which prevents the foot from slipping from where it is.

So friction can be incredibly useful, incredibly important, sometimes, depends how high they are, a matter of life and death, if the friction force is enough to maintain that grip.

Friction is also what light to match, because there's rubbing between the match surface and that kind of grippy surface that you strike the match on, and friction can generate a temperature rise, which might be enough to light like the match.

And friction could be demonstrated by just something as simple as two hands rubbing together.

You could even try that now, rub your hands on each other, and feel those surfaces gripping and rubbing on each other that you are experiencing a friction force.

Of course, we should say that friction is a contact force.

Friction only acts between surfaces that are in contact to grip or rub on each other.

Friction or grip between your hand and a jar lid is what turns the jar lid to open the jar, so you couldn't open jar lids without friction, but you couldn't close the jar lid either without friction, because it's friction between the lid and the jar, that's what makes it hard to undo.

That's what keeps the lid kind of stuck on.

And you have to push against the friction between the lid and the jar to undo the jar lid.

A friction force is what allows you to run, 'cause every time you put down, put your foot down, the foot is trying to slip backwards along the floor, but a friction force between the bottom of your foot and the floor grips your foot, and stops your foot from slipping backwards.

So friction is actually essential for how we move by walking and running.

And it's also essential for vehicles, because friction forces cause tyres to grip the road surface as they turn.

And that's what drives a car forwards.

You might have seen or experienced, you might have been in a car trying to get going on a snowy or icy surface that's if there's far less friction, the wheels might spin just on the ice without gripping instead, and that stops a car from being able to drive itself forward.

So friction's really important.

Let's have a quick check for understanding.

So we've got Miss Orange here trying to push the car.

The car's engine is off, where do friction forces act? So the question really here is asking where is there gripping or rubbing between two surfaces, because that's when you get friction when there's gripping or rubbing between two surfaces.

So what about A, between Miss Orange's feet and the road, is there gripping or rubbing there? What about B, between Miss Orange's hands and the car? Is there friction there? Is there gripping or rubbing there? What about C, between moving parts of the car? So for example, the axle turning against other moving parts of the car.

Is there friction there, gripping or rubbing? And what about D, between the tyres and the road? Is there gripping and rubbing there? So for each of those, you should kind of tick the right box, do it in your head if you need to, for each statement, is there friction there? Are you sure there's friction there? Or do you just think there's friction there? Do you think there's not friction there? Are you sure there's not friction there? Do that for each of those four places.

Pause the video now if you need to.

Okay, I'm gonna go through the answers.

So between Miss Orange's feet and the road, well, if you are pushing the car, then you are using your feet to get grip on the road surface, so you can push forward, so there is friction there, because your feet have to grip the road surface.

Between Miss Orange's hands and the car, well, their hands have to grip on the car surface to stay in place on the car surface, and not slip off, so there is friction there.

Between moving parts of the car, between any moving parts of a mechanism, there'll be a friction force acting against the motion, so yes, there'll be friction there.

And between the tyres on the road, we already said that tyres grip a road surface, so yes, there's definitely friction between tyres and the road.

So well done if you're identified that there is, in fact, friction for different reasons at all of those different points.

So let's go through in a bit more detail now.

So friction is caused by movement, rubbing between two surfaces.

There'll be a friction force between that box sliding across that tabletop.

There's the friction force.

And friction forces always act in the opposite direction to motion, so it's gonna have the effect of slowing down a moving object.

And friction is also caused though when another force attempts to produce movement.

So this box is not moving, there's a person pulling it across that table or stage, but so why is it not moving? 'Cause forces make things change.

So surely, that pulling force should be making the box move.

Well, there's another force, of course, cancelling out that pulling force.

It's the force of friction between the box and the surface.

The friction force of the surface on the box is preventing movement.

It cancels out the pulling force.

Which is what it says there.

So we should also say that the friction force between two surfaces can change size depending on the size of a pulling force.

So in this first picture, we've got a person pulling the box with a 50 newton force, and friction is at 50 newtons at that point.

But if the person pulls a bit harder, then actually, friction can get a bit bigger in response to that.

So the box doesn't start moving yet, because the forces are still balanced like friction has grown to, so it's still balances that pulling force, but there does come a point where there's a maximum friction size and if you pull harder than that, then you will get the box to start moving, 'cause friction can't increase anymore, and that maximum is different, but for every different kind of pair of surfaces that you might try and pull a box across.

So here's an example of that.

In this case, the maximum friction size looks like it's something like 100 newtons, 'cause when the person pulls with 101 newtons, friction can't increase anymore, so the person is pulling with a greater force than the friction force can be, so the forces are unbalanced, and the box can start moving or speed up from a speed of zero to whatever speed it can then slide across the floor with.

So the key thing from this slide is the size of a friction force is not set.

It can vary even between the same two surfaces depending on the situation up to a maximum, that is.

Let's do a check of what we've just gone through.

So the box in this picture is not moving, it's not being pushed either, it's not moving.

So true or false, a friction force is acting on this box.

Is that true or is that false? Five seconds to decide.

You should have decided if that's true or false.

Well done if you identified that that is false.

There's no friction false acting on that box, but what's the correct reason? So you now need to choose the correct reason.

Is it A or is it B? Is that false, because friction only acts if there's movement? Or is it false, because friction only acts if there is movement or if there's another force attempting to produce movement? Which is the correct reason why that statement was false? Five seconds to decide, A or B.

Well done if you identified the correct reason why there's no friction force acting on that box is B.

Okay, friction only acts if there's movement, or if there's another force attempting to produce movement.

And we don't have either of those situations in this case, so there's no friction force acting on that box, because it's not moving and there's no force attempting to move it either.

Here are the last few things we need to go through about friction.

So friction is the only force on this moving ice hockey puck sliding across the ice.

This means the resultant force, remember, that means the overall force acting, there's only one force acting, so the resultant force is just due to friction, 'cause there's no other forces acting horizontally.

So the resultant force is gonna make this puck slow down.

So we know that friction makes objects slow down, 'cause friction acts in the opposite direction to movement.

Let's look at another effect of friction here.

So here, we've got a sledge being pulled along by a rope, and there's also a friction force acting on the bottom of the sledge against that forwards movement.

So that sledge is gonna be speeding up, because the resultant force is forwards, because the force from the rope is bigger than friction.

So the friction force only cancels out a bit of the rope, the rope's force, and there's plenty of the ropes force left over to cause a change, 'cause forces make things change.

So that is a resultant force forwards, because these two forces don't fully cancel out.

Now, here's a sledge being dragged across grass by the same size force from the rope.

Now, this sledge would not be speeding up as quickly, because the friction force is greater.

Greater friction cancels out more of that forwards force, so there's only a smaller forwards force left over, so the resultant force is forwards but smaller, so this sledge on the grass.

It's still speeding up, 'cause there's a resultant force, and resultant force is caused a change in speed, but this sledge is gonna not speed up as quickly, 'cause the resultant force is smaller.

And then finally, this sledge on the rough track, there's a greater friction force now here which means that this sledge isn't speeding up at all, 'cause the friction force is the same size as the pulling force, so those two forces cancel each other out completely as there's no resultant force to cause a change, 'cause friction is the same size as the pulling force, so that stage will just carry on moving across the rough track, maybe bumping along a little bit, but at constant speed, because those forces are balanced.

So friction can actually have different effects.

Friction doesn't just slow things down.

Friction can also stop things speeding up or change the rate at which things speed up depending on the size of the friction force and how well it balances any forward forces.

Let's do another check then about what you think now about the force of frictions.

This is another confidence grid.

There are four statements, for each statement, you've got to decide if you're sure it's right, if you are not sure but you think it's right, if you're not sure but you think it's wrong, or if you're sure that a statement is wrong, then you should tick that box.

So I'm gonna give you five seconds to think through each statement, and come up with what you think, and then we'll go through it.

So five seconds, pause the video if you need to, off you go.

Okay, I'm gonna go through each statement now.

Make sure you've got an answer for each one.

So statement A, friction can slow down moving objects, that is definitely true.

I hope you got that one, well done if you did.

Statement B, friction pushes in the opposite direction to motion, that's definitely true, as well.

Friction always pushes against any motion.

Statement C, friction only acts when there is movement.

Well, that's false, 'cause we've seen situations where there's no movement but friction still acts, 'cause the friction prevents the movement.

So hopefully, you spotted that one was wrong.

And then statement D, the size of a friction force between two surfaces can change.

We saw that on a previous slide, the size of a friction force between two surfaces can change.

So well done if you've got lots of those right.

Let's do another check about where you've got two with your understanding of friction.

In each of these three examples, which arrow shows the direction friction acts? Is it arrow A or arrow B? For the slide, for the car that's braking, and for the climber resting with their feet against that rock face there.

So five seconds to decide for each of those, is A or B, the direction of the friction force? Five seconds, pause the video if you need to, off you go.

Right, I'll go through these now.

For the person on the slide, the force of friction on the person is going to be arrow A.

So we'll get rid of arrow B.

That is the force of friction, 'cause friction acts against movement.

And it's actually the same for the car that's braking, it's gonna be arrow A, so let's get rid of arrow B.

That is the force of friction on those screeching tyres as the car slams on its brakes.

And for the climber resting, again, it is arrow A, which is the friction force, because those the climber's feet would slip down the wall downwards.

So friction acts, friction can act to prevent movement or prevent that slipping.

And if the feet would slip downwards, then friction must act upwards like in arrow A in that situation.

So very well done, especially if you've got that last one.

Time now to do a task putting together everything you've learned so far about friction.

In this task, I would like you to draw a force arrow on each diagram to represent the friction force acting.

The length of the arrow doesn't matter for this task.

And then label the force arrow fully like we label force arrows.

You also need then, once you've drawn the force arrow for each picture to describe and explain the effect of each friction force, 'cause the effect isn't always the same.

This task is really to be designed to be done on the worksheet, which has the diagrams already printed, so you can draw arrows on.

So have a go at doing this task now.

You should pause the video, and have a go at doing all parts of the task for each diagram, off you go.

Well done for your effort with that task.

Let's go through some feedback.

So for the first example, which was a sledge sliding to a stop, you should have drawn a friction force, and may labelled it as the friction force of the ice on the sledge.

You say what provides the force and what the force acts on, the friction force of the ice on the sledge, and the effect is the sledge is gonna slow down.

And you could add a bit more detail.

The effect is that the sledge slows down, because the resultant force on the sledge is backwards.

So it changes backwards, and that's why the sledge is gonna decrease in speed.

Okay, number two, pushing a box but it doesn't move.

You should have added a friction force about there.

The friction force of the floor on the box would be a good label for that.

And what's the effect of that force? The effect of that friction force is to prevent the box from moving.

And how does it do that? Because the friction cancels out the push force, so the resultant force is zero, and the box can't start moving.

Very well done if you have an answer along those lines.

And here's some feedback from number three.

Here is a sledge being pulled along as steady speed.

There's a forward force from the rope.

You should have added a friction force about there, showing where the force acts at the bottom of the sledge.

It should be labelled the friction force of the snow or ice on the sledge.

Doesn't matter whether you've said snow or ice.

The effect of that friction force on the sledge is that the sledge is gonna stay the same speed instead of speeding up, 'cause the force from the rope would make it speed it up if there was no other force acting, but the friction cancels out that force from the rope.

Friction cancels out the pull force, so the result from force is zero and the sledge does not speed up.

It remains at steady speed.

So very well done if you got lots of your answers along the right lines.

You might want to pause the video and add to your answers to improve them.

Very well done for your effort in that first section of the lesson, we're now onto the second section of the lesson, which is about what causes friction.

So the very simple answer to that is that friction is caused by the roughness of surfaces.

How rough or smooth a surface is.

Sandpaper is very rough.

Whereas, that polished marble floor is very smooth.

Friction would be greater if an object was dragged over sandpaper than if an object was dragged over that smooth marble floor.

So it's the roughness of the sandpaper that causes greater friction.

Quick check, which surface looks like it would provide most friction? The paper, the brick, or the wood? Five seconds to decide, off you go.

Let's check, we agree, I think it looks like the brick would cause most friction, because it looks like the brick is the roughest surface to me there.

Let's look in more detail.

The roughness of a surface is caused by irregularities.

Okay, irregularities, and irregularity is something different to what's expected.

So imagine someone was painting lines on a football pitch.

You'd expect those lines to be straight.

If there was a wobble in the line, you could say that there's an irregularity in the line.

The pitch isn't up to standard.

It doesn't meet regulations as an irregularity.

So something different to what's expected, something different to a straight line, irregularities.

And you can see in the picture how the red box has irregularities on its bottom surface, and the blue surface also has irregularities on its surface, kind of little spikes and bumps.

So all surfaces have these irregularities, even surfaces that feel really smooth.

They have irregularities, too.

It's just they'll be so small you can't see them.

They're microscopic.

So here's a microscope image of paper, paper viewed under the microscope.

You can actually see the microscopic fibres that make up the paper.

So paper looks smooth to our eyes, but really, it's got these microscopic irregularities, because of its structure of the small scale, so there's no such thing as a perfectly smooth surface.

All surfaces have irregularities to some degree.

So how do those irregularities cause friction? Well, when one surface moves across another, the irregularities catch on each other.

If you look carefully at that picture, that red box is sliding to the right across the blue surface and you can see how those spiky irregularities could catch on each other.

And when each irregularity catches, there could be a small backwards force, say here, on here, and here, and here, when the irregularities catch on each other.

So that is the friction force.

The friction force is the combined effect of all of those small forces.

We just draw it as one big force.

So it's simpler to represent, but that's where a friction force comes from.

And the same thing happens when the box is pushed.

So I've now, that person's appeared pushing the box.

The irregularities provide backwards forces, that must be overcome to get the box moving.

So that's where friction forces come from.

And that friction force can increase.

When there's a greater surface area in contact, there's been more irregularities colliding at once and more little forces all adding up to make a bigger friction force with a greater surface area.

Or if objects are pressed together more tightly, you'll get a greater friction force.

That's 'cause the irregularities will require greater force to be pushed past each other if they're pushed against each other more tightly.

Okay, time for a quick check.

Have a read through all of these student statements, and identify which of them you are happy are correct.

So Aisha says, "Friction can stop an object from sliding." Laura says, "Bumps on each surface push on each other when you try to slide the box." Izzy says, "When the box slides, the little bumps catch on each other." Jacob says, "Smooth surfaces have microscopic bumps that are too small to see." And Jun says, "Every surface is a little bit rough." So for each of those statements, decide if you agree that they're correct or not.

Pause the video if you need to.

I'll give you some time to do that now.

Okay, I'll go through each in turn.

Aisha is correct, friction can stop an object from sliding.

Laura is correct, bumps on each surface, that she's used the word bumps rather than microscopic irregularities, but I know what she means.

Bumps on each surface, push on each other when you try to slide the box.

Jacob said, "Smooth surfaces have microscopic bumps that are too small to see," that's definitely true.

Jun says, "Every surface is a little bit rough." Yep, that's right because of Jacob.

Jacob has given the reason why that's true.

And Izzy says, "When the box slides, the little bumps catch on each other," and that's true, as well.

So in fact, all of these ideas were correct or on the right lines.

Well done if you agreed with all of them.

Right, so you're ready now to do a task.

Oh, watch that little video of the boat just being pushed across the surface.

One, two, three, off it goes, but then it slows down and stops.

So your task is to explain what caused that toy boat to slow down and stop.

Now, the desk and the lower surface of the boat both seems smooth.

That's shown in the right hand picture there.

So explain what caused this toy boat to slow down and stop.

You should write your answer as bullet points, and try and think of perhaps three or four different things you can say that explains what caused that toy boat to slow down and stop when it was pushed across that desk.

Pause the video now, get your pen, have a go at writing that explanation, off you go.

Okay, I'm gonna give some feedback on your answers.

So here are some example points you could have included in your answers.

Once I've gone through them all, it might be a good idea to pause the video, and make any adjustments or additions to your answer to improve it based on what I've said.

So here are some things you could have said.

A friction force acts on the boat from the desk surface, because there is movement or rubbing between the two surfaces.

The friction force acts backwards, which is in the opposite direction to motion, and there's no forwards force once the boat's been let go.

So the resultant force is backwards, so the boat slows down.

That's why there's a change in speed.

Why does that happen? Why is there a friction force? It's 'cause both surfaces actually have microscopic irregularities that are too small to see.

So when one surface moves across another, the irregularities catch on each other, and when each irregularity catches, there's a small backwards force, and the friction force is the combined effect of all of these small backwards forces.

So very, very well done if you included lots of those ideas.

Pause the video now, and see if you can add to your answer to improve it.

Well done for your efforts so far in the lesson.

Let's do the last section on how friction forces can be reduced.

So smoother surfaces produce less friction, we know that, but what's the reason why? It's because on a smoother surface, the irregularities are smaller.

So have a look at the picture, and just look at the difference between a smoother surface.

So if the red blocks bottom surface was smoother compared to a rougher surface.

If that red block's bottom surface was rougher, the irregularities would be larger.

So a smoother surface versus a rougher surface.

That's gonna cause more friction, 'cause the irregularities will catch more with bigger irregularities.

So the first way you could reduce friction is to make a surface smoother if you can, for example, by sanding it down like in that picture.

Here's another way you could reduce the friction between two surfaces, you could add a lubricant, between the two surfaces.

A lubricant is a liquid added between two surfaces to reduce the friction.

And how does that work? Well, it's not just 'cause liquids are more slippery or liquids can flow.

It's because the lubricant actually pushes or keeps the surfaces slightly further apart.

So there's got to be space for that thin layer of liquid, as well.

So the irregularities are then gonna catch on each other less if the two surfaces are just slightly further apart, 'cause there's a layer, thin layer of lubricant just keeping 'em slightly further apart than they otherwise would be.

So if the irregularities catch on each other less or not at all, then the forces will be less, 'cause there's less catching.

So oils and polishes are examples of lubricants.

Liquids added between two surfaces to reduce friction.

So let's do a check, which of these liquids are acting as a lubricant? Choose as many as you think are acting as lubricants.

I'll give you five seconds to decide.

Let's see how you got on.

So the soap acts as a lubricant.

Adding the soap will help dirt slip off your hand.

So hand soap is acting as a lubricant.

The snowy slash on the road is reducing friction.

It's kind of a liquid which is getting between the two surfaces, the tyre surface and the road reducing friction, so that is acting as a lubricant.

Car engine oil is added into parts of the engine to reduce the friction between moving parts of the engine, so that is a lubricant.

Now, people get engine oil mixed up with petrol.

Petrol's the fuel that's added to the car to burnt, to be burnt, 'cause fuel is a store of energy, which is when it's burnt, that's how the engine works to make the car move.

So it's not to do with reducing friction.

Petrol, but engine oil is, that's two different liquids, can be added to cars.

And bike chain oil, so oil you put on the bike chain, that reduces the friction between the chain as it moves over the cogs of the bike, so it reduces friction between the moving parts.

So four of those were acting as lubricants.

Well done if you identified any of those four.

Another quick check then, which is the best explanation of how lubricants reduce friction? Is it A, lubricants make surfaces slippery? B, lubricants make movement between surfaces smoother? Or C, lubricants keep surfaces slightly further apart, which is the best explanation of how lubricants reduce friction? Five seconds to decide.

Well done if you identified the best explanation is C, lubricants keep surfaces slightly further apart.

Yes, they are also a bit slippery, and yes, they kind of do make the movement smoother, but how do they make the movement smoother? 'Cause they keep the surfaces slightly further apart, so the irregularities catch on each other a bit less, reducing the tiny backwards forces which add up to make the friction force.

Well done if you've got C.

So a good lubricant should do three things.

It should lubricate.

That's means reduce the total backwards force, the total friction force, it should lubricate.

A good lubricant should also stay in place.

It's no good if it just slips off, where you want it to be to reduce the friction.

And a good lubricant should not cause rusting of metal parts, so for example, water is part of the cause or can help speed up the rusting process of metals.

So water is not a good lubricant to use on metal objects.

So oils, which don't cause rusting and they're not linked to rusting are better options to use as lubricants on things like bike chains and car engines.

So lubrication reduces the damage to wear and tear, because the forces between moving parts, the frictional forces between moving parts are smaller, so there's less damage over time.

Well done, so it's time to look at this task.

This task is about a water slide.

It's a water slide in an outdoor playground.

The water is usually turned off, but children can still use the slide.

And on some days in summer, the water is turned on to make it a water slide.

So two parts to this task, they're both written parts, so you'll need a pen and you'll need to write written answers to question one and question two.

Question one, explain why children do not speed up much on the slide when it's dry.

You need to write a written answer to that.

Using bullet points is a good idea.

Use all the ideas from this lesson.

And part two, question two, explain why the slide is faster when the water is turned on.

So you need to write a written answer to that, as well.

So you should pause the video now, and have a go at writing your two answers, off you go.

Well done for your effort on that task.

I'm going to go through some feedback now.

So question one, explain why children do not speed up much on the slide when it's dry.

Here are some example points you could have included.

So hopefully, your answer includes at least some of these points.

Children using the slide do not speed up much, because a friction force will act on them in the opposite direction to their motion.

That's a really simple thing to say.

They don't speed up much, 'cause of friction, and friction acts opposite to the motion, so they don't speed up much.

So the friction force causes their speed to slow, or the friction force, so you could say going into a bit more detail, the friction force reduces the resultant force, so they don't speed up as quickly.

Or another option, the friction force balances the downwards force, so the resultant force is zero, and their speed does not increase.

All of those are valid answers for what friction might do when a child goes down the slide.

And a final bit of explanation you could have added about why friction acts.

Friction acts, because the slide surface and the child's clothes have tiny irregularities that catch causing backwards forces, which add up to create the friction force.

You might wanna pause the video now, and add to your answer anything you think could improve it.

Here's some feedback for question two.

Now, explain why the slide is faster when the water is turned on.

Here are some example points, hopefully, you might have included.

So the water is acting as a lubricant, reducing the friction, get that keyword from the lesson in there, lubricant.

Why does the water act as a lubricant? Because a thin layer of water means the child is slightly further away from the slide surface, so that irregularities on the surfaces catch on each other less reducing the backwards forces that add up to create the friction.

Well done if you included some of those ideas.

And then because the water is there acting as a lubricant, that means the friction force is smaller.

Hopefully, you said something along those lines, and that means the child doesn't slow down as much, or it means the backwards forces are smaller, so the resultant downwards force is greater, and they speed up more quickly.

So very well done if you included lots of those ideas.

Pause the video now to add to your answer.

So well done for completing this lesson on friction.

Here is a summary.

Friction is the force of one surface gripping or rubbing on another.

A friction force always acts in the opposite direction to movement or the opposite direction to attempted movement.

Friction always prevents or slows movement between surfaces.

Friction is caused by the roughness of surfaces, those microscopic irregularities that surfaces have.

And friction forces can be reduced by making surfaces smoother, or by adding a lubricant, which is a liquid that keeps the surfaces slightly further apart, so they catch less.