Hello there, my name's Mr. Forbes and I'll be leading you through this lesson on streamlining, which is part of the moving by force unit.

By the end of this lesson, you're gonna be able to explain what streamlining is and how it reduces the drag force acting on objects moving through things like air or water.

And these are the keywords that'll help you understand the lesson.

The first of 'em is drag, the force on an object moving through a fluid, like a liquid or a gas.

Air resistance, which is the drag force when you're moving through air.

Streamlined, which is altering the shape of an object so it moves through the air more easily.

And water resistance, which is the drag force acting on an object moving in water.

And here's an explanation of those keywords.

You can return to the slide at any point if you need some clarification.

The lesson today's in three parts.

In the first part, we're gonna try and see what drag forces are and what causes them.

In the second part, we're gonna see how we can reduce the drag forces by streamlining and that allows objects to move quicker through the air more easily.

And in the third part, we're gonna move on and look at the difference between the drag forces in air and in water.

So let's go on with the first part, which is all about drag forces.

Now, you've probably been on a rollercoaster, or some other kind of fast-moving fairground ride before, and you've ended up looking something like this.

What's happening here is you're moving so fast through the air that the air particles are producing a noticeable drag force on your body that's pushing back your hair and pushing back the skin on your face.

The reason there's a drag force acting on you when you move through the air is because the air's made up of tiny particles, something like this.

Air particles are much, much smaller than this, so small that they're invisible, but they're still there.

And as you move through them, you've gotta push them out the way.

And although each of those particles is very small and you only need a very small force to push it out of the way, you've gotta push an awful lot of them.

When you push them with the tiny forces needed, those particles are gonna push back on you.

And that's the origin of the drag force.

Drag force that acts on you when you're moving through the air is called air resistance.

And the amount of air resistance you experience depends on how fast you're going.

When you're moving faster, you get more air resistance.

So you can imagine yourself riding on a bike at a low speed.

When you're at a low speed, you're only pushing a small amount of air out the way every second.

And so you get a small amount of air resistance 'cause you're not pushing that many particles.

When you travel faster, you've gotta push more air out the way and that means there's gonna be a higher air resistance because you're pushing more and more particles every second out of the way.

And also, those particles are gonna have to be pushed harder 'cause they're moving faster relative to you.

And so you get higher force each time you're pushing the particle out the way.

Those two factors add together and give you a higher air resistance at higher speed.

It's time for our first check now.

Why does air resistance decrease as you slow down at the end of a rollercoaster ride? And I'd like you to pick two reasons from this list.

Is it the air particles get smaller, there are fewer air particles being pushed out of the way every second? Each collision is at a lower speed, so produces a smaller force or the force is more spread out on your face.

I'd like you to pause the video, select two reasons and then restart.

Okay, welcome back.

The two reasons you should have chosen are these two.

There are fewer air particles being pushed out of the way each second.

Because you're moving through the air slower, there's less particles.

The second reason is each collision's at a lower speed, producing a smaller force.

So congratulations if you got both of those right.

I've got a second check for you here, which is about using force diagrams to decide when an object's moving fastest.

The figures show the air resistance acting on moving cars.

In which of those figures is the car moving fastest? So look at the diagrams carefully, use the information on them to help you decide.

Pause the video, make a selection, and then restart when you're ready.

Right, the correct answer there was B.

You can tell that 'cause the force arrow is largest there, the air resistance is much larger than the other two.

So the car must be moving quickly through the air.

So, so far we've seen what happened when you move through the air, but you get similar effects when the air moves past you.

Moving air causes a force on stationary objects.

You've probably seen that when the wind blows like this.

The wind is blowing against those trees, causing a force to act on them in this direction.

Similarly, you'll see wind turbines spinning around because the air is blowing past them and causing a force on their blades and that'll generate some electricity for us.

The force of moving air can be so large, it can lift things.

Let's have a look at a video of some indoor skydiving to see the effect.

(upbeat music) In the video, you saw the force, it was so large it could lift up a couple of people.

When you're moving very quickly through the air or the air is moving very quickly past you, you can generate large forces.

Okay, we're up for the first task now and it's all about understanding how particles of air cause drag forces.

Some pupils are using a ball pool to help understand drag.

They're thinking about moving through the air and they want to explain why drag gets bigger when you walk faster through the pool.

What I'd like you to decide is what do the coloured balls represent? What does a person do to the balls as they walk along? What do the balls do to the person who's walking? And if the person walks faster, the drag force on them increases.

How can you use the model to explain what happens and why that is? I'd like you to pause the video, spend a little time answering those and then restart when you're ready.

Welcome back.

Let's work through those answers.

So for the first one, what do the coloured balls represent? Well, each of those balls represents an air particle.

So the whole ball pool represents a large number of air particles.

The second thing, what does the person do to the balls? Well, as they walk along, they've gotta push them out the way.

In order to move forward, you've gotta push those balls out the way.

The third, what do the balls do to the person? Well, if you push the balls, they're gonna push back against the person.

So the answer there is the balls push back on the person with the same sort of force in the opposite direction.

Now, how can the model be used to explain why drag increases? Well, if the person walks faster, they're pushing more balls out of the way each second and they're having to use a larger force to push those balls out of the way and that means that the balls are gonna push back on a person with a great force.

And so that represents a larger drag force acting on them.

Well done if you've got that.

Okay, we're gonna be moving on to the second part of the lesson now, which is all about how you can reduce drag by streamlining and that allows us to make things move faster through the air or use smaller forces to be able to move through the air.

If you've seen some very old films, you've probably seen some cars that look a little bit like this.

These are the very earliest designs and they moved at very low speeds so they didn't need to be shaped so they could move through the air very quickly at all.

More modern cars look something like this and they travel at much higher speeds and they've gotta be specially designed so that when they move through the air, the air flows past them more easily.

Otherwise you'd need huge forces to keep them moving.

Those cars are described as being streamlined and they're streamlined to reduce the drag forces acting on them.

So you can see the shapes, they're smooth curves.

We use streamlining to reduce the drag force on moving objects.

The streamlined shapes are smooth and narrow and they're designed to allow objects to move through the air more easily.

So you can see an example like this, this train.

The train is sleek, narrow, and it's got that smooth curve on the front to allow it to move through the air more easily because it doesn't have to push as much air out the way.

That allows it to reach a much higher speed.

As you've seen already, streamlined objects are often curved.

Curved shapes allow the air to flow over them instead of having to push directly backwards out the way, you know, will just slide past more easily.

They're often shapes like this and the airflow will go over the top or to the sides instead of having to be pushed directly backwards.

So curved shapes are the best for moving through air.

So a quick check now.

I'd like you to think about what you've learned so far and look at this cyclist.

How is that cyclist making themselves more streamlined? I'd like you to pause the video, write out three or four things that you see in the figure and then restart.

Welcome back, let's have a look at some of the examples you could have chosen.

So first of all, the cycling helmet.

That's made of a hard plastic and it allows the air to flow to the sides or over the top because of its smooth curved shape.

The second thing the cyclist is doing is they're bunching up into a narrow shape and that means they are thinner and go through the air more easily.

The third feature is they're wearing tight clothes, they're wearing Lycra and that clings to the skin and allows the air to flow past it easily without any crumples or ripples or things like that in it.

And finally, they're in a low posture, they've hunched down so that they're reducing their shape again to be the smallest possible to fit through the air.

So well done if you've got three or four of those.

As you saw, cyclists can adjust some of their features so that they can move through the air more quickly, and racing cyclist wants to do that as much as possible.

So they can do things like using a very thin, narrow bicycle frame and that'll cut through the air without causing too much air resistance.

They can wear cycling helmets that allow the air to flow smoothly across the top of them, or this helmet here's got some holes in to allow the head to stay cool as well.

And they can also wear tight-fitting clothes and that allows the air to flow past them more easily.

Putting all those features together will allow them to reach higher top speeds.

Okay, check to see if you understand that.

I'd like you to answer this true or false question.

Racing cyclists increase their drag by wearing streamlined helmets.

Is that true or false? And I'd also like you to justify your answer using one of the two options.

Streamlining increases drag forces at high speeds or streamlining allows air to flow more easily over the helmet.

Pause the video, make your two selections and then restart.

Welcome back.

You should have realised that was false.

Racing cyclists want to decrease their drag by streamlined helmets.

So the answer you should have given for the justification is streamlining allows the air to flow more easily over the helmet.

Well done if you got both of those.

We've seen that modern cars are streamlined to reduce drag and you can see an example of a modern family car here.

It's got a curved front, it's fairly narrow and you can also see it's curved a little bit at the back as well.

All those features help with the streamlining and help make it move through the air more smoothly.

If you've got a racing car or a sports car, you might have something a bit more like this.

This has got extreme levels of streamlining.

It's very, very sleek.

It's very low level.

You can see you're not gonna sit in it comfortably because it's got a very low roof and that feature of the low roof allows the air to flow past it even better.

Right, to check if you understood that, I'd like you to look at this sports car.

It's an old classic.

I'd like you to try and identify the features that make it more streamlined.

So pause the video, find three or four features and then restart and we'll go through them.

Okay, let's have a look at features that you should have spotted.

Let's go through about four of them.

First of all, it's smooth and curved.

It's even more curved than that Lamborghini from the previous slide.

You've got smooth curves to the side and that will allow the air to flow past those much more easily.

It's got a very narrow front and this car is very small, and so it's not gonna be pushing as much air out the way.

It's got retractable headlights, something you don't see very often anymore, but those headlights would pop up at night, reducing the streamlining.

But when you want to go faster during the day, those can be lowered and that will make it more sleek.

And finally, it's very, very low indeed.

This car is very difficult to sit in because you're so low down inside it.

So well done if you spotted those four features.

It's not only sports cars that need to be streamlined.

Even lorries do.

These lorries are very tall and very wide, and so they'd have a huge amount of drag when they're moving through the air.

They've gotta push a huge amount of air out the way.

If you look carefully at the diagram, you can see there's one feature that allows them to move through the air a little bit more easily, and that's the curve at the top.

That curve directs the air over the cargo, reducing the overall drag, making the lorry be able to travel a little bit more smoothly.

In the examples we've seen so far, we've tried to decrease the drag force on something to allow it to move faster but that's not always the case.

Sometimes we want to increase the drag forces to slow something down.

A parachute is a great example of that.

It allows a drag force to increase massively and slow something down.

So if you're flying a jet plane and you you need to land it in a short distance, you can open a parachute and that parachute will increase the drag and allow the plane to stop very quickly.

Similarly, if something's falling from space, like this space capsule, we can open a parachute and slow it down just before it reaches the ground.

It still hits you at quite a speed, but it's much less than if it didn't have a parachute.

Okay, a test of your understanding of whether you understand drag forces now.

Which two features of this sports utility vehicle would cause it to have more drag than a normal car moving at the same speed? I'd like you to select two from the list.

It's heavier, it's wider, it's taller, it has a larger engine.

So pause the video, select two, and then restart please.

Hello again, and hopefully you selected these two features.

It's wider and it's taller.

Both of those features would increase the drag on it.

It being heavier and having a larger engine, both of those facts are true, but they don't affect the drag force on the car.

So one done if you've got the right two.

Okay, we're up to the task now.

We're gonna check if you understood streamlining properly.

I've got four cycling helmets shown below, A, B, C, and D.

First one, smooth, soft plastic.

B is smooth, hard plastic.

C is woven cloth.

And D is hard plastic with some air gaps for ventilation.

I want you to state which of those helmets produces the smallest drag.

And I'd like to explain why the other three helmets produce a larger drag.

Okay, so pause the video, answer those two questions and restart when you're ready.

Okay, welcome back.

You should have chosen A.

That'll produce the smallest drag force.

It's a small, soft plastic helmet.

And so the reasons now.

Helmet B is taller and it's got a flatter front and that will cause it to push more air out of the way compared to helmet A.

So there's a larger drag.

C has got a rougher texture and that'll trap air, not allowing it to move to the side easily, and therefore, it'll give it bigger drag as well.

And helmet D has got those ventilation gaps and that's gonna allow air in, gonna trap the air, gonna make it more difficult to move through it.

So that's gonna create a larger drag as well.

Okay, we're gonna move on to the final part of the lesson now, and that's about comparing drag in air and drag in water.

The drag force you have when you're moving through water is called water resistance.

And when you're moving at the same speed, you get much higher water resistance than you do on air resistance.

And that's because of this.

If you look carefully, you can see the air particles and the water particles there.

The water particles are much, much closer together than the air particles.

So if you imagine trying to move through them, there's many, many more of them you'll need to shove out the way and that's gonna create a much larger drag force.

And there's a second reason as well.

Water particles are held together by some small forces and you've gotta overcome those forces as well.

So you need to move more water particles out the way and you've gotta overcome those small extra forces and that creates a larger drag force.

Okay, a check of that now.

I'd like you to decide why water resistance is greater than air resistance for objects moving at the same speed.

And I'd like you to pick two reasons.

Is it because air particles are further apart than water particles, so fewer to be pushed out the way? Is it because air particles are moving about more quickly and get out the way easier? Or is it because water particles are forces between them, pulling them together? So pause the video, select two, and restart.

Okay, the two correct answers there were A and C.

Well done if you got those two.

Now, because air resistance is much smaller than water resistance, a ball will fall faster through the air, it'll fall through water, so the ball will take longer to fall through the water than it takes to fall through the air.

I've got an example here.

I've got two balls, same size falling through air and water.

They've both got the same gravitational force acting on them, so they pull down on the same size force, but the one in water has a much larger drag than the one in the air.

And that means you've got a smaller resultant force acting on the ball moving through the water.

That small resultant force will mean the ball will speed up more slowly and won't reach the same speed.

Okay, to check if you've understood that, I'd like you to have a look at this question.

A ball has been dropped in three different liquids and the position of the ball 0.

1 seconds after being released is shown in all three.

So you can see on there A, B, and C.

I'd like you to decide which liquid produced the biggest drag, please.

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

Welcome back.

You should have selected B.

The reason for that is although the gravitational force on all three is the same, the drag forces are different.

And so that gives different resultant forces.

And the resultant force on B is smaller and that means that the object is gonna end up moving slower.

Well done if you've got that.

The maximum speed you can travel in water is actually much, much slower than the maximum speed you can travel in air and the reason for that is because the drag force in the water is much, much higher.

Fast boats like the speedboat here need to produce very large forces and they can't get up to the same sorts of speeds as a car travelling through the air.

I've got a table here of the speed records to compare.

The land speed record for a car, 1,200 kilometres an hour, although admittedly that was rocket powered.

The surface water speed again for the fastest boat, much smaller, 500 kilometres an hour, and if you were completely submerged under the water, so you have to push through it all, you've got a top speed of something like 80 kilometres an hour, much, much smaller.

And that's because you've got a much larger drag force inside the water than you've got when you're just in air.

Okay, we've reached the final task of the lesson now and it's all about world records.

The world record for 100-meter sprint is less than 10 seconds, but the world record for a 100-meter swim is 66 seconds roughly.

I'd like you to use the idea of drag forces to explain why those records are so very different than each other.

So I'd like you to pause the video, write out an explanation for that, and then restart when you've got one.

Okay, welcome back.

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

Your explanation could have included things like this.

The drag force in the water is much, much larger than the drag force in the air, and the swimmer's gonna have to produce a very large driving force to be able to move through the water.

And even if the swimmer can produce the same size force as the sprinter, they're not gonna be able to get up to the same speed because that drag force is gonna be working against them much harder.

So they're gonna go at much slower speed and that's why they take six times as long to travel the 100 metres.

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 quick summary.

An object moving through a liquid or a gas will feel a drag force.

If you're in air, that's called air resistance.

If you're in water, it's called water resistance.

And for both of those, the faster you go, the greater the drag force is going to be.

We use streamlining to reduce the drag force on moving vehicles, and that allows 'em to move faster or more easily through the air.

The reason streamlining works is because the shapes push less air or water out the way.

Instead, they push the air to the side or above instead of directly backwards.

Well done in reaching the end of the lesson.

Hopefully you'll see me again in the next one.

Bye for now.