Hello there, I'm Mr. Forbes, and welcome to this final lesson from the hidden forces unit.

This lesson's all about pressure at different depths and different heights, and in it we'll be looking at how water causes high pressure and objects as you go beneath the surface, and how air pressure decreases as you get higher and higher.

By the end of this lesson, you'll be able to describe how the pressure in liquid like water can produce forces that act on objects and give us things like upthrust.

You'll also be able to describe how the pressure varies with the depth of the water and how air pressure varies with height.

The key words you need to understand to get the most from the lesson shown here.

The first is upthrust, and that's a force produced by an object resting on the surface of water or beneath water.

It's an upward force produced by liquids.

Pressure.

You looked at in earlier lessons, and that's the effect of a force acting over an area.

Depth is just how deep below the surface of something you are.

And atmospheric pressure is the pressure caused by all the air molecules in your hitting things and producing a pressure on surfaces.

And here's a set of explanations of those keywords that you can return to at any time during the lesson if you get confused.

This lesson's in three parts.

And in the first part, we're gonna look at water pressure and help to calculate forces produced by that water pressure.

And we've been examining things floating on the surface of water to do that.

In the second part of the lesson, we're gonna look at how pressure changes as you go beneath the surface of the water, such as going under the sea.

And we'll see the effect of that pressure on things like submarines and divers.

And in the third part of the lesson, we'll go the opposite direction.

We'll go upwards and see what happens to atmospheric pressure as you get higher and higher.

So if you're ready, let's begin with the first part.

Water pressure.

Right, let's start by looking at a raft floating on the surface of water.

So we've got a stationary raft here, it's not moving left or right, and it's not moving up or down.

So we should know that the forces on it are balanced for that to happen.

We've got the weight of the raft downwards because a gravitational force pulls the raft towards the centre of the Earth, and that raft must then be pushing on the water beneath it.

And that's the weight force.

But there's also another force that must be acting upwards to balance that force out.

And that force is called the upthrust force.

The upthrust force is produced by the water because the raft push pushes downwards and the water pushes back with the same size force.

That's the upthrust.

So let's check if you remember some of the basics about forces acting on an object that's stationary.

So I've got a raft here and it's got a weight of 400 newtons.

What size is the upthrust force keeping the raft floating? And I've drawn that as a purple arrow on the diagram.

So pause the video, hit your selection from A, B, and C, and then restart.

Welcome back.

You should have chosen exactly 400 newtons.

If the force was a different size than that, then they would be unbalanced and the raft wouldn't be floating.

It would either be sinking, or more mysteriously, rising above the surface of the water.

So the force has to be exactly 400 newtons for it to float on the surface if its weight is 400 newtons.

Well done if you got that.

So let's have a look at where the upthrust force is coming from.

upthrust is caused by the pressure of the water acting on that raft.

You looked at pressure in earlier lessons where you found out that when a force acts over an area, it produces a pressure, but the opposite's true as well.

When the pressure acts over an area, it'll produce a force.

So we get an equation for pressure that you may have seen.

Pressure is forced divided by area.

And if we start to rearrange that equation, we can try and get an expression for the force as well.

So I'm going to shift the equation around to move area to the left of it.

So we've got force divided by area is pressure.

Then I'm gonna multiply both sides by area.

And if you look carefully, you'll see I've got divided by area and multiply by area on the left hand side there.

So those two are gonna cancel out.

It's gonna give me a final equation that shows me that the force produced is the pressure times the area.

So the upthrust is going to be the pressure of the water acting on the raft multiplied by the area of the raft.

Right, let's see if you can apply that force equals pressure times the area equation now.

I've got water pushing up on this raft with a pressure of 18 newtons per square metre.

And you can see there's a raft there floating in the water.

And the area of the raft touching the water is four square metres.

So I want you to calculate the size of the upthrust force.

Is it A, 20 newtons? B, 18 newtons? Or C, 320 newtons? So pause the video, try and use the equation to find that out, and then restart.

Okay, welcome back.

Hopefully you chose the bottom of those options.

C, 320 newtons.

I'll show you how we get the answer now.

We write out the equation force is pressure times area, and because the force is the upthrust, we'll use that word and we're substituting the values from the question though.

The pressure's 18 newtons per square metre, and it's four metres squared for the area.

So we take those two values, multiply them together, and we get an upthrust of 320 newtons.

Well done if you got that.

Now there's a limit to the maximum upthrust force you can get on a raft because the pressure at the surface of the water is constant.

And so we are only gonna be able to get an upthrust that depends on that pressure and the size of the raft.

So if I've got a small raft like this, I've got the upthrust and the weight acting on it equal and opposite.

If I put a box on top of that raft like this, then I'm gonna get a larger weight into the water, that's going to have to produce a larger upthrust if the raft is still floating.

But there's a maximum limit to that.

If I put something too heavy on the raft, a car like this, then the weight is going to be so large that I don't get an upthrust large enough to keep the raft afloat.

Because that upthrust has a limit.

And so the raft is gonna sink into the water.

So to avoid that sinking problem, I need to use a larger raft.

The larger the area of the raft, the greater the upthrust that I'm gonna be able to get from it.

The maximum upthrust, that is.

Because the force that can produce the upthrust is gonna be the pressure, which is constant at the surface of water, times the area of the raft.

So I've got a small raft here, and it's gonna have a small maximum upthrust.

If you get a larger raft, I'm gonna have a larger maximum upthrust.

And an even larger raft, we're gonna be able to get a larger upthrust before it sinks.

So let's check if you understand about the upthrust forces and how it's connected to area and pressure.

So I've got these ships, they're all the same weight and they're all the same width, and that's very important.

They're the same width when they're unloaded.

Which one's going to be able to hold the most cargo without sinking? So have a look carefully at the diagrams, pause the video, make your selection, and then restart.

Welcome back.

Hopefully you selected ship A.

And the reason you should have selected that one is because it's got the largest area, because it's longer than the other two ships.

It's got the largest area, and that's going to allow it to have the greatest upthrust.

So the upthrust is greater when you've got a large area.

So A was the right answer.

Well done if you got that.

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

And I've got a table here and I want you to complete it.

So the table shows the weight and the areas of the bottom of some objects floating on the surface of the water.

And I want you to use that pressure equation, it's on earlier slides, to complete the table.

So pause the video, complete the table, and then restart when you're done.

Okay, welcome back.

Let's have a look at how we should have done that.

For the first row of the table, we're looking for pressure.

So we can write up the equation is forced divided by area.

Force of 600 newtons, area of 0.

5 metre squared, 1,200 pascals.

So we fill that into the table there.

And we do the same procedure for the second one.

And that'll give us a value of 30 pascals using the data in that row.

The next one, we need the other version of the equation, force is pressure times area.

So force is 1000 times 2.

5, the area.

2,500 newtons, and that fills in there.

And for the next one, we've got to do a little bit more manipulation because we're looking for the area.

So we can write up the pressure equation.

the force is pressure times area, put in the values, and a bit of manipulation shows the area is 17 square metres.

So we fill that in there.

Well done if you got all four of those.

Okay, we're ready for the second part of the lesson now, and that's about pressure under the sea.

Or any body of water, really.

And we're underneath the water surface.

You can feel a pressure acting on us.

So we're gonna take a look at that.

When you're beneath the surface of the water, there's a pressure that acts on you from all directions.

And divers under the sea feel that pressure pushing in on them.

They can feel it trying to crush them.

Now you may have experienced that pressure yourself a little bit if you go to the bottom of the swimming pool, sitting on the bottom of the swimming pool, you can feel the weight or the pressure of that water pushing on your body.

So as I said, that pressure beneath the sea is caused by the weight of all that water above you.

And when you are deep underwater, there's a huge mass of water above you pushing downwards.

And that will create a very large pressure acting on somebody.

So as you go even a few metres beneath the surface of the water, you feel an incredible pressure pushing inwards on you.

So I've got a diver here a few metres under the surface of the water and there's a column of water directly above them that's going to be pushing downwards on them, creating a very large pressure.

And the reason that pressure is very noticeable is because water is quite dense, and that dense water is gonna have quite a large weight, and that weight is gonna push down on that diver, making it very uncomfortable for them.

So I've got a submarine here deep under the ocean.

And that submarine's going to be under enormous pressure.

What's gonna happen is the weight of the water above it is gonna be pushing down on it, creating a pressure on its top surface, something like this.

So it's spread all over that top surface and very, very large pressure.

But that's not the only direction the pressure's going to act on.

When you're submerged in a liquid, the pressure acts from all directions.

And so there's going to be a pressure as well from beneath the submarine.

And that pressure is again gonna be pushing inwards on the submarine.

And it's not only the top and the bottom, it's the sides and the front and the back.

So the pressure acts in all directions, and that pressure is due to billions and billions and billions of water particles bumping into the surface of that submarine.

Each of those particles causing a small force.

All of those forces add up and create a pressure that's all around the submarine.

And the submarine's gotta be very, very strong to withstand that pressure from all directions.

The pressure caused by water increases with the depth.

So the deeper you go into the water, the more water's above you, and the greater the pressure's going to be.

So if I'm near the surface of the water, like this fish here, there's only a small amount of water above me.

And so the pressure is not particularly high.

But a few centimetres deeper, and I've got an increase in pressure acting on me because there's a greater mass of water above me pushing down.

So I've got a higher pressure there.

And as I go deeper again, there's a higher pressure.

And deeper again and deeper again.

And after only a metre or so, there's a very high pressure acting on me.

So pressure increases with the depth of the water.

So you've seen that the pressure in water increases with depth, but it's the same at a specific depth as well.

So we've got two fish here that are on that purple line.

And because they're at the same depth in water there's gonna be the same volume of water above them and the same mass of water above them.

And that's gonna produce the same downward force on them.

And therefore you're going to get an equal pressure at that depth.

So all along the purple dash line, the pressure's going to be the same.

So both of those fishes are gonna experience the same pressure because there's the same weight of water above them.

But if you look deeper in the pond at the black dotted line, there's going to be a higher pressure there.

So if the fish moves down to that level, the pressure's going to be greater.

And if both of them are there, the pressure's gonna be the same on both of them again.

Okay, let's check if you understand what affects pressure.

I've got three fish in some water here, fish X, Z, and Y, and fish Y is deeper in the water than the other two.

So I would like you to decide which two of these statements is correct.

Is it A, the pressure on all the fish is the same? Is it B, the pressure on fish X is the greatest? Is it C, the pressure on fish Y is the greatest? Or is it D, the pressure on fish X is the same as the pressure on fish Z? So decide which two of those are correct please.

Pause the video, make your selections, and restart.

Okay, welcome back.

Well the two options you should have chosen are these, the pressure on fish y is the greatest, and that's because it's deeper in the water than the other two.

So there's gonna be a higher pressure 'cause there's a greater weight of water above it.

And the other one you should selected was the bottom one, X and Z, those two fish, they're at the same depth.

So the pressure on both of those must be the same.

Well done if you got those two.

The pressure in water is caused by the weight of that water acting on you.

But even when you're not under water, you are actually under quite a large amount of pressure because of the atmosphere above you.

And we can look at that later in the lesson.

But if you are stood near the surface of the water, there is actually a pressure of 100,000 newton per square metre acting on your body.

But that pressure increases very, very quickly as you go beneath the surface of water.

If you go down one metre, it increases to 110,000 newtons per square metre.

So that's quite a large increase in pressure, a 10% increase just by going down one metre.

If you go down a second metre, it increases to 120,000 newtons.

So that increase in pressure is very, very rapid.

You only have to go down 10 metres to double the pressure that's acting on you.

So the pressure increases by 10,000 newton per square metre for every metre you go underwater.

Another check of your understanding of pressure here.

What I'd like you to do is look carefully at the three diagrams of a submarine here.

And I'd like you to just use those diagrams to decide which of those submarines must be at the greatest depth.

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

Welcome back.

Hopefully you chose option C there.

And you should have been able to identify that because the fourth arrows are larger, there's a greater pressure there.

So that submarine must be at the greater depth if there's a greater pressure on it.

A's got the smallest pressure.

And B, somewhere in the middle.

Well done if you selected C.

And another check here, dams are used to hold back water in reservoirs.

And the dams are constructed so they're much thicker at the bottom than they are at the top.

So I've got a diagram of that here.

The dam's holding back a large amount of water.

So it's quite deep on one side, and it's thicker at the bottom than it is at the top.

And I'd like you to explain why a dam is thicker at the bottom than it is at the top to me please.

So pause the video, write out a brief explanation, and then restart.

Okay, welcome back.

Hopefully your explanations are gonna be something like this.

So as you go deeper in the water, the pressure increases, and that means we are going to get larger forces acting on the dam deeper into the water.

There's a higher pressure though, so there's larger forces.

And we write that out like this.

The water pressure at the bottom of the dam is much greater than the pressure at the top.

So that's the first key point.

And the second one is this will produce large forces.

So the dam needs to be stronger and thicker.

So those forces need to be withstood by the dam.

So it needs to be much thicker there at the bottom.

Well done if you got explanation something like that.

So as I mentioned at the beginning of the lesson, water pressure acts all directions.

So this diver is deep underwater and there's gonna be pressure pushing down on them and from the sides and even from the bottom.

So you're getting pressure in all directions pushing inwards and that's gonna make it very hard to breathe underwater.

Because as you are moving your lungs in and out, you're going to have to push against that pressure.

So divers experience some great difficulty in breathing, and that's one of the limits to how deep we can go is can you still breathe at that depth? And that idea leads into this task as well.

I've got some pupils here discussing what it would be like if you're at the bottom of the sea, if you're standing very deep in the ocean.

So we've got four different statements, and I'd like you to explain who is correct and why they're correct.

And you can read through their different opinions there.

And I'd like you to also decide, for those people who are incorrect, how you could correct their answers.

So explain who's correct and give reasons why they are, and what would you say to help those who are incorrect to explain why they're incorrect.

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

Okay, welcome back.

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

So first thing we needed to do was to explain who's correct and why.

So Aisha was correct.

You're going to need to be in a protective vessel if you're very, very deep in the water because it's gonna be impossible to breathe in and out because the pressure's going be gonna be just too high.

So the pressure could be too high to survive in very deep water without being inside the vessel.

The second person who was right was Alex, and that's because the water pressure's going to be pushing the clothes onto her, squashing them on.

And so she's going to feel that those clothes are very tight.

I wonder if you got those two.

And so let's have a look at the two students who had incorrect explanations.

We got Jun here, and the pressure would change as she walks on the bottom.

Well that's not true because the pressure's equal.

So as long as it's not going deeper into the water or higher up in the water, the pressure's going to be the same.

And Lucas said it would be hard to walk.

Well, it's not really hard to walk 'cause of the weight of the water.

It will be difficult to walk because you've got to push through it.

So you're gonna get water resistance, but that's not because of the pressure, that's just the water resistance.

Well done if you got those two.

Right now it's time to move on to the final part of the lesson.

And we're gonna look at atmospheric pressure.

So we saw that as you go deeper into water, the pressure increases.

But we're gonna see what happens as you go higher and higher into the atmosphere.

So let's start with that.

First thing you need to know about the atmosphere is that it reaches up to about 100 kilometres above the surface of the Earth.

Now, there's no exact boundary, because what happens is the air gets thinner and thinner and eventually there's almost no air at all.

And that happens at about 100 kilometres above the surface.

And as that's a nice round number, we say the atmosphere reaches up to about that height.

The air in the atmosphere pushes downwards because it's got a weight, and that causes atmospheric pressure to act on all objects inside it.

Atmospheric pressure decreases with the height.

So as you travel upwards through the air, the pressure gets less and less because there's less air above you.

It's basically the equivalent of rising up through the water where we saw the pressure increases with depth, so it would decrease as you move towards the surface.

So the same thing happens in the atmosphere.

The pressure gets less as you go up in it.

If you are stood at sea level, then the pressure acting on your body is 100,000 newtons per square metre, or 100 kilonewtons per square metre.

So there is quite a large pressure acting on you due to the weight of the air above you.

There's 100 kilometres worth of Earth directly above your head if you're at sea level.

If you climb higher in the atmosphere, there's less air above your head and the pressure falls.

If you move up just nine kilometres, by climbing Mount Everest for example, the pressure will drop to about a third of that, 34 kilonewtons per square metre.

And if you go a bit higher, say in an aeroplane, the pressure drops again.

So at 10 kilometres, the pressure's only 26 kilonewtons per square metre.

So pressure's decreasing with height as you go higher in the air.

Okay, I've got quite a challenging question for you here now.

I'd like you to imagine being in an aeroplane.

So we've got Lucas in an aeroplane here and he notices that his bag of crisps, when it's unopened, puffs up.

So he's taken off and the bag of crisps has puffed up and it's a bit like a balloon.

When he opens them, he hears a pop.

And he's trying to explain what's happening.

And what I'd like you to do is to fill in the blanks in his explanation he's got there.

So I'd like you to pause the video, read what he said, try and fill in those blanks, and restart please.

Okay, welcome back.

Lucas's explanation should have been something like this.

The pressure inside the sealed bag of crisps was greater than the pressure in the aeroplane.

So the pressure inside that bag was higher, and that's why it was sort of pushing outwards.

The pressure in the bag didn't change, so the pressure in the aeroplane must have decreased.

So as he took off, the aeroplane has gone higher into the sky and so the pressure has decreased on the aeroplane.

And when he opened the crisps, he had air rushing out of that bag.

And that's what made the popping sound, the sudden rush of air as he opened the bag.

Well done if you got an explanation like that.

The pressure changes you get as you change height and air are much smaller than the pressure changes you'd get if you move through the same sort of height in water.

And that's because air is not very dense.

I've got 1000 metres of air here, and if I rise up that level, I've got a pressure change of 10,000 newtons per square metre.

But to get the same pressure change in water, I only have to move through one metre of water, as we saw earlier.

So pressure changes in air are much more gradual than pressure changes in water because air is much less dense than water is.

I mentioned earlier that you are under pressure right now just being close to sea level.

There's a pressure of 100 kilonewtons per square metre acting on you, and that's quite a large pressure.

Now if I've got a can like this, I've got air pressure acting on the outside of it, pushing inwards with that very, very high pressure.

But that can doesn't get crushed by the air pressure because there's a pressure acting outwards as well.

And that outward pressure balances the inwards pressure.

So the can's perfectly fine, it's not getting crushed.

And the same thing's happening with your body.

There's a huge pressure pushing inwards on you, but your body pushes out with an equal pressure.

And so all of that's balanced and nothing happens.

But if I could remove the air from inside the can, then you'd see the effect of that air pressure.

So let's watch a video where I use a vacuum pump to suck out all the air from the can and see what happens to the can.

(vacuum whirring) So you saw in the video the effect of that air pressure.

It was very large pressure pushing inwards on the can.

There was no pressure inside the can pushing outwards to balance that off.

And so the can was completely crushed.

So air pressure can be very, very damaging.

There's another way of demonstrating air pressure, and this is what you can try yourself at home.

I can take a glass of water, completely fill that glass with water and then place a piece of card over it and then turn it upside down.

And normally you'd expect the weight of the water to cause that card to be pushed off and the water would fall out and make a mess everywhere.

But the pressure from the air is great enough to push upwards with an equal size force to the weight of the water, and that water will stay in the glass if you do it correctly.

So we're gonna watch a video of that happen now and see if the water stays in the glass.

Okay, that was successful.

And you can try the same thing yourself if you like, but be careful and don't make a mess.

Okay, I've got the final task of the lesson here, and it's all about a weather balloon.

Now a weather balloon is something you feel with helium gas and it rises up into the atmosphere carrying sensors with it to detect what's happening with the weather and the wind and things like that.

So a weather balloon is filled with helium gas at sea level and it's sealed and then it's released.

The balloon rises high into the atmosphere.

What I'd like you to do is to explain why the balloon expands.

And you can see I've drawn the balloon at different heights there, and it's expanding as it rises in the atmosphere.

And I'd like you to use those diagrams to help you through your explanation.

So pause the video, work out an explanation, and then restart please.

Okay, welcome back.

Let's have a look at the explanations for that here.

So your answer should have been something like this.

As the balloon rises up, the pressure outside the balloon gets less and less, but the pressure inside the balloon doesn't decrease by as much.

So you have a larger pressure inside the balloon than outside the balloon as it's going higher.

And that means that that extra pressure can push outwards, and that's gonna cause the balloon to stretch to expand.

So that's why it gets bigger.

If you use some sorts of force diagram arrows, then you could have drawn something like this showing that the pressure outside the balloon is decreasing by making the force's arrows there a bit small on it.

So well done if you got explanation something like that.

Okay, we're nearly at the end of the lesson now, so let's have a quick summary of all the information we've covered.

Water pressure causes an upthrust that pushes up on objects on the surface of the water.

So you can see in the diagram there, got a little raft floating on the surface of the water, and there's an upthrust that acts on it.

And that upthrust is because the water produces the pressure on the bottom of the raft, and that pressure produces a force.

The larger the surface area of an object that the water's pushing on, the bigger the upthrust.

So a bigger raft would be able to provide a large upthrust when necessary.

Pressure in water increases with depth.

So as you go deeper underwater, you get a higher pressure.

And atmospheric pressure decreases with height.

And you can picture that as increasing with depth.

The deeper you go into the atmosphere, the higher the pressure.

Okay, that's the end of the lesson.

Well done for reaching it.

And I'll see you in the next one.

Bye bye.