Lesson video

Hello there, I'm Mr. Forbes, and welcome to this lesson from the "Hidden Forces" unit, which is all about pressure.

Now pressure is the effect of a force acting over an area, and in this lesson you're going to learn how to calculate pressure, and the effects of pressure on objects.

By the end of this lesson you're gonna be able to describe pressure.

And pressure is the effect of a force acting on an area.

So whenever you have a force acts on an object, it produces a pressure, and that pressure depends upon the size of the force and the area as well.

So let's get started with that.

And these are the keywords you need to understand for this lesson, the first of which is pressure.

As I've said, pressure is the effect of a force acting on an area.

Concentrated means there's more of something acting in a certain place, so once something's concentrated, there's more of it acting on an area for when we're doing pressure.

And then the other two are units of pressure: newtons per square centimetre, and newtons per square metre, which is also called pascal.

Both of those are units of pressure.

And here's a set of explanations for those keywords that you can refer back to during the lessons if you need to.

So we're gonna start our look at pressure by imagining pushing downwards on a balloon, a bit like this.

So you place your hand on the top of a balloon and push downwards.

When you push downwards on that balloon, you're gonna create a force on it, and that's gonna produce a pressure acting on the balloon.

And when you squish it, the balloon will change shape, you're putting the pressure on it, and it changes the shape of the balloon, but it's unlikely to explode or pop unless you put a really large force on it.

The force is spread out over a large area so it doesn't create what's called a high pressure.

It's spread out and produces a lower pressure.

And we can see a little video of the effect of that here.

(balloon squeaking) A light force on the balloon is causing it to get squashed, but it doesn't pop.

And as you saw, the balloon didn't pop, even though I put quite a large force on it, because the pressure I was producing on the balloon wasn't high enough to damage it.

Now instead of using a large force, let's use a small area.

You can see what happens quite simply with that animation.

What I'm doing is using a small force concentrated onto the sharp end of a pin, and that pin creates a very high pressure on the balloon surface, and that tears it.

So let's see that again in a bit more detail by watching the video and listening to the sound.

The ends of this skewer have a very small surface area.

(balloon popping) So the small force was concentrated on a very small area, and that caused the material of the balloon to rip, and it popped.

So we've got a high pressure there.

As I mentioned, the effect of the force acting on an area is called pressure.

And the larger the force that acts on the area, the greater the pressure you're going to get.

So I've got two areas here, both the same, and if I put a small force on the left area, then I'm going to generate a low pressure 'cause there's a small force over that large area, giving me a low pressure.

Now using the same area, I can put a larger force acting on it, and that large force acting on that same area will produce a higher pressure.

So I'm getting a pressure based upon the size of the force over the same areas.

If the force is acting on a smaller area, it will give a higher pressure.

So I've got a large area here and a small area.

And if I take an equal force, so let's put the force on both of those areas there, on the large area, the force is less concentrated, it's more spread out, there's a smaller force per square centimetre there, and that's gonna give me a low pressure, whereas in the other side I've got an equal force, but it's now concentrated more on that small area, it's about a third of the size, so it's gonna give me a higher pressure.

So the size of the pressure depends upon the area as well as the size of the force.

Let's check if you've got a good understanding of what pressure is by looking at this question here.

Which of these combinations of force and area will give the highest pressure? Is it diagram A, a large force acting on a large area? B, a small force acting on a large area? C, a large force acting on a small area? Or D, a small force acting on a small area? Pause the video and make your decision, and then restart and we'll check the answer.

Okay, welcome back.

The answer to that was C.

So if you understand pressure, the pressure is greater for a larger force, and it's also greater for a smaller area, so the combination that'll give the largest pressure overall is a large force and a small area.

Well done if you've got that.

As you saw with the balloon, even if you've got a small force, if you can concentrate that into a very small area, you can produce a large pressure, and that large pressure will damage surfaces.

For example, when you've got scissors, you don't use a very large force, but the edges of the scissors are sharp, and have got a very, very small area, and so those sharp blades will have that very small area, and that will produce a very large pressure even for a small force, and so you can quite easily cut through the paper.

By just squeezing your fingers together, you can cut through card and other things as well.

So high pressures can easily damage surfaces.

Anything that's got a point on the end of it has a very small area.

So I've got the edge of a dart here, and the tip of that dart has got an area of much, much less than one square millimetre.

So it's very, very sharp indeed.

When I throw that dart, even though I don't use a large force, that point is gonna easily be able to create a very large pressure into something like a dart board, cutting deeply into it, and so the dart will stick into it like this.

So when the dart flies towards the board, it quite easily goes through its surface.

And if you drop it even onto wood or something like that, it will quite easily go into the surface 'cause it produces a high pressure because of that very small area.

Okay, I'd like to check your understanding of pressure now.

I've got a nail here held by some pliers, and I would like you to answer this.

When a nail is hammered into wood, the pressure on the top surface of the nail, that's the flat bit there, is greater than the pressure at the point.

Is that true or false? So pause the video, and select your answer, and then restart.

Okay, hopefully you selected false for that.

The pressure isn't greater at the top surface than it is at the point.

I'd like you now to justify the answer, so choose one of these two explanations, please.

Is it the force is the same but the area is much smaller at the point, so the force is more concentrated? Or is it because the force is greater at the pointed end of the nail, so this produces a larger pressure? So again, pause the video, make your selection, and then restart.

Welcome back.

Hopefully you selected the first of those options.

The force is the same, you're hitting the top of the nail with a certain size force, and all of that force is passed through the nail to the point.

So the force is the same at both points, but the pointed end has got a smaller area, so the force is more concentrated, and that gives a higher pressure.

Well done if you selected that.

Now sometimes you want to reduce the pressure acting on things, you want to spread it out by using a larger area.

So one example of that is when you're walking on snow, you can see here some normal-sized shoes.

If you walk on snow with normal-sized shoes, you'll sink in it.

And normally that's not a problem if the snow's not too deep.

But if it is deep snow, you don't want to sink into it, you'll use a snowshoe instead.

So these are some snowshoes, and as you can see they have much, much larger areas then normal-sized shoes, eight or nine times the area.

And what that will do is reduce the pressure so you can walk on the snow without sinking into it.

Here's a quick video showing you some people walking on snow and I'd like you to watch it.

And as you can see, they walk very easily on the snow with these large shoes.

Okay, let's check your understanding of pressure on walking on soft surfaces.

And to do that, let's have a look at some camels.

So why don't camels sink into the sand as they walk? Is it A, because they have low weight? Is it B, because their feet have a large surface area? Or is it C, they move so quickly they don't have time to sink? So pause the video, make a selection, and restart.

Okay, hopefully you selected B as the answer, they've got large area on their feet.

You can see that in the picture, but if you watch carefully this video, you'll see as the camels step on the sand, the area of their feet increases, they spread out and so the pressure they exert on the sand is much less.

So let's have a watch of that.

Okay, time for a check of your understanding of pressure now with task A.

You can see here in the picture somebody being rescued from thin ice.

Thin ice is exceptionally dangerous.

If it breaks and you fall beneath it, you're in icy cold water, so you need to be rescued quickly.

But rescue teams take their time to do that 'cause they don't wanna crack the ice themselves.

So during a rescue on thin ice, the rescue team lie flat or use boards to slide across the ice, and you can see in the picture that they're using especially designed raft to do that.

That takes a lot more time than walking across the ice.

So I'd like you to explain why is it safer for the rescue team to lie down and slide across the ice instead of walking on it.

And I'd also like you to use a diagram to do that.

So pause the video, answer those two parts, and then restart.

Welcome back.

Well hopefully your explanation was something like this.

When somebody's lying down, they make a larger area of contact with the ice, and that means that their weight is spread out over a larger area, and the pressure on the ice will be less, so the ice is less likely to break.

Now I did ask you to do a diagram to help with that explanation, so I've one here.

This is somebody standing on the ice, and all of their weight is acting on that one foot, and that will create a high pressure on the ice, and it's likely to break.

But if they're lying down on something like a board, all of their weight is spread over a much larger area, and that gives a lower pressure.

So well done if your answers look anything like that.

Okay, we're gonna move on to the second part of the lesson now, and that's all about how to calculate pressure.

To do this section, you might want to get a calculator.

So once you've got one, let's begin.

So you've already got an understanding of what affects pressure.

When you've got a small area, you can have a high pressure.

When you've got a large force, you get high pressure.

So we're gonna define pressure now, and the pressure is the force acting per each unit of area.

And as an equation, we express that as pressure is equal to force divided by area.

We most often use symbols to represent these things, so in symbols that's p equals F divided by A.

And pressure is measured in newtons per square centimetre here, force is measured in newtons, and area is measured in square centimetres.

So when I've got an area of one square centimetre, and I put a force of one newton on it, I get a pressure of one newton per square centimetre.

Let's have a look at a calculation of pressure now.

So I've got an elephant here, and they're walking across flat ground.

I've got the elephant with a weight of 51,000 newtons, and that's a typical weight for a medium-sized elephant, and each feet, all four of them together have a combined area of 1,200 centimetres squared.

What pressure does that elephant put on the ground? So the stages in that calculation of this, first right up the equation, pressure is force divided by area, then you can identify those two values, force and the area, in the question and substitute them in.

So now we've got the pressure is 51,000 divided by 1,200.

And finally we need to do that calculation, and comes up with 42.

5.

But we've forgotten to put the unit here, so we need to put that in as well.

And this was a number of newtons divided by an area in centimetre squared, so that will give us a unit of newtons per centimetre squared.

We can have a look at another example now.

This is a more typical one.

So we've got a book resting on a table, and it's got a weight of 15 newtons, and a surface area of 300 centimetres squared, it is in contact with the table.

And I want to work out the pressure on the table.

So what I do is I write up the equation just as I did before, pressure equals force divided by area.

I look carefully at the question and identify the force, 15 newtons, and the area, 300 centimetre squared, and put those values into the equation, and then I do the calculation to get 0.

05.

As before, I've gotta put the unit in, it's newtons per centimetre squared just like this.

Now it's time for you to have a go.

I'd like you to do the same procedure with a question.

The same book, so it's got the same weight, is turned so it rests on its edge, and that's got an area of 30 centimetres squared.

I'd like to know what the pressure on the table is now.

So pause the video, follow through the same procedure, and see if you can work out the pressure for me, and then restart.

Okay, the steps you should have taken here, write out the equation, pressure is force divided by area, substitute in the two values, remembering that the force, the weight of the book was still 15 newtons, but the area this time is 30 centimetres squared, so that should give a pressure of 0.

5 newtons per centimetre squared.

So well done if you've got that.

What we're gonna do now is measure the pressure that you can actually produce.

We're gonna do a simple experiment to do that.

So we're going to measure how much pressure your hand can exert or put on a surface.

And to do that we need two things, we need to know the area of your hand, and we need to know what force you can put on it.

So the first thing we're gonna do is to measure the size of the force your hand can put on a surface.

So to do that, the simplest thing to do is to push down on some bathroom scales.

Once you push down on those scales, you can see the force, if the scales are in newtons, or if it's measuring in kilogrammes, you can calculate the force by looking at the number of kilogrammes and multiplying by 10.

So if we watch this quick video, it'll show you how to do that.

Carefully push downwards with the palm of your hand, trying to get a steady reading.

Make sure you don't push too hard.

If your scales are measuring kilogrammes, you need to multiply the value by 10 to get a value in newtons.

And the second thing you need to be able to do is to measure the area of your hand in centimetre squared.

So to do that, what you need to do is get a sheet of paper that's got a grid on it in one centimetre squared, and just draw around the outline of your hand, which is a bit like this.

So let's watch a video of it.

Place the palm of your hand as carefully as you can in the centre of a sheet of paper with one centimetre squares on it, hold it still.

Then draw around the outline of your hand trying to keep your fingers together, and connect the bottom edge from one to the other like this to complete the outline.

Count up the one centimetre squares, numbering a few as you go, so you don't lose count, and go all the way to the bottom of the hand to find a total area.

So using that technique, let's find out what pressure you can put on a surface.

I'd like you to follow these steps, please.

Draw around your hand on a sheet of squared paper with one centimetre squares, add up the squares counting them to find out how much area your hand covers on that sheet of paper.

Push down on some bathroom scales with the flat of your hand and find the force you push with.

Now if the scales are measuring newtons, just use that, but if they're measuring kilogrammes, just multiply that number by 10, and that'll give you the force that you're pushing down with, and then calculate the pressure you produce using the equation pressure equals force divided by area.

So I'd like you to follow those four stages now, please, to find the pressure you can put on a surface pushing downwards with your hand.

So pause the video, do that task, and then restart when you're done.

Okay, welcome back, and this is the results of me trying the experiment.

So there's an outline of my hand, and what I've done is I've counted up those squares, so you should have got a shape like that and counted 'em up.

My hand's got an area of 163 centimetre squared, once I've counted those squares.

The next thing I did is I pushed down on the scales, and I pushed down, well, I was actually trying to push down with a force of 200 newtons, but I got 19.

9 kilogrammes, which is only 199 newtons.

But that'll do.

And then I calculated the pressure that I produced using the area and the force.

So the pressure is the force divided by the area, that was 199 divided by 163, and that meant that I put a pressure of 1.

22 newtons per centimes squared by pushing down gently with my hand.

Your answer will be different, but well done if you followed that procedure.

Okay, we onto the final part of the lesson now, and this is called calculating different pressures.

And in it we're gonna look at how to convert between the different ways we can measure pressure.

So far you've measured pressure in newtons per centimetre squared, but you also need to be able to measure pressure in newtons per metre squared.

So we'll see how to convert between those two units.

So in physics the standard unit of area that we use is the square metre, it's not the square centimetre.

And the standard unit force we use is the newton.

So normally we'd like to calculate pressure using those two standard units of newtons and square metres, and get a pressure based upon that.

So if you imagine I've got a square metre here, and I push down with a force of one newton, what I'm doing is producing a pressure of one newton on every square metre, so that's one newton per square metre.

And normally we measure pressure in that unit of newtons per square metre.

Now that unit is always, sorry, that unit has also got a standard name, and that's called the pascal.

So the pascal is basically exactly the same as one newton per square metre, one pascal, one newtons per square metre.

Let's have a look at calculating pressure based upon square metres and newtons.

So I've got a snowmobile here, and it's got a weight of 2,400 newtons, and it's got a surface area in contact with the ground of 0.

25 square metres there.

What's the pressure it puts on the ground? Now to calculate pressure using these values, I just use the same equation.

So I write down pressure is force divided by area, and I put the force in in newtons, but this time I put the area in square metres.

So the pressure is 2,400 divided by 0.

25.

And if we do that mathematics, I get 9,600.

I've still gotta put the unit there, and as I used newtons and square metres, the pressure this time is in newtons per square metre like that.

So I'd like you to try this example now, please.

I've got the same snowmobile, but this time a person of weight 600 newtons sits on it, how much pressure does it exert on the ground now? So think carefully about that, pause the video, try the calculation, and see what your answer is, then restart.

Okay, the first thing to do is to understand that the force in the scenario has changed now.

We've got the snowmobile's weight of 2,400 newtons, and also the person sitting on it's weight of 600 newtons, so we've got a new force that's 3,000 newtons, the sum of those two are the values.

Then we can just use the same pressure equation as before.

Pressure is force divided by area.

The area of the snowmobile in contact with the ground hasn't changed, so we can use the value from earlier.

So we put the force in as 3,000 newtons, and the area is 0.

25 square metres.

Do the calculation and that gives us a pressure of 12,000 newtons per square metre.

So it's increased 'cause there's a greater weight acting on the same area.

Well done if you've got that.

As I mentioned earlier, sometimes you've gotta be able to convert between the two units of pressure we've seen so far.

So you can measure pressure in newtons per centimetre squared, and pressure in newtons per metre squared.

And we need to be able to convert.

So let's have a look at one square metre.

So this is one metre squared, and you should know that one square metre has got two sides, one side's 100 centimetres long, and the other side 100 centimetres long as well.

So the total area of that in square centimetres is 100 centimetres times 100 centimetres, and that gives one square metre is equal to 10,000 square centimetres.

And that can make it a little bit confusing and difficult to convert between the units because you're not multiplying by 100 or dividing by 100, you're multiplying and dividing by 10,000 each time.

So pressure in newtons per centimetre squared is always much bigger than the same pressure in newton per metre squared.

And the two ways to convert is to convert from newton per square metre to newton per centimetre squared, multiply the pressure by 10,000, and to convert the other way, you've got to divide the pressure by 10,000.

Okay, let's see if you can do one of those conversions for me now, please.

I'd like you to convert a pressure of 40,000 newton per centimetre squared into pascals, or newtons per metre squared.

Is it A, 40,000 pascals, B, 4,000 pascals, C, 4 pascals, or D, 0.

4 pascals.

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

Okay, welcome back.

Your answer to that should be four pascals, and the mathematics behind that, the pressure is 40,000 newtons per centimetre squared, and to convert to newtons per metre squared, you have to divide by 10,000, and that gives four newtons per metre squared, and four newtons per metre squared is four pascals.

Well done for getting that right.

Okay, we've reached the final task now.

So I'd like you to answer this pair of questions for me here.

So I've got an apartment that's got a wooden floor, and wooden floors scratch easily.

So this wooden floor can take a maximum pressure of 80,000 pascals before it becomes damaged.

And I've got a settee, as shown in the picture there, it's got four small feet, each of which has got an area of 25 centimetre squared, and that settee weigh 1,200 newtons.

I'd like you to show me that the settee will damage the floor.

So I want you to work out and prove that the settee will damage the floor if it's just sitting on it normally.

And then I'd like you to to suggest some ways that that settee could be adapted so that it can be used on the floor without damaging it.

So pause the video, and try and work out the solutions to those two questions, and then restart when you wanna see the solutions.

Okay, here's the solution to that.

So let's have a look at part one, show the settee will damage the wooden floor.

What we do, first of all, is calculate the area of the feet.

It's four feet, each of which has got an area of 25 centimetre squared.

So that gives us a total area for the feet of 100 centimetre squared.

Then we can use the pressure equation based upon the force and the area, so pressure is force divided by area, pressure is 1,200, that's the weight of the settee, divided by 100, which is the area of the feet, that gives us a pressure of 12 newtons per centimetre squared.

Now the second part of it was about damage.

And we get damage if the pressure is over 80,000 newtons per metre squared.

Converting the 80,000 newtons per metre squared into newtons per centimetre squared gives a value of eight newtons per centimetre squared.

That's lower than the pressure that's being produced by the feet.

So that settee is gonna damage the floor.

So the second part of that task was how you could adapt the settee so that you could use it on the floor.

And you can see there I popped on the diagram a little closeup of one of the feet of the settee.

And the thing we could do is this, we could decrease the pressure if you could increase the area of contact with the floor.

So that narrow bit of the leg at the bottom touching the floor is the problem.

One thing we could do to do that is putting something underneath it so that you spread the pressure out over the surface.

So if you put something like a plastic or metal disc at the bottom, increasing the area of the feet, I could reduce the pressure it's putting on the floor.

A bit like those people travelling across the ice earlier in the lesson, I'm spreading the force over a greater area, reducing the pressure.

So well done if you came up with that solution.

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

So let's have a look at what you should have learned during it.

A force acting over a surface area produces a pressure.

So when that force acts, it's spread out over an area, and we call that overall effect pressure.

If that force is concentrated on a small area, it gives a high pressure, and the opposite's true as well.

If the force is spread over a larger area, it gives a smaller pressure.

Pressure's calculated using this equation, pressure equals force divided by area, and pressure is usually measured in newtons per square metre.

But it's also sometimes measured in newton per square centimetre if you're dealing with small areas.

So I've got an example here.

I've got an area of one square metre, and a force of five newtons acting on it.

So that's gonna give me a pressure of five newtons per square metre.

Converting that to newtons per centimetre squared gives me a force of 0.

0005 newtons per centimetre squared, I've divided by 10,000 there.

And finally, one newton per square metre is also called one pascal.

So well done for reaching the end of this lesson.

Hopefully I'll see you in the next one as well.

Bye-bye.