Lesson video

Hello there, I'm Mr. Forbes, and welcome to this lesson from the particle explanations of density and pressure unit.

This lesson's called "Upthrust," and it's all about the force that allows objects to float in fluids.

By the end of this lesson, you're going to be able to describe how pressure changes with depth in a fluid and how those differences in pressure at different depths cause an upthrust to act on an object inside a fluid.

The keywords you need to help you through this lesson are shown here.

The first is pressure, and that's produced by the forces from particles within a fluid acting on each other or on a surface.

Particles, which is the term we use for atoms or molecules that make up matter.

Upthrust, which is the force acting upwards on an object that's in a liquid or on a fluid due to pressure differences between the top and bottom surfaces.

And density, which is the mass per unit volume of a substance.

And we should know that a less dense substance will float on top of a more dense substance as well.

The lesson's in three parts.

And in the first part, we'll look at the differences in pressure at different depths in a fluid and why that happens.

In the second part of the lesson, we'll look at the upthrust, which is the force that will act on an object submerged in a fluid, and why that's caused by differences in pressure between its surfaces.

And in the final part, we'll look at the different factors that affect the floating and sinking of an object and its density in terms of the types of particles that are in it and there's spacing between them.

So when you're ready, let's start by looking at pressure and depth.

The pressure underwater is caused by the weight of the water acting downwards on objects submerged in it.

So if I've got a submarine here, there's going to be a force of the water acting downwards.

The weight of all that water's gonna act on that submarine and it's gonna create pressure.

Because it's quite high up in the water there, it's gonna be lower pressure.

But as the submarine goes deeper in the water, there's going to be more water and more weight of water above it.

That's gonna generate a higher pressure on the submarine.

The extra weight is increasing the pressure on that submarine.

We can demonstrate that idea with a can full of water.

So we start with an empty can and then fill it to the top with water, and there's a small hole in the side of the can.

And what's gonna happen is fairly obvious.

The water is going to escape through that hole.

So when the can is filled with the water, we're gonna get water jetting out.

And it's gonna go quite far because there's gonna be a high pressure 'cause there's quite a lot of water above the level of that hole.

But as that can slowly empties and the water level decreases, then the water's not gonna fly out at the same speed.

It's gonna be a bit slower because the pressure's lower now.

There's less water above the hole.

So the weight of the water above the hole is decreased, the pressures decreased, and the water doesn't travel as far.

Let's see if you understand that idea.

I've got three identical cans here, but you can't see into them to see the water level.

And they've each got a small hole on the side at the same height.

In which of those three cans is the water level deepest? So pause the video, make your decision and restart, please.

Welcome back.

Hopefully you selected c.

The water's going out the furthest here, and the pressure increases with depth so that water's gone out furthest because the pressure is greatest in that can, so that must mean that the water depth must be greatest in that can as well.

Well done if you've got that.

Now the reason the water pressure increases with depth is because there's a greater weight of water pushing down.

And at greater depth, the particles are gonna be pushed closer together by that greater force.

So there's gonna be bigger forces between them.

So let's say I've got some particles in shallow water.

These particles are very close together, but there's still some gaps between them.

But if I go deeper into the water, deeper below the surface, the forces are gonna be greater, the particles are gonna be compacted or pushed closer together, so we've got a higher pressure in all directions deeper in the water.

So in the shallow water, I've got fairly small forces between the particles.

They're not pushed together as much.

But deeper in the water, I've got larger forces, so a greater pressure, and that pressure acts in all directions on all the particles.

So a question for you now.

Which diagram most accurately shows the direction of pressure at a pointing in a fluid? So I've got three possible answers there.

I'd like you to select one.

Pause the video, make your selection and restart, please.

Welcome back.

You should have selected answer c.

And that's because pressure acts equally in all directions.

In the other two diagrams, the pressure just seems to be acting in one direction, which is incorrect.

So well done if you've got that one.

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

And I've got a single question in two parts here for you.

I've got a diver working underwater at a depth of 20 metres trying to fix a broken pipeline.

Describe the cause of the high pressure acting on the diver, and then b, explain what would happen to the pressure if they swam even deeper into the water, please.

So pause the video, write out your two answers, and then restart, please.

Welcome back.

Hopefully your descriptions are like this.

The cause of the high pressure is water particles are moving around pushing against the diver.

The force of the particles on the diver are spread over an area, and that causes the pressure on him.

And the pressure is high because there's a great weight of water acting.

And for part b, the pressure will increase because there's a greater weight of water acting downwards surrounding the diver.

Well done if you've got answers something like that.

And now it's time to move on to the second part of the lesson.

And in this one, we're going to explain why there's an upthrust force acting on an object that's in water.

So let's get on with that.

If you try and push a ball full of air underwater, you'll feel that there's an upward force acting on it, pushing on it, and bringing it back to the surface when you let go.

And that's because when you submerge a ball in water, there'll be pressure acting on it in all directions, and that pressure will act inwards on the ball.

But as we'll see, that pressure is not equal in every direction.

And so the pressure on a submerged ball is not equal in all the directions.

If we look at the pressure on the sides of the ball here, the pressure is equal on the two sides, and that's because they're at the same depth within the water.

So those two pressures act on each other, and they sort of cancel each other out, but the ball is being compressed by those.

But at the top of the ball, that's higher up in the water, so there's actually gonna be a lower pressure there because it's at less depth.

So I've got a lower pressure there, but at the bottom of the ball that's deeper in the water where the pressure's higher, so we've got a higher pressure on the ball.

So overall, those pressure differences are going to produce a force that acts upwards because there's a higher pressure at the bottom of the ball than at the top of the ball.

The resultant force pushing upwards due to those pressure difference is called the upthrust.

So we've got the two forces on here.

I've got the upthrust acting upwards to accelerate the ball upwards, and I've got the weight of the ball acting downwards.

And if we find the resultant of those two forces, it allows us to find out which way the ball is going to accelerate.

Is it gonna sink downwards or is it gonna float upwards? That depends on which is bigger, upthrust or weight, or if they're equal.

So let's see if you can decide which way an object's going to accelerate when it's underwater.

I've got three balls here all underwater, and I've marked the size of the upthrust and the weight for each of them.

I want you to decide if the ball will accelerate upwards, downwards, or not at all.

So pause the video, make your decisions for each of the ball, and then restart, please.

Welcome back.

So going through each of them, the first one, that's going to accelerate upwards because the upwards force, the upthrust, is greater than the weight.

The second ball is not going to accelerate at all.

The upthrust and the weight are equal, so there's no acceleration.

And then the final one, well, that's gonna accelerate downwards.

It's going to sink because the weight is greater than the upthrust.

Well done if you've got those three.

The size of the acceleration can be found using Newton's Second Law of motion, which is this.

The acceleration is the resultant force divided by the mass, or if we use symbols, a equals F over m where acceleration, a, is measured in metres per second squared, force is measured in newtons, that's symbol F, and mass, m, is measured in kilogrammes.

So we're gonna try and use that expression to calculate some accelerations now.

So I've got a ball of mass 0.

5 kilogrammes and a weight of 5 newtons, and there's an upthrust force of 7 newtons acting on it.

I'm going to calculate the vertical acceleration of the ball.

So the first thing I do is I find the resultant force.

And the resultant force here is me taking one force from the other, and that gives me a force of 2.

0 newtons acting upwards because the upwards force is slightly larger than the downwards force.

Now I can use that to find the acceleration.

The acceleration is force divided by mass.

Put those numbers in, 2 newtons, remember I'm using the resultant force, and there's the mass, 0.

5 kilogrammes.

And that gives me an acceleration of 4.

0 metres per second squared.

And that's in the upwards direction.

Now it's your turn.

I'd like you to calculate the vertical acceleration of the brick.

So we've got a brick of mass 2 kilogrammes, it's got a weight of 20 newtons, and an upthrust force of 8 newtons acting on it.

So pause the video, calculate the acceleration, and then restart, please.

Welcome back.

Well, the resultant force here is found to be minus 12 newtons.

I've got a force acting downwards.

And I can then use that force to calculate the acceleration.

And it's minus 6.

0 metres per second squared.

I've got a force, sorry, an acceleration, 6.

0 metres per second downwards, and that's what the minus sign indicated.

Well done if you've got that.

Upthrust forces don't just act on objects within the water, they'll act on an object floating on the surface of the water as well.

So I've got a raft here, and I've got its weight and its upthrust.

The pressure of the water on the bottom of the raft is greater than the pressure of the air on the top.

And what that's going to do is allow the weight of the raft on the water be equal to the upthrust provided that the water pressure is acting on the bottom.

Now a question for you.

I've got a boat of weight 50,000 newtons, and it's floating on the surface of a lake.

What size is the upthrust acting on it? So pause the video, make your selection from those three, and restart, please.

Welcome back.

It should be exactly 50,000 newtons.

The boat's floating, so there's no resultant force on it.

It's not rising up and it's not sinking downwards, so the weight and the upthrust must be equal in size.

Okay, it's time for the second task.

And what I'd like you to do is to answer these two questions.

I'd like you to explain why it's difficult to hold a beach ball under water.

So if you've ever tried it, it's very, very difficult.

And for part two, I'd like you to look at that submarine.

It's travelling forwards at a constant depth under the sea surface, and you've got its weight there.

I'd like you to state the size of the upthrust force and explain the cause of the upthrust force on the submarine as well, please.

So pause the video, answer those questions, and restart.

Welcome back.

Well, let's have a look at the first one.

Your explanation should be something about a very large upthrust force acting on a ball when it's underwater due to those pressure differences at the top and the bottom of the ball.

And for the second one, if you stated the size of the upthrust, it should be equal to the weight.

So it's exactly the same value, 2 million newtons, and the cause of the upthrust force is the difference in pressure between the top and bottom surface of the submarine.

And that gives a resultant force that acts in the upwards direction.

We call that upthrust.

Well done if you've got those.

And now we're onto the final part of the lesson.

And we're going to look at why some materials will float on water and some won't.

So let's go on with that.

Any substance will sink in water if it's more dense than the water.

So a stone will sink in water because it's more dense.

So if I place a stone in water, it will fall to the bottom of the water.

And that's because the stone has a higher density than the water.

And the reason the stone has got higher density than water is this.

It's made of particles which are heavier than water particles, so each individual particle in the stone is heavier than the one single water particle.

And the second reason is the particles are also packed more closely together in the stone.

So overall, the stone has more mass per cubic centimetre than water, and that's what causes it to sink in the water.

Let's see if you understood that explanation.

I'd like you to complete these statements by using just the words greater and smaller to fill in the gaps.

So pause the video, decide which word fits which gap, and then restart, please.

Welcome back.

Let's go through those.

So rock has a greater density than water because it has a greater mass in a particular volume.

This is because each particle of rock has a greater mass than a particle of water.

And the particle gaps, sorry, the gaps between the particles in the rock are smaller than the particles in water.

So well done if you've got those.

Now let's see why a substance floats on water.

And it floats because it's less dense than the water.

So we can have oil, and oil will float on the water because it's less dense.

So if I spill oil on water, it'll float on the top like that.

And the oil has a lower density than water because even though it's made of particles that have a greater mass than the water particles, they're much further apart.

They're spaced out more than the particles in water.

So the individual particles are heavier, but they're more spaced out, and that means that the oil has less mass per cubic centimetre than water, and so it floats.

Now let's look at a situation where the particles are identical.

So we've got liquid water and solid water, that's ice, are both made from exactly the same molecules, H2O molecules.

If you put a piece of ice in water, the ice will float on the water.

Now, ice is less dense than the water, even though it's made from the same particles.

So particles, each individual one, it's got the same mass, but the ice is less dense.

And the reason it'll float is the particles in the solid ice are spaced further apart than those in liquid water.

And because they're further apart, there's less of them in each unit of volume, and so it's got a lower density.

Okay, let's see if you understood that explanation.

I'd like you to complete these statements by adding the words greater and smaller to each of the gaps.

So pause the video, read through them and fill in the gaps, and then restart, please.

Welcome back.

Well, let's have a look at the solutions.

Ice has a smaller density than water because it has a smaller mass in a particular volume.

That means the gaps between the particles in the ice must be greater than the gaps between the particles in the water.

Well done if you got those answers.

And now it's time for the final task of the lesson.

So I've got some pupils, and they're trying to make models to try and explain why some substances have a higher density than others.

And they're using little balls with sticks between them to join them together.

So what I'd like you to do is to state three ways that they're good models for explaining density and three ways where they're not accurate models for explaining density.

And then try to use the particle model to explain why some substances have a higher density than others.

So pause the video, answer those three questions, and restart, please.

Okay, welcome back.

And here's some explanations why they're good models.

I'm not gonna read through them all, but you've got several answers there that'll explain why they are good models and it will give some ideas.

Restart.

Welcome back, and here's some ways that that was a good model for explaining density.

And you can see I've listed three of them there.

I'm not gonna read them all out, you can read for yourself, but they can be used to explain some of the concepts of density.

So well done if you've got answers a bit like this.

And here are the limitations of the model, reasons why they're not an accurate model of density.

And you've got several examples there to read through.

Your answer could have contained any of those.

So well done if you've got those.

And here's the answer to the final part, using the particle model to explain why some substances have higher density than others.

And it's to do with the amount of mass in a certain volume.

And that'll be to do with the size of the particles and their spacing.

So well done if you've got answers that look like this.

And now we've reached the end of the lesson, so let's have a quick summary of everything we've learned.

The pressure in a liquid increases with depth, or the deeper you go, the greater the pressure.

And that's due to the additional weight of the liquid above which pushes particles of liquid closer together.

So they're gonna be squashed more, there's gonna be greater forces, and that's gonna increase the pressure.

Pressure differences between the top and the bottom of an object inside a fluid produce the upthrust.

So the pressure's gonna be greatest at the bottom of the object and lower at the top of the object, and that gives upthrust.

And the resultant force of the upthrust and the weight can cause the object to accelerate, so to rise upwards or start to fall downwards.

A less dense material will float on a more dense one.

And the density depends on the mass and the spacing of the individual particles.

Well done for reaching the end of this lesson.

I'll see you in the next one.