Lesson video

Hello there, I'm Mr. Forbes, and welcome to this lesson from the Hidden Forces Unit.

The lesson's all about floating and sinking and the different factors that affect whether or not an object will float or sink in water, such as the density and the shape of the object.

By the end of this lesson, you're going to be able to explain why some objects float and why some objects don't float.

You're gonna be able to relate that back to the density of the object and also to its shape.

So when you're ready, let's get started.

So let's all look at the keywords that you'll need to understand to help you with this lesson.

The first of them is upthrust.

Upthrust is a force that acts on an object when it's floating in a liquid such as water.

And as its name suggests, that force acts upwards.

Sometimes, this is called buoyancy force.

Displaced means to be pushed aside.

So if an amount of water is displaced, that amount water is pushed to the side.

And in this lesson, we'll see that that displaced water is responsible for causing an object to float.

A displacement can is a device we can use to measure the amount of displaced water, and we'll see how that works later.

And density, the density of an object tells you how heavy it's for its volume.

And in this lesson, what we need to know is that if an object is less dense than water, it'll float, and if it's more dense than water, it will sink.

Here's some explanations of those keywords, and you can return to this slide at any point in the lesson if you wanna check them out again.

This lesson's in three parts.

In the first part of the lesson, we're gonna look at what floating means.

So we're gonna look at some scenarios why object float or don't float, and make sure that we understand what I mean by the phrase floating on water.

In the second part of the lesson, we're gonna cover an investigation that allows us to look at objects floating and measure the amount of water floating objects and sinking objects displace.

And in the third part of the lesson, we'll look at a rule for floating.

So we've got a general rule that'll tell us whether an object will float or not based upon its density and its shape.

So when you're ready, let's start with floating on water.

A boat is designed so that it floats on water.

And to be able to float on water, it needs to have a force acting on it that opposes the gravitational force that pulls it down.

That gravitational force causes the boat's weight and that would make it sink if that was the only vertical force acting on the boat.

So obviously, there needs to be another vertical force that acts against it that keeps the boat floating.

And that vertical force acts with the same size as the weight of the boat, but acts upwards in the opposite direction there.

That upwards force is called the upthrust or the buoyancy force.

So a boat is described as being buoyant if it floats on water because the buoyancy force or the upthrust force is the same size as the gravitational force.

So as long as those two forces are the same size, that boat will continue to float on the surface of the water.

If the force is acting on the boat aren't balanced, I mean the vertical forces, the upwards and downward ones, then the boat's going to start to move.

The first scenario is pretty straightforward.

If, for example, the weight of the boat was greater than upthrust, and that could happen if, say there was a hole in the boat and it started to take on water and its weight starts to increase, then that boat would start to sink and it continues to sink until it reached bottom of the sea.

The second scenario is less common to notice, but imagine you're on a boat and then you step off it and the weight of the boat will suddenly decrease 'cause your weight isn't pushing downwards on the boat as well.

So what happens in that case is the upthrust is temporarily slightly larger than the weight of the boat and the boat would bob upwards, and that can be awkward when you getting out of a boat.

You can often feel that effect where the boat bobs upwards and you get off balance and hopefully you don't fall in.

So let's see if you understand what I mean by floating on water.

I've got some fruit to end vegetables here and they're all in or on some water, and quite simply, you like you to identify which of these objects are floating on the water and which aren't.

So pause the video and make your selections and then restart.

Okay, as I said, that was a fairly straightforward one.

We can look at each of those objects.

Start with A, which is a strawberry, that's floating.

It's floating on the surface of the water.

B is an apple, and that's floating on the surface of the water as well.

C is a potato, and that isn't floating, that's sunk to the bottom.

So that must mean that its weight is greater than the upthrust force acting on it.

D, that's a pear, and that's floating on the surface.

So in that scenario, we must have the weight and the upthrust force being the same size.

And E is a carrot which is sunk, and that means that its weight is greater than the upthrust force as well.

So well done if you've got all of those.

And this is another check to see if you understand what I mean by floating on surface of water.

I've got a picture of an iceberg here, and there's two parts labelled X and Y.

And I'd like you to decide which parts of that iceberg is floating.

Is it A, only part X is floating, B, only part Y is floating, C, those parts, X and Y, are floating, or D, no part of the iceberg are floating.

So have a think about that, pause the video, make a selection, and then restart.

Okay, welcome back.

And you should have selected the C, both parts of the iceberg are floating.

So even though some of it's beneath the surface of the water, the overall iceberg is floating.

So when I describe something as floating on water, I mean any object that's at the surface of the water, with at least part of it above the surface, and it doesn't matter that some of it's beneath the surface as well, as long as it doesn't reach all the way down to the bottom of the water and to the solid surface beneath that.

So a floating object, just has to have part of it, above the top of the water.

So well then if you've got that.

Okay, let's have a look at some of the possible things that pupils might think about floating in different depth of water.

So we've got a first example here.

It's easier to float in deep swimming pool than it is in a shallow one.

So the deeper the water, the easier it's to float in it.

Or perhaps the opposite might be true, you'll sink in deeper water.

It's much easier to think if the water is deeper 'cause it pulls you down more.

Or possibly a third example, it doesn't matter how deep the water is, the force on you will be the same so you'll still float at the same depth in the same way when you're in the water.

There's only one thing we can do to check those out, and that's let's do a test and see.

Okay, let's have a look at a way of testing to see which of those ideas right.

I've got a little green ball here placed in a large measuring cylinder, and obviously, it'll rest on the bottom of the measuring cylinder.

And I've chosen the ball so that it will float in water.

So I'll pour a bit of water in, and the ball floats on the surface of it.

As you can see there, half of it's beneath the surface and half is above.

So what would happen if it add more water to it? Well, add a bit more, and it still floats in the same way.

Well, what happens if you add a little bit more again? It's still floating in the same way.

I've still got half of the ball above the water and half of the ball beneath it.

So it floats identically in all laws.

And even if you add a bit more, there's no effect.

So the depth of the water doesn't appear to have any effect on how well an object will float in it.

Well, that's just me talking through.

Let's have a quick look at a video to demonstrate that is actually what happens.

So when more water is added, the ball will always float, and half of it beneath the surface and half above.

So let's check that video out.

Okay, you should have seen in the video that I used a pen, 'cause I didn't have a ball handy, but it still floated in the same way no matter how deep the water was.

The same amount of pen was above the water as below the surface of the water.

So the depth of the water really doesn't seem to have any effect there.

Let's check if you understood the conclusion to that experiment.

I've got the results there on the right.

So what I'd like you to do is to decide which of these is true.

The deeper the water, the more difficult it is for something to float in it, the deeper the water, the easier it is for something to float in it, or the depth of the water does not affect how something floats in it.

So pause the video and make your selection, and then restart when you're ready.

Okay, and as we watched the video, we saw the actual result.

It's the depth of water does not affect how something floats in it.

It doesn't matter how deep that water is, it'll still produce the same upwards force on you, and so you'll float in the same way or sink in the same way.

Well done if you got that.

Let's check if you understand the consequences of the results there.

So we've got a potato, and potatoes sink in shallow water.

What will happen to the potato if I put it in deep water? Whether it be A, it could float on the surface, or B, sink to the bottom.

So pause the video, make selection, and restart.

Okay, welcome back.

You should have selected B, the potato still sinks.

It doesn't matter that the water's deeper, or even if it was shallower, the potato would always sink in it.

So well done if you've got that.

Here's a slightly trickier one.

I've got an apple and that's floating in shallow water.

Which of the three scenarios will happen when I put it in deep water? Is it A, B, or C? So look at those pictures carefully, pause the video, make your selection, and then restart.

Welcome back.

Well, that was a little bit more difficult because you have to look more carefully at the diagrams. In that scenario, you should have selected B, the apple still floats, and the reason you should select B instead of A or C is because the apple in B is still floating at the same depth in the water.

And the first scenario, the apple has raised up a little bit, and in the final one, C there, it's sunk down a bit more, and that really shouldn't happen.

What depths of water shouldn't have an effect on how high up the apple is against the surface? So well done if you chose B.

Okay, we've reached the first task with lesson now, and this is all about objects that are floating.

So I've got a scenario here with three submarines, okay, A, B, and C.

What I'd like you to do is look careful at those diagrams and what the submarines are doing.

So A is not moving on the surface, B is sinking downwards, and C is rising upwards.

And I'd like you to draw the vertical forces acting on each of those submarines and then decide on which of the submarines is the upthrust and the weight equal.

Okay, so pause the video, try and complete that task, and restart when you're done.

Welcome back.

Let's have a look at the solution to that, starting with submarine A, that's not moving, and that means that there's no result force acting on it.

So in that scenario, the upthrust in the weight must be equal to each other.

Submarine B, that's sinking, it's moving downwards, and that must mean that there's a greater force acting down than upwards if it's speeding up and going downwards.

Scenario C, that's rising.

So it's speeding upwards.

So there's a greater upwards force acting on it than downwards force.

So I've got a larger upthrusts in the weight there.

And I did ask you to mark which one was the upthrust and the weight was the same, and that is the first as a submarine, submarine A.

Well done if you've got all that information correct.

Okay, we're ready to move on to the second part of the lesson now.

We're gonna look at how we can investigate whether objects float or sink, and see if we can start to identify some of the factors that affect whether it'll float or sink or not.

So let's go.

Okay, I'm gonna start with two objects that I know float in water.

So what I've got got here are three identical beakers, each containing the same amount of water.

So they're all level at the moment.

What I do is I place the green ball into the central beaker there, and you can see what's happened is the water level has risen.

Water's moved to the sides because the ball's pushed down, and it's pushed some of the water out the way.

We say that water's been displaced, and that displaced water because of the ball has made the water level rise there.

I can imagine what happens if I put the red ball in.

The red ball floats as well, but it's got a bigger volume, it's gonna push down on the water, and it's going to make that water get displaced to the side in the same way.

But this time, it's a bigger ball.

It's displaced more water.

I've got a greater amount of displaced water.

So a larger object has caused a greater water rise.

Now because those balls are floating, there must be that the upthrust force that they're producing, sorry, the water is producing, must be equal to the weight of the balls.

So in that scenario, I've got the upthrust force equal to the weight of the objects.

So what happens if I've got objects that sink in water? Do I get the same sort of effect? Well, I've got two objects here that I know will sink in water, two balls again, I'm gonna place the pink ball into the central beaker.

As I place it in, then it's still gonna displace some of the water.

So the water level has still risen in the scenario.

We've got a higher level of water, but the ball is sunk to the bottom.

And similarly, if I put the golden ball in, that causes the object that sort of the water to be displaced again.

And because it's got a larger volume, it's gonna push more water out the way.

So we've got a higher level of water because of that larger object there and a greater water rise.

The difference in this scenario is the force is acting on the balls.

In this scenario, the upthrust that the water's providing is smaller than the weight.

It's not able to keep that object floating.

So the upthrust is less than the weight of the object.

So you've seen that whether an object floats or not, it's gonna displace water when you put it into the water.

And what we need to do is have a way of measuring how much water gets displaced by objects that float and objects that sink.

And the device we used to do that is called a displacement can.

And it's basically a metal can with a little spout on the side, and it allows us to collect the water that's displaced and to then measure it later.

So it looks a little bit like that.

What you do with it is you fill it with some water right up to the spout, okay? So generally what we do is we overfill it and let some of the water pour out until the water level falls back down to exactly where that spout is.

So it's full, basically.

If I put any more water in, it'll start pouring up the side.

Then what I do is I put the object into that water very carefully, making sure don't splash, and the water level will rise.

So it's above the level where the spout is.

And because it's above the level where the spout is, it'll start to pour out of it again.

So the water will gradually pour out and I collect it in a small beaker pour on the side.

And over time, that water level will gradually fall until where it's back exactly in level with the spout again, and it'll stop pouring out of it.

And what I've done then is I've measured or collected all the water that would been displaced and I can go on and measure it later.

So I've collected that water in the beaker at the end there and I can go and measure its mass or its volume or something like that.

Okay, let's check if you understand how a displacement can is used.

Which of these displacement can is ready to measure the water that's displaced when I put that orange ball in it? Is it displacement can A, B, or C? So pause the video and make your selection by looking careful at the diagrams and then restart.

Okay, welcome back.

You should have selected displacement can C in that scenario.

And the reason for that is because we need the displacement can to be full to spout.

So it's got water all the way up to the edge of the spout and we need an empty collection beaker.

So well done if you selected C.

Okay, let's have a look at some suggestions that pupils have made about a relationship between the water that's displaced and whether an object will float or it'll sink.

So the first one is this one, things that float will displace more water than things that sink.

So anything that will float on the surface of the water will always displace a larger volume or mass of water than anything that sinks in the water.

Oh, we've got this possibility.

The volume of the water and the object will always be the same.

So no matter whether it floats or it sinks, you'll always get the same volume of water displaced by that object.

Third possibility, the weight of the displaced water and the weight of the object will be the same.

So in that scenario, it doesn't matter whether it floats or sinks, but you'll always get the same weight of water as the weight of the object.

And there's only one way to find out which of those things should be true and that's to do a test.

So in the experiment, what I'm gonna do is measure the mass of the water displaced.

I'm not gonna measure the volume this time.

I'm gonna measure what mass of water is displaced by an object that floats and what mass of water is displaced by an object that sinks.

So to do that, I need a balance.

Now I'm gonna be measuring the water that is displaced from that displacement can.

And to do that, I need to measure the mass of the beaker first.

If I measure the mass of the beaker first and then I measure the mass of the beaker that's got water in it.

At the end of the experiment like this, you'll notice there's an increase in mass, and that increase in mass is going to be the mass of the water that's inside the beaker.

So that allows me to measure the mass of the water.

In this scenario, what I do is just subtract the larger number, sorry, subtract the smaller number, the large number, the empty beaker from the large beaker, and that will leave me with the mass of the water that's in the beaker.

So in this example, 41.

1 grammes of water are in that beaker.

Let's see if you understand how to do that.

So I've got another scenario here.

I've got the empty beaker and I've got the full beaker, and I've got the mass of both of those.

And what I'd like you to do is to work out the mass of the water in the beaker from those two reading speeds.

So pause the video, work out the mass of the water, and then restart when you're ready.

Okay, what you should have done is just subtract those two values.

If you subtract the 110.

2 grammes from the 132.

6 grammes, you get 22.

4 grammes.

So well done If you've got that.

That's the mass of the water in the beaker.

Okay, it's time for you to actually carry out that experiment now.

What I'd like you to do is to choose some objects that might float and sink, I'd like you to measure their masses, and then I'd like you to measure the mass of the water that they displaced when you put them in a displacement can.

You can watch a quick video to show how that process is done.

Let's look at how to use a displacement can.

Here it is, it's like a metal cylinder with a spout.

And what we can do, we can fill that up with water, and we can use it to measure how much water an object will push out of the way when we pop it inside the displacement can, and that's what we're going to do now.

First of all, we need to fill it with water.

We're going to overfill it, so some water will come out of the spout, and that means we know it's a completely full to the top.

So here's the water just coming out of the spout now.

Wait till that stops dripping and then we'll get rid of that excess water.

So we've now got a dis displacement can that's completely full to the top with water.

And before we start, we're going to weigh the mass of the beaker, which is 62 grammes, and we're going to record that.

We'll place the beaker back underneath the spout.

And now we're ready to take our measurements.

What we're going to measure is how much water a metal ball will push out of the way.

So here's the ball, it's made out of steel, and we measure its mass, and that comes out at 260 grammes.

Now that's gonna sink in the water.

It's tied to a piece of string so we can gently lower it into the water without splashing.

And by lowering it gently, the water will come out of the spout until it's completely underneath the water.

Again, we wait till it stops dripping, and then we can measure the mass of that water.

But it's also got the beaker with it.

And the beaker and the water together weigh 104 grammes.

The water on own will weigh 42 grammes.

And so we've used a displacement can to measure the mass of the water that a steel ball has pushed out of the way.

Okay, hopefully that video showed you how to carry the experiment clearly.

So what I'd like you to do now is to pause the video, carry out out the experiment, recording your results on a table like this one, and then restart the video once you're done.

Okay, welcome back.

Hopefully you collected your results, and they look a little bit like this.

Obviously, you'll use different objects, but you should have got a similar sort of results with the mass of the objects and the mass of the water displaced.

Well done if you did.

Okay, now it's time for the final part of lesson.

What we're gonna do is look at the data we collected from the experiment and see if we can come up with a general rule about whether an object will float or sink based upon the data we collected there.

So let's have a look at that.

So we're gonna start with this example table of data.

And as you can see, I've got several objects that float and several that sink.

I don't really need to know what the objects were, I just really need to know about the mass of the object and the mass of the water that they displaced.

And what we can do is to find a general rule is we can compare the masses of the objects and the mass of the water that they displaced.

Now as you can see there's two categories there are objects that sink and objects that float.

So what we should do is compare the ones that float together and see if there's a pattern there and compare the ones that sink and see if there's a different pattern there.

So that's what we're going to do.

Right, so I want you to try and find a conclusion for the objects that floated.

So I've got my data table here and I've highlighted the objects that floated.

They were objects, B, C, F, and G, and I'd like you to decide which of the two conclusions about those objects are correct.

Is A, floating objects displace a greater mass of water than their own mass, or is it B, floating objects displace the same mass of water as their own mass.

So I'll pause the video, make your selection of the conclusion, and then restart when you're ready.

Okay, welcome back.

Let's have a look at that.

Well, looking at the data, you can see in all of those cases, the floating objects have displaced the same mass of water as their own mass.

So for example, object C floated, it has a massive 8.

5 grammes, and it displaced 8.

5 grammes.

Object G, 6.

8 grammes mass, and it displaced 6.

8 grammes of water.

So well done if you've selected that option.

Okay, we're gonna do the same thing for objects that sink.

So I've got my data table again and I've highlighted the objects that sunk in the water, and I'd like you to try and reach a conclusion about that by comparing no mass and the mass of the water they displaced.

Okay, so is the conclusion, objects that sink displaced less mass of water than their own mass, or objects that sink displays the same mass of water as their own mass.

So pause the video, select your conclusion, and then restart.

Okay, welcome back.

Let's have a look at that.

The conclusion you should have selected was A, and we can see that in the data.

If you look at object A, it's got a massive of 12.

2 grammes and it only displaced 9.

9 grammes of water and it sank.

Or if we look at object H, that's got 14.

2 grammes of mass, but it could only displace 11.

1 grammes of water.

So the conclusion, you should have reached was conclusion A.

Well done if you've got that.

So we've got our two conclusions and we can come up with a general rule about floating and sinking based on them.

So the first one is for objects that float.

Now an object will float if its shape can push its own massive water out of the way.

So in all the scenarios of floating, you saw that the object displaced the same mass of water as itself.

So an equal mass and weight of water was displaced to the mass and weight of the object that floated in it.

In the scenarios where the object sinks, then the object couldn't push out enough water, it could only push out a smaller mass and weight of water than its own mass and weight, and so it sank.

The water couldn't provide a large enough upthrust because it didn't push enough water out the way.

So there's our general rule about floating and sinking based upon the data we saw in the experiment.

Right, let's check whether or not you can use that rule about floating and sinking to help Izzy here.

Izzy's forgotten to write down whether the objects floated or sank or not during that experiment.

So she's got a results table, but she just didn't complete the last part of it though.

What I'd like to do is to use the rule to decide whether or not each of those six objects, A to F, floated or sank.

So pause the video and restart when you're ready.

Okay, so using the rule.

Welcome back.

Let's use the rule to decide.

Welcome back.

Let's use the rule to help Izzy out.

So in the first scenario, object A, that floats.

It displaces the same mass of water as its own mass.

The second scenario, that's B, that's sank because it couldn't displace enough water, and we can use the same rule to continue with the rest.

That floats.

D sinks.

E floats 'cause it displaces its own mass again.

And finally, F, doesn't quite float.

So that one sinks because it displaces 17.

9 grammes instead of 17.

2.

So it was just on the edge of floating, but couldn't quite do it.

Wondering if you've got those? So we're seeing that some objects will float or sink, but one of the factors we've not looked at yet is the shape of the object.

So if I take a ball of clay and put it into some water, a 10 gramme ball here, it'll sink to the bottom.

But if I take exactly the same amount of clay and I shape it into a sort of dish or boat shape, it'll float on the surface of the water.

The reason behind that is because when the clay is in a little ball, it's got a small volume, or when it's shaped into a sort of dish, it's got a higher volume.

The volume of the boat is much larger than the volume of that little ball.

And so it's got a greater area in contact with the water and it allows that water to create a larger force on the boat.

And so the boat will float on the surface.

So that boat has got a lower density than the solid ball of clay, and that allows it to float.

You probably already know, ships are made of steel, and steel is much denser than water.

It's got a higher density than water.

So if I get a solid block of steel and place it on the surface of water, it'll sink straight to the bottom.

That's because that solid block of steel has got a small volume.

But if I shape that steel into a boat shape, then it will float on the surface of the water because its density is lower than the density of water now because it contains a large volume, most of which is actually just air.

So just by shaping the steel, I get an object that's got a lower average density than the water, and it'll float on its surface.

So as long as the overall density of the object, including all the materials in it, is less than the density of the water, then I'll get an object that will float.

So ships will float on water.

Okay, we're onto the final task of the lesson now, and what I'd like you to do is this.

I've got some pupils and they've been given a ball of modelling clay, and the density of that clay is greater than the density of water.

I'd like you to explain to the pupils what will happen if you put that ball as it is into a bowl of water.

And then I'd like you to explain what the pupils can do to make the clay float on the surface of the water.

So pause the video and answer those two questions and then restart when you're ready.

Okay, welcome back.

So here's the answers to those two.

First of all, the clay will sink if it's just in the ball shape because the water can't produce an upthrust that's as large as the weight of the clay.

And the second of those answers, what they should do is make it into some sort of boat shape.

They'll increase the volume.

That will make the density less, and so it's less than the density water.

So eventually, it'll float.

So well done if you've got those two answers.

Right, we're at the end of lesson now, so here's a summary of all the information.

Right, we're at the end of the lesson now.

So here's a summary of all the information you've learned.

So first of all, an object will float on the surface of the water when the upthrust is equal to its weight.

We can use a displacement can to measure the amount of water and object will displace.

Whether it floats or it sinks, we can still measure the displaced water by collecting it with a little beaker.

And a general rule, an object will float if its shape allows it to push its own mass of water out of the way.

Otherwise, it'll sink in the water.

And finally, an object with a density that's less than water will float on the surface, an object with a density that's greater than that water will sink.

So well done for reaching the end of the lesson and I hope see you in the next lesson.

Goodbye.