Lesson video

Hi everyone, my name is Ms. Hummel and together we'll be answering the question which factors affect an object's ability to float? In this lesson, we'll be discussing upthrust or buoyancy in more detail.

We'll discuss what density is and how it affects floating.

And we will also discuss the link between weight and upthrust.

Finally, we will make some predictions which we will test.

Our lesson will follow this structure.

We will begin by recapping what upthrust, a type of contact force is then we will discuss what density is.

We will use that knowledge to discuss why some things float.

And finally, we will complete some investigations and predictions.

In this lesson, you will need an exercise book or paper, a pencil or pen, a coloured pencil or pen, and a ruler.

If you haven't got those things, pause the video now and go get them.

Here are our star words, which are the most important words of our entire lesson.

I'm going to say them and ask you to repeat them after me.

When I point at myself, it will be my turn.

And when I pointed at you, it will be your turn.

Forces, upthrust, weight, gravity, density, float, and sink.

Great, the words upthrust, weight, density and gravity will help us understand what factors affect whether an object will float or sink.

For our first part of the lesson, we're going to recap the concept of upthrust.

Upthrust is a type of contact force.

An object that is partly or completely submerged.

That means under the water experiences a greater pressure on its bottom surface than on its top surface.

This causes a resultant force upwards.

This force is called upthrust, which is also known as buoyancy.

I would now like you to pause the video, to answer this question.

Can you think of items which you know float? And how about some items which you know sink? You can resume once you've finished thinking about this.

We're now going to examine the difficult concept of density.

Density is defined as how much stuff is packed into a particular area.

Now point the cube which has a larger density.

You should have pointed to the one on the right.

Now why do you think that's the one that has the larger density? It has the larger density because it has more stuff packed into the same area as the one on the left.

A denser object also weighs more than the same amount of a less dense object.

Which of these balls has a higher density? Point at the one with the higher density.

Now, hopefully you said bowling ball.

What that means is that the bowling ball weighs more and has more stuff packed into it than the beach ball.

I would now like you to pause the video to complete this question.

Can you think of items which are very dense? Now these are items that may look small, but are actually very heavy.

And then can you think of some items which are not very dense? So those are items that maybe look larger, but are actually really light.

You can resume once you're finished with this think task.

For dense items, you may have thought of things like coins.

They sure do get heavy if you get a lot of change.

For some items which are not very dense, you may have thought of inflatable objects, which are filled with air.

Now that we know what density is, we can begin to understand why some items float.

An object in a liquid is displacing, which is the same thing as moving some of the liquid.

Let's have a look at how that works in practise.

You should be able to see a glass.

And I drew a little line where the water's currently kind of standing.

I have a ping pong ball here, and if I force it down, you'll notice that the rest of the water is rising.

So as I push it up, it kind of goes back to where it was.

When I'm pushing it down, it's rising.

That means that the space that was used up by the water is now used up by the ping pong ball and the rest of the water is displaced.

That means that it has moved.

In our diagram, you can see where the ping pong ball is and the same little gap that we can see where the ping pong ball is under the line of water is the same amount of water that has risen up on the sides of the ping pong.

The thrust force, which is the force that was pushing up the ping pong ball when I was trying to kind of force it in the water is equal to the size of the weight of the liquid that was moved by the object.

So when I put the ping pong ball just to the very surface and pushed it down just a little bit, I could feel a force pushing the ping pong ball up and making it rise.

However, as I went further down with the ball, there was more water that was displaced.

And therefore that would have had a higher weight.

And what I found was as I was pushing it lower, the force that I could feel for upthrust was getting stronger.

At any one point, an object in water will also have gravitational force pushing it down, and upthrust pushing it up.

In this case, when I was doing it with the ping pong ball, it had my manual force, which is not the same as gravity.

So at any one point there's gravitational force acting on an object, but then if it's in water, there will also be some upthrust force pushing it up.

If the force for gravity is the same as the force for upthrust, then it's going to float because the forces are balanced.

So we can say that if the thrust is equal to the weight of the object, the object will float.

Remember that weight is the force of gravity acting on an object.

We can also say that if the upthrust is less than the weight of the object, then the object will sink.

Because in this case, the forces were unbalanced.

If my weight, which is the force of gravity on an object is stronger than my upthrust, then my object is going to continue to sink.

Now you may have been wondering how density plays a role into all of this.

If the density of the object is greater than the density of the liquid, the object could never displace or move enough liquid to create an upthrust that will hold its weight up, so it sinks.

Therefore, the items that will float have a density that is the same or lighter than that of water.

When I began investigating why some things float, why some things float and others don't, the first thing that came to my mind was ice.

In my head, I was thinking, "Ice must be really dense "because it's a heavy object." However, ice is actually 9% less dense than water, which is why it ends up floating.

If the density is less, it means that the mass of the displaced water is more than the mass of the object, which is inside the water, in this case ice.

This will therefore mean that it floats and rises.

Oil also has a lower, different density than water.

Because of this, it does not incorporate or mix well with the water.

And this is also the case with vinegar and oil.

I often use vinegar and oil for my salads, and I always want to mix them so it's well distributed, but I always find that I need to shake it really hard for the particles of the oil and the particles of the vinegar to distribute a little bit more evenly.

Your activity is going to be to explain why the cork is floating and the metal is sinking.

You can use words like density, water displaced.

So thinking about how much water has been moved and weight.

For the final part of our lesson, we're going to complete some investigation and predictions.

You're going to want to have a table that looks like this.

If it's not in front of you, you're going to need to draw one with a ruler.

Now on one hand, on the left hand side, it has the objects that we're going to be investigating, which are coin, paperclip, small piece of paper, blue tack and pencil.

Then we've got a prediction.

Do you think it's going to float or sink? Then we've got an explanation.

So you say why you think that is.

Why do you think it's going to float or sink? And finally, it's got a column at the very right that says, "Were you correct?" Where you're either going to tick or cross whether you were correct in your prediction.

I would now like you to pause the video and make sure that you've completed the prediction column and the explanation column.

You can resume once you're finished, and I will investigate whether those objects float or sink.

Okay, so what I've got with me is a large jar of water.

It's actually very heavy.

We're going to start with the coins, so think about your prediction, whether it was that it's going to float or whether it's going to sink.

And now you're going to watch what happens when I put the coin in the water.

Here's my coin.

As you can see, it went straight to the bottom and it sunk.

If you thought it would sink, you can give yourself a tick.

If you thought it would float, you're going to have to give yourself a cross or you're going to have to correct your work.

Next, let's look at a paperclip.

Again, I've got my jar, I've got my paperclip and I'm going to drop it.

And it went straight to the bottom again, just like the coin did.

So if you thought it would sink, give yourself a tick.

And if you thought it would float, give yourself a cross and correct your work.

Next, we've got a small piece of paper, which I've got right here.

When I look at this I keep thinking I might drop it.

Okay, so I've got my paper, let's see if it sinks or floats.

Now it seems like it is floating.

Now, once it gets wet, do you think it would continue to float? Let's find out.

So I'm going to get it wet.

I'm going to try again.

It is still floating.

Now eventually, it would disintegrate because the material of paper does absorb some water.

Now give yourself a tick if you said it would float and correct your work, if you thought it would sink.

Next, one of my favourite things to fidget with is a little piece of blue tack.

I'm going to go ahead and put it in my jar.

And it's sunk straight to the bottom.

So if you thought that it would sink, make sure to give yourself a tick and if you thought it would float, correct your work.

Finally, and the one I'm the most interested in, we're going to test whether a pencil will float or sink.

I've got my pencil.

I'm going to put it in the water and it's floating.

You can see it right there.

If you thought it was going to float, give yourself a tick.

And if you thought it was going to sink, cross it out and make sure you correct your work.

Now because I already did this, you don't need to repeat the same experiment.

However, if there are a few items around your house, which you would like to experiment with, you certainly can.

Just make sure it's nothing that you're going to care about if it gets ruined in the water.

Now we can say that the coin sunk because the density of the coin was greater than the density of the water.

We can also say that it's sunk because the weight of the coin was greater than the weight that was displaced of the water.

We can say the same thing for the paperclip and the blue tack.

However, we can say that the density of the paper and the pencil, which was wooden was less than the density of water.

We have now finished our lesson.

And it's time for you to complete your exit quiz.

You need to exit the video and complete the quiz to test your knowledge and understanding of this lesson.

I hope you enjoyed this lesson, bye.