Lesson video

Hi there.

My name is Mr. Tilstone.

I'm a teacher and I'm really excited because I get to spend this math lesson with you.

If I've met you before, it's nice to see you again and if I haven't met you before, it's nice to meet you.

If you're ready, I'm ready.

Let's begin.

The outcome of today's lesson is "I know that perimeter is the distance around a 2D shape." And we're going to have lots of fun with this concept today.

It's going to be very hands-on.

Our keywords are just the one.

My turn, perimeter, your turn- What does that mean? You're going to be exploring this a lot over the coming lessons, but perimeter is this.

The distance around a two dimensional shape is called the perimeter, and we can see an example there.

Our lesson today is split into two cycles.

The first will be what is perimeter? Because this might be the very first time you've met that word.

And the second will be comparing perimeters.

But let's start by exploring the question, what is perimeter? In this lesson, you're going to meet Laura and Lucas.

Have you met them before? They're here today to give us a helping hand with our maths.

How many ways can you think to run your finger around the outside of a piece of paper? Have you got a piece of paper there in front of you? Have a go.

How many ways can you think to run your finger around the outside of it? Well, here's what you could do.

You could start in one corner and go clockwise, finishing where you started, just like this.

You've run your finger around the outside of that paper.

There's more than one way to do that, isn't there? Can you think of a different way, as you find one yourself? What about this? You could do the same thing, but anticlockwise.

Go the other way.

Let's have a look.

And again, you've run your finger around the outside of that paper.

Can you think of another way? What about this? The same thing, but starting in a different corner? Did you think of that one? Here we go.

You could do this.

There's a special name for what you've explored here, and it's perimeter.

So we've got the stem sentence.

The distance around the mm is its perimeter.

Can you say that with me please? Are you ready? The distance around the mm is its perimeter.

Now just you.

Off you go.

Great stuff.

You're going to be saying that quite a lot.

So in this case, the distance around the paper is its perimeter.

Laura is investigating the perimeter, that word again, of this leaf face.

Now you might be in a position where you've got some leaves in front of you now and you can do the same thing.

That would be great.

Like before, she can run her fingers around the edge of the leaf, just like this.

And we can use that stem sentence again.

The distance around the edge of the leaf is its perimeter.

She draws around the outline of the leaf.

And you could do that again.

If you've got a leaf and you've got a pencil, you could draw around the outline, just like this.

The outline that she's drawn still shows its perimeter, so we've still got that same stem sentence.

The distance around the edge of the leaf is its perimeter.

Now if she takes some string and goes around the outline.

Have you got some string in front of you? Because you could do the same thing and it would be fantastic if you did.

She could wrap string around that outline that she's drawn and we can still say the same thing.

It's still showing the perimeter.

The distance around the edge of the leaf is its perimeter.

That's showing the perimeter of the leaf.

Now she turns the string back into a straight line just like this.

Is that still showing the perimeter of the leaf? Yes it is.

It's still showing the perimeter of the leaf.

That's still the distance around the edge of the leaf.

Perimeter is a measure of length.

Let's do a quick check for understanding.

Which of these correctly shows the perimeter of the triangle? So imagine you've got a triangle in front of you and you're running your fingers around it.

Which of them shows it being done properly? Can you explain why the other options do not show perimeter? Pause the video.

Let's have a look.

Well, A and D have not gone all the way around the shape.

Did you notice? A almost went all the way around but not quite.

So that's not the perimeter.

C hasn't followed the outside lines of the shape.

It's a bit wiggly, isn't it? So it's almost a perimeter, but it's not quite.

E has gone round one of the sides twice.

So that's not the perimeter, that's more than the perimeter.

The distance around the triangle is its perimeter.

Which one's showing that? That's B.

Well done if you said B, you're on track.

We've got some tasks.

Draw around the outline of three objects, and these are just some examples for you.

It could be a paper plate, a number block if you've got some of those, an envelope, a leaf, anything really, to create a 2D shape.

Say the stem sentence to describe each one.

So that stem sentence, again, the distance around the mm is its perimeter.

So fill that in.

Number two, use string.

If you've got string to create the perimeter of the same objects, either by placing the string around the object or around its outline.

Again, creating a 2D shape.

Say that stem sentence again each time.

I think you're going to have fun with this.

And then number three, "Lucas has been asked to show the perimeter of the number block using string.

Has he been successful? Explain." Have fun and I'll see you soon for some feedback.

How did you get on exploring the perimeter of different objects? Let's have a look.

So these are some potential objects that you might have chosen to draw around.

In this case, the distance around the plate is its perimeter.

In this case the distance around the number block is its perimeter.

And in this case the distance around the envelope is its perimeter.

And then you use string or wool or something like that to create the perimeter of those same objects by wrapping it around it.

So again, the distance around the plate is its perimeter, the distance around the number block is its perimeter and the distance around the envelope is its perimeter.

That's just three that I've chosen.

They're all showing perimeter.

Now you may have turned the string into a straight line afterwards and it's still showing the perimeter.

So well done if he did that.

And was Lucas successful in showing the perimeter of that number block with string? No.

Well done though to Lucas because he got some bits right.

He knew that perimeter is the distance around the shape, but he hasn't quite gone all the way around the shape.

So the full perimeter is not being shown.

That is not the perimeter of that number block.

It's almost.

You're doing really well so far.

Shall we move on to cycle two? That's comparing perimeters.

Sometimes it's easy to compare the perimeters of two shapes just by looking.

Let's do an example.

Here's a tablet.

I'm sure you've either got one of these or seen one of those.

You might have one in your classroom, you might have one at home.

It's a tablet.

And I can see some different 2D shapes within that tablet.

Now we're going to explore those.

We're gonna use this stem sentence now, slightly different.

Mm has a shorter perimeter than mm, because the distance around it is shorter and the other stem sentence is going to be the same thing but sort of in reverse.

Mm has a longer perimeter than mm, because the distance around it is longer.

So we can compare two perimeters in two different ways.

So we're going to look at that home button there.

What shape is that? Well, that's a circle.

And we're going to look at the shape around this screen.

Now, would you say they've got different perimeters? Yes, I would.

I'd say they've got very different perimeters.

One's clearly got a bigger perimeter, a longer perimeter than the other one.

Let's use their stem sentences.

Which one's got a shorter perimeter? The home button has got a shorter perimeter than the screen because the distance around it is shorter.

Now let's reverse that.

The screen has a longer perimeter than the home button because the distance around it is longer.

And once again you could explore that with your finger.

It wouldn't take long, would it, to go around the outside of the home button? It would take longer to go around the outside of the screen.

Let's compare two different shapes on the tablet.

This time the screen has a shorter perimeter than the casing because the distance around it is shorter.

And likewise, the casing has a longer perimeter than the screen because the distance around it is longer.

And you can see that it's longer.

Laura and Lucas are thinking about the perimeters of these leaves.

So they've got two different leaves.

And again, you might have two different leaves in your classroom or in front of you now.

That would be brilliant.

So they're thinking about those perimeters and Laura says, "I think leaf A will have a longer perimeter because it's longer." Yes, that leaf is longer.

Hmm.

Do you agree with her? And Lucas says, "I think leaf B will have a longer perimeter because it's wider." Yes, it is wider.

Hmm.

What do you think? Do you agree with Laura or Lucas or perhaps neither of them? What do you think? Is there a way they could check? Perimeter is the distance around a shape.

And we can see that lady bird going around that leaf.

That's the perimeter.

The lady bird has just walked the perimeter of the leaf.

Hmm.

What did you notice there when the lady bird explored the perimeter of that leaf? Was it the same? No, the lady bird has further to travel around leaf B.

Therefore, leaf B has a longer perimeter.

This could also be shown with string.

So you've probably not got a lady bird in front of you to walk around the perimeter, but you probably have got some string and you could explore it that way.

So we could say mm has a shorter perimeter than mm because the distance around it is shorter.

Which one has got the shorter distance? A or B? And likewise we could flip that around and say mm has a longer perimeter than mm because the distance around it is longer.

Can you fill those stem sentences in? Leaf A has a shorter perimeter than leaf B because the distance around it is shorter.

And let's flip it.

Leaf B has a longer perimeter than leaf A because the distance around it is longer.

Lucas has been using number rods.

Have you got some of those in your classroom? Because they're really helpful for exploring perimeter if you have.

Lucas has created a regular polygon using number rods.

You might have been learning about regular polygons recently.

They've got the same length sides and the same interior angles.

And so he's created one using number rods and he turns it into a straight line to show the perimeter.

Let's have a look at that.

There we go.

One rod, two rods, three rods.

And both of those are showing the perimeter of the triangle.

Laura creates an irregular polygon.

So what's her polygon going to be like? Maybe different length sides, maybe different interior angles.

Maybe both.

Here we go.

So the side lengths are the same but the interior angles aren't.

That's an irregular polygon.

It's difficult to see whether this shape has a longer or shorter perimeter than the triangle.

So Laura shows her perimeter as a straight line as well.

So she's going to unfurl hers just like this, just like before.

And she's got a straight line.

Ah, now we can compare them.

Alright, let's use those stem sentences.

So we've got Laura's shape and we've got Lucas's shape.

But Lucas's polygon has a longer perimeter than Laura's polygon because the distance around it is longer.

Flip it around please.

Laura's polygon has a shorter perimeter than Lucas's polygon because the distance around it is shorter.

Let's have a check which of these is true.

Have a look at those shapes.

Is it true to say the triangle has a longer perimeter than the square? Is it true to say the square has a longer perimeter than the triangle? Is it true to say the triangle and the square have got the same perimeter? Now there's a little bit of a clue.

What do you notice about the side lengths of those shapes? Hmm.

Pause the video and off you go.

What do you think? This one.

The square has a longer perimeter than the triangle.

So if you notice, they've got the same length sides, but the square's got one more, so it's got to be longer.

It's time for some practise.

For each pair of shapes, tick the one with the longer perimeter.

And how do you know? Look at the different 2D shapes on this football pitch and compare their perimeters in as many ways as you can.

And we've got those stem sentences.

Mm has a longer perimeter than mm and mm has a shorter perimeter than mm.

So we've got some different parts.

So A is the perimeter of the whole pitch.

B is the perimeter of one half of the pitch.

C is the perimeter of the centre circle.

D is the perimeter of the penalty arc and E is the perimeter of the goal box.

So you don't need to write down that whole phrase each time.

Just the letters is fine, please.

Number three, choose one of the objects from task A.

So it might be the envelope or the number block or whatever you want.

Use string to compare its perimeter with the perimeter of the other objects.

Or if you like, you can choose some new ones and then say those stem sentences.

And finally, number four, use number rods if you have them to create pairs of polygons just like Lucas and Laura were doing.

Compare their perimeters by turning them into a straight line and say those stem sentences.

Have fun and I'll see you shortly.

Welcome back.

How did you get on and did you have fun? Let's have a look.

For each pair of shapes, tick the one with the longer perimeter and how do you know? Well, with A, the shapes are the same.

They're both equilateral triangles, but one is clearly bigger, so therefore it must have a longer perimeter.

What about B? Well, the shapes are very similar, aren't they? They're almost identical.

But have you noticed, the one on the right's got an extra bit on, like an extra arrowhead, which has increased its perimeter.

So that's got to have the longer perimeter.

And for C, they're two different shapes, aren't they? But the second shape has clearly got a much longer perimeter.

It would take you longer to run your finger around the outside of that shape, wouldn't you agree? And then the football pitch, let's have a look at that.

There are lots and lots and lots of possible answers here.

Here's just a few.

You might have said E has a smaller perimeter than A.

So that's the goal box has a smaller perimeter than the whole pitch.

You might have said B has a longer perimeter than C.

So one half of the pitch has a longer perimeter than the centre circle.

Many, many possible options there.

Well done if you got lots of them.

And number three, the objects from task A.

You were using string to compare them.

So here we go.

There's two of our objects and we could say the envelope has a longer perimeter than the number block, because the distance around it is longer.

You can see that when you unfurl that string.

And then you can flip that around and say the number block has a shorter perimeter than the envelope because the distance around it is shorter.

And then using those number rods to create pairs of polygons.

Here we go.

When we turn them into straight lines, that's what these look like.

That's just two examples.

You might have used very different shapes to that, but in this case we can say the triangle has a longer perimeter than the square because the distance around it is longer.

And then we can flip that around and say the square has a shorter perimeter than the triangle because the distance around it is shorter.

We've come to the end of the lesson.

That went really quickly and I've had lots of fun there and I hope you have too.

Today's lesson has been knowing that perimeter is a distance around a 2D shape.

Do you think you know that now? I think you do.

I think you've done really well.

All 2D shapes have got a perimeter so you can run your finger around the outside of all 2D shapes.

Perimeter is a measure of length.

It's the distance around the outside of a shape.

And if you can go away knowing that, you've done really well in today's lesson.

It's been a real pleasure spending this lesson with you and I really do hope I get the chance to spend another math lesson with you in the very near future.

But until then, enjoy the rest of your day.

Take care and goodbye.