Lesson video

Hello there, my name is Mr. Tilstone.

I'm a teacher and the thing I like teaching most just has to be maths.

So I'm really excited to be spending this maths lesson with you.

We're going to be learning about perimeter today.

Perhaps you've been learning about perimeter recently and you already know quite a lot.

Well, let's see if we can teach you even more if you're ready, I'm ready, let's begin.

The outcome of today's lesson is I know that perimeter is measured in units of length and can be found by counting or measuring units.

So hopefully you've had some recent experience exploring the concept of perimeter and you know a little bit about what that is.

Maybe through string or drawing around outlines, that kind of thing.

Now we're going to look at the units of length that can be used for perimeter.

And we've got two key words.

My turn perimeter, your turn, my turn unit, your turn.

What do those words mean? Let's have a reminder.

The distance around a two dimensional shape is called the perimeter.

So here's a rectangle and here's a distance around it.

And that's the perimeter.

And a unit is one of something that defines the one that you are counting in.

So we're going to be exploring units in this lesson.

The lesson's split into two cycles.

The first cycle will be measuring perimeter by counting and the second measuring perimeter using centimetres.

So we're going to get quite specific.

So let's start if you are ready by measuring perimeter by counting.

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 and very good, they are too.

Got a number block here.

You might have one of these in front of you.

And if you have, why not use it? Why not explore along with us? What can you say about this number shape? What do you think? Anything? Well, Lucas says the outline of the shape is a rectangle.

So it has a perimeter.

Yes it does.

It's a polygon, it is shape a 2D shape it's got a perimeter.

The sides aren't the same length.

No, that's true.

But the shape is split into squares which do have the same side lengths.

Yes, can you see? So it's made of two different squares and because they're squares, the side lengths are the same.

Here we go, look, so that's one side length.

That's another, the same.

Another that's the same.

Another that's the same.

Another that's the same.

And another that's the same.

The perimeter can be thought of as six equal parts just like we've just seen.

At the moment the size of these parts is unknown.

We've not measured them, but they can be called units.

So that's quite a general term.

And we've got a stem sentence here.

I'll say it, then we'll say it, then you'll say it.

The shape has a perimeter of mm units.

Shall we say that together? You ready? The shape has a perimeter of mm units.

Now just you.

You ready? Go Fab, and we're going to explore that stem sentence.

In this case it's got a perimeter of six units.

We counted it six times, six equal parts.

Let's have a look at this.

This is a new number shape.

What's the same and what's different about this shape compared with the one that you've just seen what's it got in common? And again, if you've got those shapes, you can compare them in front of you now.

What's the same and what's different? And we've got that stem sentence again that you might wish to explore.

Well, it's rectilinear, which the last one was as well, but it's not a rectangle, which the last one was.

So rectilinear but not a rectangle.

It is still split into squares just like the other one was.

So the perimeter can still be counted in units just like we did before.

Maybe it's a different number this time.

Now let's have a look.

So 1, 2, 3, 4, 5, 6, 7, 8.

We're ready to complete that stem sentence.

Shall we do it together? The shape has a perimeter of 8 units.

Lucas says, I wonder if the perimeter will be different if I start counting from a different vertex.

What do you think? If he changes the vertex, will that give him a different number? Hmm.

Should we see? So this time's gone from this vertex, so the bottom right hand corner as we can see it.

So that's 1 count with me, 2, 3, 4, 5, 6, 7, 8 hasn't changed exactly the same as before.

It still gave the same perimeter.

So let's do that stem sentence again.

This shape has a perimeter of 8 units.

So it didn't matter which vertex we started from the perimeter was the same.

Okay, so we've got Laura and Lucas and we've got that same stem sentence, new shape.

Lucas says, I counted the units this shape has a perimeter of 8 units.

Laura says, I counted the units.

This shape has a perimeter of 10 units.

Hmm.

So either one of them is wrong or they're both wrong.

Who's correct, if anybody? And can you explain why? Let's have a think.

Why might one have said the wrong answer here.

Let's have a look.

Lucas was correct, so it was 8 units.

Laura may have counted one of the sides twice.

Hmm, to give her a bigger number than she needed.

That would make sense, wouldn't it? That would be why she got 10.

A good tip is to sketch a starting point so that you know when to stop counting.

It doesn't have to be from that vertex, it can be from any of the vertices, but we've got a starting point so therefore we've got a finishing point as well.

We're not gonna count them twice like Laura did.

So there we go.

Top tip.

So 1, 2, 3, 4, 5, 6, 7, 8.

So we know to stop there because that's where we started.

So that's a really good tip.

So the shape has a perimeter of 8 units.

You could instead put a dot against each unit so that you know you've counted to them all.

That's a really good tip too.

Let's have a look at that in action.

1, 2, 3, 4, 5, 6, 7, 8.

Now we can be absolutely sure that we've counted all of them and that we haven't counted any of them more than once.

So the shape has a perimeter of 8 units.

So some really good ways to check that you're doing it properly.

Let's have a check, which of these answers is correct for the perimeter and why might each of the incorrect answers have been given? Okay, so we've got A 6, B 8, C 10 and D 11.

Only one's right, but can you figure out why they might have got the wrong answer? Use that stem sentence and pause the video.

Let's have a look.

So you can see we've got a little starting point there.

It was drawn on that vertex.

It could have been drawn from any of the vertices, but we use that one.

That's a start point and a stop point.

When we count them all, we've got 10.

So 10 is the correct answer using that stem sentence.

The shape has a perimeter of 10 units, but I wonder where we've got 6, 8 or 11 from.

I think they're plausible wrong answer.

Let's see.

So if you said 6, that's a number of sides, not the perimeter, that's 6 sided shape, but that's not what we're asking for.

8, perhaps one of the sides of the shapes been missed so the units were not counted, that's possible, isn't it? Not counted all of them.

And 11, perhaps one of the sides of the shapes been counted twice so that its units were counted twice.

But having that start and stop point should stop that from happening.

Lucas wants to measure the perimeter of his mini whiteboard.

Why might that be difficult? Hmm.

Think about what's different between the mini whiteboard and those number shapes.

Think about what this hasn't got that they did have that made it easier.

Unlike the number shapes, this shape is not split into squares.

So it's actually difficult to count the units.

It's just one big rectangle.

You can't see the squares inside it.

It would be helpful if we could, but we can't.

Can we still measure the perimeter, I wonder in units? Lucas has got a really good idea.

He says I have some matchsticks that are equal in size.

I could lay them down and count each one as a unit.

Ha, that's a great idea.

Long as they're equal in size, that will work.

Let's have a look, so there's a matchstick 1, count with me, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14.

Let's do the stem sentence together.

The shape has a perimeter of 14 units.

That was a really great idea from Lucas.

Let's have a check.

What's the perimeter of this trading card? You can't do it yet, but if I click again, you can see we've got some matchsticks to help us.

So what is the perimeter of that trading card? Now a little tip, make sure you're not counting the sides more than once and make sure that you are counting all of the sides as well.

Use that stem sentence and pause the video.

Let's see.

Well you could have started from any of the vertices really.

I'm going to start from the top left one.

So 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.

The shape as a perimeter of 10 units.

And if you've got the number 10, well done.

You are on track.

Okay, we've got two different shapes here.

What's the same and what's different about shapes A and B? Hmm, we've got our stem sentence again.

The shape has a perimeter of mm units.

Well know if that's going to be helpful.

What's the same and what's different? They look very, very similar don't they? So they different about how they look though as well.

Have you seen it? Have you spotted it? Well they're both rectangles are exactly the same size, so they will have the same perimeter.

But what's different? The squares inside shape A can be seen but the squares inside B cannot be seen.

Does this matter? Are you still going to be able to count the perimeter? What do you think? Yes you are.

It's the sides of the squares that are counted and we can still do that even when we can't see the squares inside the shape.

So 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.

It was exactly the same process for both.

Let's do the stem sentence.

The shape has a perimeter of 10 units.

Let's have a check.

What's the perimeter of this shape? So once again you can't see the squares inside it but doesn't matter.

See if you can still count the perimeter anyway.

And can you use that stem sentence please? Pause the video.

Did you manage it? I wonder which vertex you started from.

Doesn't matter as long as you're finished at the same one.

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12.

The shape has a perimeter of 12 units.

Well done if you've got that, you are definitely on track.

It's time for some practise.

Task one, what is the perimeter of each of the number shapes? And you may wish if you've got some in your classroom to use real number shapes to help.

That way you can maybe put your finger on one of the vertices so that you'd know to start and stop at the same place.

And you're going to use a stem sentence.

The shape has a perimeter of mm units.

Number two, give the perimeter of these shapes by counting and then you're gonna use a stem sentence each time the shape has a perimeter of mm units.

Remember again to have a start and stop point, you might want to use dots or you might want to just put a little mark for a starting and stopping point, up to you.

But don't count the sides more than once and make sure that you have counted all of the sides.

Number three, the shape picture represents the outline of the Oak Academy field as seen from above.

So like a bird's eye view, is Lucas correct? Explain.

Let's see what Lucas has got to say.

He says, I've counted the squares around the outline and the school field has a perimeter of 30 units.

Hmm, okay, see if you agree or disagree and then explain why.

And number four, use equal sized matchstick or straws or something like that.

They've gotta be equal sized to find the perimeter of the outlines of some different rectilinear objects in your classroom.

Some examples of rectilinear objects are sticky notes, tablets, that kind of thing.

And you going to use this stem sentence again that mm has a perimeter of mm units.

So have fun with that.

I'll see you soon for some feedback.

Pause the video.

Welcome back, let's have a look, how did you get on? So number one, we're gonna use that stem sentence.

So if we've got starting point, we'll have a finishing point too 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 shape has a perimeter of 10 units.

This one's got 12, this one's also got 12.

They've got the same perimeter even though they're different shapes.

You might have explored that before.

This one's got a perimeter of 14 units.

This one's also the perimeter of 14 units.

Number 2, give the perimeter of these shapes.

So for A, the shape has a perimeter of 12 units for B, the shape has a perimeter of 12 units, even though they're different shapes so they've got the same perimeter for C, quite a lot to count there.

So we're going to make sure that we've got a starting and stopping point.

It's a good tip for any shape as well.

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20.

This shape has a perimeter of 20 units.

And what about D? Let's have a look at D.

That's got a perimeter of 12 units.

And then Lucas counted too many squares.

He shouldn't have counted the corner squares 'cause they're not sides of the shape so they don't count.

They're not sides of the shape are they? They're linked to the vertices of the shape, but they're not sides so he shouldn't have counted them.

And it's counted that one as one unit.

But can you say it's actually got two countable sides? The perimeter is actually 26 units.

So very well done if you explain why he was wrong and especially well done if you said how many it did have, brilliant.

And then in this case that tablet has a perimeter of about 14 units.

It's not exact look.

Can you see one of 'em goes over a little bit, but we can say it's about 14 units.

I think you are ready for cycle two.

You're doing really well.

This is measuring perimeter using centimetres.

Lucas has just learned that the squares in his maths book have a length of one centimetre each.

And that might be the case in your maths book as well or it might not.

So this shape has a perimeter of 8 units.

That's what we've already established.

But because they're centimetres, each of those lines is 1 centimetre.

We can use centimetres instead.

So we don't need to say unit in this case we can say centimetres, it's been renamed.

So the shape has a perimeter of 8 centimetres.

That unit's been named.

Well let's have a quick check.

What is the perimeter of this shape? This time, use centimetres as your unit.

Pause the video.

Let's have a look.

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12.

The shape has a perimeter of 12 centimetres.

Well done if he said that word.

Now Lauren knows that perimeter can be measured in centimetres she's eager to find out the perimeter of this leaf.

If you've still got some leaves your classroom, you could do this too, that would be fantastic.

How could she do it? Hmm, but it's not rectilinear how could she measure the perimeter of that leaf in centimetres? Have you got a good idea? Hmm? Let me give you a clue.

What if you use string, hmm? She draws around the outline of the leaf You've probably done that before, there we go.

Take the leaf away.

Don't need the leaf anymore.

That's showing the perimeter.

Now she takes some string, she goes round the outline like so, now she turns the string back into a straight line.

Again.

you've probably done this before.

Now what could you do with the straight line? She can measure the straight line.

There you go, measure it.

And in this case, can you see what it measures? So we've started at the zero on the ruler and it's 14 centimetres.

So we can say the leaf has a perimeter of 14 centimetres.

Let's have a check, explain to a partner how string can be used to find the perimeter of an object's outline in centimetres.

Pause the video and explain that as clearly as you can, please.

What did you say? It might have been something a bit like this.

Number one, draw around the objects outline.

Number two, use string to go around the outline.

Three, turn the string back into a straight line.

And four, measure the string using a ruler, metre stick or tape measure.

And that works with any objects, not just leaves.

Time for some final practise I think number one, give the perimeter of these shapes in centimetres.

So they're rectilinear shapes.

Can you give them in centimetres please? And use that stem sentence each time.

The shape has a perimeter of mm centimetres.

And number two, use string to find the perimeters of the outlines of various leaves or similar objects in centimetres.

Have fun with that.

I think you will.

Pause the video and I will see you soon.

How did you get on? Did you have fun exploring those objects with string? Hopefully you did.

So the perimeter of these shapes in centimetres A, the shape has a perimeter of 16 centimetres.

You can count 16 of those centimetres.

This shape has a perimeter of 20 centimetres.

And C, this shape has a perimeter of 22 centimetres.

And again, just like before, maybe you made a little mark for a start and stop point or maybe you put dots each time you counted something like that to make sure you counted them all.

And number two, use strength to find the perimeters of the outlines of various leaves or similar objects in centimetres.

So we have got a leaf here, in this example, the string that went around the outline of the leaf made a straight line and look, it measured 55 centimetres.

That's just one example.

You might have used leaves, you might not.

And you might have had different numbers to that.

You probably did.

And we've come to the end of the lesson.

Today's lesson has been knowing that perimeter is measured in units of length and specifically you looked at centimetres.

It can be found by counting or measuring units.

So you've done lots of counting today.

Perimeter is measured in units.

The units can be informal ones and simply called units.

That's quite a general term or standard units of length such as centimetres.

And there are other ones as well.

We looked at centimetres today, but it can be measured in things like millimetres, kilometres, that kind of thing.

Counting the units around the outside of a shape gives the perimeter, you've done lots of that today.

The shape pictured here has a perimeter of 12 centimetres.

Perimeter can also be measured in standard units such as centimetres, which you've done today by using rulers or tape measures.

I've had such great fun with you today and I hope you've had fun too exploring perimeter with standard units.

Hope you enjoy the rest of your day.

Take care and goodbye.