Lesson video

Hi, everybody.

We are going to be moving on with our unit today and we're going to be looking at calculating the perimeter of 2D shapes in centimetres and millimetres.

So, before we begin, put on your hats, cho, tighten those ties, c-c-cho and tell the computer, now I'm a mathematician.

So, here is today's lesson agenda.

First off, we're going to do a quick do now, to identify 2D shapes, we're then going to do our star words then we're going to calculate the perimeter of 2D shapes together, before moving on to your independent task.

But before we begin this lesson, please make sure you have a pencil, a piece of paper and a ruler, if not, please pause this video now and go get them.

Great, let's get started.

Here we have a selection of six 2D shapes, pause this video and identify their names.

Great job, we have a rectangle, a pentagon with five sides, a triangle, a square, a hexagon with six sides and a circle.

Here are our star words.

We have perimeter, measure, calculate, total distance and altogether.

Remember, that we need to make sure that we use our star words, let's practise them again.

Perimeter, measure, calculate, total distance, altogether, great.

Here we have Billy the Mole, and Billy the Mole walked the perimeter of this equilateral triangle, but what do you think perimeter means? Well, perimeter means that's he's taking a walk the whole way round the outside of our triangle.

So, the perimeter is the distance around the shape.

So, imagine your Billy the Mole walking around our perimeter.

So, how do we find the perimeter of this shape? We know that one side is six centimetres, the second side is six centimetres and the third side is eight centimetres.

So, pretend your this teeny weeny ant and he's going to walk the perimeter of this shape.

So, he's going up six centimetres, down six centimetres and across eight centimetres.

So, we actually need to add them together, so 6 add 6 is equal to 12 and 12 add eight is equal to 20.

Therefore, the perimeter of this shape is six, ooh! The total perimeter of this shape is 20 centimetres altogether.

Here, we have another triangle.

The first side is six centimetres, the second side is six centimetres and the third side is six centimetres.

Our little ant is going to go for walk all around the perimeter of this shape.

So, we need to add, six add six add six, six add six is equal to 12, and 12 add six is equal to 18, therefore, the total perimeter of this shape is 18 centimetres altogether.

How about this one? Pause the video and give it a go.

Super job, remember that little ant, he's going to be walking up six centimetres, across 10 centimetres, down six centimetres and across 10 centimetres.

So, we're going to do some adding.

Six add six is equal to 12 and 10 add 10 is equal to 20, have I finished? No, I haven't finished, I need to add those two together, 20 add 12 is equal to 32 centimetres, therefore, the total perimeter of this shape is 32 centimetres.

Here we have a shape.

Hmm, what shape does this look like? What shape, what shape? Ooh, it's a square but it's on its side, and each of its sides will be the same.

So, they are all seven centimetres, so we need to remember our little ant going on a walk around the perimeter of the shape.

So, seven add seven is equal to 14, seven add seven is equal to 14, add the two together and you get 28 centimetres.

Therefore, the total perimeter of this square is 28 centimetres.

Whew, now this is a shape, let's count the sides.

One, two, three, four, five, six, seven, eight.

This is an eight sided shape and this eight sided shape is called an octagon.

And what I know about an octagon is that all the sides are going to be the same length on this octagon.

Now remember, you're a little ant, off you go on your long walk.

Now, because it's an eight sided shape, we know we need to add four, eight times or we can do four times eight or eight times four, which is equal to 34.

Therefore, the total perimeter of this octagon is 32 centimetres.

Now, it's your turn.

Here we have four different shapes, now, some of them are in centimetres and some of them are in millimetres.

Give it a go, pause this video.

Great job, everyone, super, super work.

Let's keep going.

So, here we have our shapes and quickly we have, the total perimeter of the first triangle's 20 centimetres, the total perimeter of the square is 16 centimetres, the total perimeter of the second triangle is nine centimetres or 90 millimetres, and finally, the total perimeter of the rectangle is 70 millimetres or seven centimetres.

Give yourselves, one, two, three, a one, two, three.

Great job, now, here we have a square on a piece of grid paper, how on earth are we going to measure the perimeter of this shape? So, each square is one centimetre and my rectangle is four squares by three squares, so what is the perimeter of my rectangle? So, I know I have to go across four, down three, across four and up three.

So, we need to add, four add four add three add three.

Four add four is equal to eight, so we add three is equal to six and when you add the two together, it comes to 14 centimetres.

Therefore, the perimeter of my rectangle is 14 centimetres.

Super, for you're independent task today, you are going to find the perimeters for these four shapes.

Remember, each square is one centimetre, so be very, very careful when you're doing your adding up.

Remember to pause the video.

Super job, everyone, let's go through these together.

The perimeter of shape A is 24 centimetres.

The perimeter of shape B is 26 centimetres.

The perimeter of shape C is 40 centimetres.

And the perimeter of shape D is 46 centimetres.

Time do some ticking and fixing.

Well done, everyone, today has been a super, super lesson.

If you'd like to, please ask your parent or carer to share your work on Instagram, Facebook or Twitter, tagging @OakNational and #LearnWithOak.

See you again soon.