Teacher talks: Video playlist

[THEME MUSIC] [SCHOOL BELL RING] Hello, I’m Mr Smith, and together we’re going to explore perimeter on a grid.

Perimeter is part of the strand of measurement in mathematics.

Perimeter is the path that goes around a two dimensional object.

We often think of it as the length around the outside of a shape.

If I was to walk around the outside of a field, I would be walking around the perimeter of the field.

Have a look at this shape.

I have drawn a rectangle onto a grid with squares.

Can we work out what the perimeter is around the outside of the rectangle using the squares to help us?

Let’s work it out together.

So, this side is three squares long.

Then, four squares, three squares, and finally four squares.

So, altogether the perimeter is 14 squares in total.

If each square represents 1cm, the perimeter will be 14cm.

We can record it like this: Perimeter equals 14cm.

Let’s try another example.

I’m going to show you a shape on a grid.

On this grid each square is equivalent to 1cm.

See if you can work out the perimeter.

Mm, what’s the perimeter of this rectangle, if each grid square is 1cm?

Remember, you can press pause to give yourself time to work it out.

How did you do?

Let’s see.

We’ve got 5cm along the top and bottom, and then the sides of 4cm each.

So, the sum we need to do is five, add five, add four, add four.

I worked that out to be 18cm.

Well done if you got that answer too.

I write it down like this:

Perimeter equals 18cm.

Have you noticed an interesting fact about the sides of the rectangles?

In the examples we’ve seen so far, two of the sides have the same length.

I wonder if there’s a special type of rectangle where all of the sides would be the same in length.

Have you worked it out yet?

Yes, of course — it’s a square.

In this example each side is 4cm.

To work out the perimeter

we can either count up in fours —

four, add four, add four, add four —

or we can multiply:

Four multiplied by four.

Whichever method we use, we get to the same answer: Sixteen.

So, our perimeter is 16cm.

This time I have a different challenge for you.

You need something to write with and something to write on.

Pause the video if you need to go and grab a pencil and paper.

Now draw a rectangle.

Any rectangle will do.

Now, imagine this rectangle

has a perimeter of 10cm.

Can you work out what the length of each side might be?

Yeah, and you guessed it.

There is more than one answer.

Press pause while you solve the problem.

How did you get on?

Here is one possible solution:

In this rectangle the two shorter sides are 2cm.

And if we add those together we get 4cm.

That leaves us with the remainder of 6cm.

So, the other two sides are 3cm each.

Compare your rectangle to mine.

What’s the same

and what is different?

I think we’ve got perimeters sussed.

Remember, perimeter goes around the outside of a shape.

Why not see if you can work out the perimeter of some of the items you’ve got lying around at home?

Some will be harder than others.

Happy practising!