Video Transcript
How many straight edges does this
shape have?
We can see a picture of a 3D or a
solid shape here. Can you think of any objects that
are the same as this shape? Looks the same as perhaps a block
of wood or maybe a long brick. The name of this shape is a
cuboid. We know that it’s a cuboid because
it has rectangular faces. It has four of these long
rectangular faces all the way around. And then, at each end is what looks
like it might be a square face. But we can just call them
rectangles because squares are a special type of rectangle anyway.
So, this shape, with rectangular
faces along the way around, is a cuboid. But although it’s helpful for us to
think about the faces of this shape, the question doesn’t ask us about its
faces. We need to say how many edges this
shape has. Do you remember what the edge of a
3D shape is? An edge of a 3D shape is the place
where two of its faces meet. It’s where the faces touch. So, if we pick two of the faces on
this particular cuboid, we’ve got this long rectangular face here and the square
face on the end.
Can you see the place where these
two faces meet? It’s along here. This is one of the edges of the
cuboid. If you imagine this cuboid is a
long cardboard box and you open it out and flatten it, we’d be able to see this
edge. It would be one of the folds. It’s where two faces meet. Did you notice this edge was one of
the black lines that makes up our picture? But unfortunately, because this is
a picture of a solid shape, we can’t see all of the edges. Some of them are hidden around the
back. Let’s try and visualize what our
shape looks like around the back. And we’ll use dotted lines to draw
in the extra edges.
Well, from what we know about
cuboids, the face on the bottom of this shape is exactly the same as the face on the
top. It’s going to be another
rectangle. In a way, this cuboid doesn’t
really have a top and a bottom because all we’d have to do is turn it upside down;
it’d look exactly the same. So, let’s draw in our missing sides
of this rectangle on the bottom of the shape, one at the side and one along the
back. And the sides of our rectangular
face become the edges of this 3D shape.
We’ve got one more edge that’s
missing. Can you spot where it belongs? It’s round the back on the left
here. And it shows us that the end of our
cuboid, there’s a square face. Can you see now that drawing in
these dotted lines really helps us to visualize the shape? We don’t need to imagine what it
looks like around the back; we can see.
Now that we’ve drawn in all of the
edges, let’s count them. There are four straight edges that
go all the way around this square face on the right. Then, there are another four edges
around the opposite end of the cuboid. And then, finally, there are four
more much longer edges that run along the length of the cuboid. So, that’s three groups of
four. We know that three times four
equals 12. So, we can say that the number of
edges that this shape has is 12.
