### Video Transcript

Roll, Stack, and Slide

In this video, we’re going to learn
how to sort out 3D shapes based on whether we can roll them, whether we’re able to
stack them, or whether the shapes will slide. Some shapes only do one of these
things, some do two, and some even do all three.

Let’s begin by thinking about these
three doing words: roll, stack, and slide. Now, we know when something rolls,
it turns over again and again and again. Now, for a 3D shape to roll, it
needs to have a curved surface. That’s what makes it roll. This bowling ball is a sphere. And one thing we know about spheres
is that they have a curved surface all the way around. So when we let go of our bowling
ball at the end of the alley, it’s going to start to roll. If a shape has a curved surface, it
rolls.

Now, when something slides, it
moves smoothly across a surface without turning over and over. There’s a word that rhymes with
slide that means a very similar thing, glide. Now, for a 3D shape to slide, it
needs to have a flat surface. This cuboid shape isn’t going to
roll, but it will slide if we push it. This is because it has a flat
surface. In fact, cuboids have lots of flat
surfaces.

When we stack something, we put it
on top of something else. For us to be able to stack a 3D
shape, it needs to have a flat surface at the top and also at the bottom. This tin of beans is a
cylinder. Although it has a curved surface
all the way around, it has a flat surface at one end and another flat surface at the
other. This means that we can stack
cylinders. And that’s why when we go to the
supermarket, we often see tins that are stacked up.

Now, we did say at the start of the
video that some shapes can do all three. They can roll, they can slide, and
they can be stacked. And cylinders are interesting
because they’re one of these shapes. We already know that they can be
stacked. If we put them standing up on their
flat surface and push them, they’ll slide. But cylinders also have a curved
surface too. And if we turn this cylinder on its
side and push it, it’s going to roll. So a cylinder is a really good
example of a shape that we can roll, stack, and slide.

Do you think you know enough now to
be able to tell whether we can roll, stack, or slide a shape? Let’s try some questions and
see.

Where does this shape belong? Can roll or cannot roll.

In this question, we can see a
picture of a cylinder. We can tell it’s a cylinder because
it has a curved surface all the way around. But there are two flat surfaces
too, one at each end. Our question asks us, where does
this shape belong? And we’re given two possible
groups. Does it belong with the shapes that
can roll? Or does it belong with the shapes
that cannot roll?

We know when something rolls, it
turns over and over. And we know that a 3D shape can
roll if it has a curved surface. That’s why if we look at the shapes
in the first group, they each have a curved surface. This is what helps them turn over
and over as they roll along. If you look at the shapes in the
second group, we can’t see any curved surfaces at all. That’s why they cannot roll.

So to help us decide where to put
our cylinder, we could simply ask, does a cylinder have a curved surface? The answer is yes. A cylinder does have a curved
surface all the way around. So if we turn our cylinder on its
side, we can make it roll. Our shape belongs in the group of
3D shapes that can roll.

Which group does this shape belong
to? Cannot slide or can slide.

In this question, we’re shown a
picture of a 3D shape. If we look at it carefully, we can
see that it has a curved surface all the way around. At one end, there’s a flat
surface. And at the other end, it goes to a
point. It’s a cone. We need to think carefully about
whether cones can slide or not because we’re given two groups. And we need to decide which one to
put our cone in. Shall we put it with the shapes
that can slide? Or what about this sphere here
which cannot slide?

We know that when something slides,
it moves smoothly across a surface. It’s not the same as rolling over
and over. And so we know that 3D shapes can
slide only if they have a flat surface. That’s why this sphere is in the
“cannot slide” group. It doesn’t have a flat surface at
all. It’s completely curved all the way
around. And when we push it, it rolls. But if we look at our two shapes in
the “can slide” group, we can see that they both have flat surfaces. So by pushing them, we can make
them slide across a tabletop.

Now, let’s look at our cone. Does it have a flat surface? Yes, it does. It’s on the bottom of this
particular picture. So if we put our cone on the
tabletop like this and push it, it’s going to slide. It has a flat surface. And so because our 3D shape has a
flat surface, the group that it belongs to is the group of shapes that can
slide.

Think about shapes that can roll,
stack, and slide. One of these shapes can roll and
slide. Which one? One of these shapes can slide and
stack. Which one?

This question encourages us to
think about whether shapes can roll, stack, and slide. And you know some shapes can do
more than one of these things. The first part of the question asks
us to find one of the shapes that can both roll and slide. And we’re given two shapes to look
at. The green shape is a cube, and the
red shape is a cone.

Now, which of these two shapes can
do two things, roll and also slide? Now, for a 3D shape to slide along
if we push it, it’s going to need to have a flat surface. But we can see that both of our
shapes have at least one flat surface. So they’re both going to be able to
slide. But for a 3D shape to roll, it’s
going to need to turn over and over again. And for this, it’s going to need a
curved surface. Which of our two shapes has a
curved surface? It’s the cone. It has a flat surface and a curved
surface. And if we stand it upon its flat
surface, it will slide. But if we turn it onto its curved
surface on the side, we can make it roll too.

In the second part of the question,
we’re shown two more 3D shapes. The first shape is not a cuboid,
but it might remind us of one. Looks a little bit like a cuboid,
doesn’t it? And we know the name of our second
shape. It’s a pyramid. And we’re told that one of these
shapes can slide and also stack. Is it our unusual yellow shape or
our purple pyramid?

Well, we already know that to slide
a shape, it needs a flat surface. And both of these shapes do have a
flat surface. If we put them on a tabletop and
push them, they’d both slide. But which one could we stack on top
of each other? Well, for us to be able to stack a
3D shape, it needs to have flat surfaces at either end. Our pyramid might have a flat
surface at one end, but at the other end there’s a point. Can you imagine trying to balance
one pyramid on top of another? We can’t stack pyramids.

But if we look carefully at our
first shape, we can see that it does have flat surfaces opposite each other. Because it’s such an unusual shape,
it might make a bit of a wonky tower, but we can still stack it. The shape that can both roll and
slide is the red cone, and the shape that can both slide and also stack is the
yellow shape.

So what do we learn in this
video? We’ve learned how to sort 3D shapes
based on whether they can roll. And for shapes to roll, we know
they need to have a curved surface. Whether they can stack — we’ve
learned that to do this, we need flat surfaces at both ends. Or whether they can slide — and to
do this, we’ve learned that a 3D shape needs a flat surface to slide on.