Video: Identifying 3D Shapes matching a Description

There is a mystery shape in the bag. Choose from the following clues the one that will not help you guess the name of the shape. [A] It has a curved surface. [B] It has 2 flat faces. [C] It is small. Based on these clues, what is the shape in the bag?

03:22

Video Transcript

There is a mystery shape in this bag. Choose from the following clues the one that will not help you guess the name of the shape. It has a curved surface. It has two flat faces. Or it is small. Based on these clues, what is the shape in the bag?

And then we’re given three pictures of 3D shapes to choose from. How good are you at identifying 3D shapes? And what if you can’t see them? We’re told that there’s a mystery shape in this bag, and we can’t see it, can we? Now, we’re given 3 different clues to help us identify the shape. It has a curved surface, it has two flat faces, and it is small. Now, all three of these clues are true of our shape, but they’re not all helpful. We’re told to choose from these clues the one that’s not going to help us guess the name of the shape. Can you spot which one it is?

What are the sorts of things that we look for if we’re trying to work out the name of a 3D shape? Well, things like whether the shape has a curved surface or not and the number of faces that it has. That’s important too. These are the things that make a particular type of 3D shape, that shape. But if we look at our last clue, it’s talking about the size of the shape.

Now, if I said to you: “I’ve got a mystery shape that I want you to guess. It’s small. What do you think it is?,” you’d say to me: “That’s not enough information. Any shape could be small.” The size of a shape isn’t a property that we use when we’re trying to identify 3D shapes. Some shapes are big. Some shapes are small. Some shapes are somewhere in between. The clue that’s not really going to help us guess the name of the shape is it is small.

In the last part of the question, we need to identify the shape. Based on these clues, what is the shape in the bag? And we’ve got three possible shapes to choose from. Do you know the names of them? The blue shape is a cone. And we can see that that’s got a curved surface, but then so has our second shape, which is a cylinder, and also our third shape, which is also a cylinder. Our second clue tells us that the shape has two flat faces. Now, if we look at our first shape, we can see that it has a flat face at one end. But at the other end, it goes to a point. So this only has one flat face. This isn’t the shape we’re looking for.

But our second shape does have two flat faces. Cylinders have a flat face at either end, don’t they? Because our last shape is a cylinder too, we can see its two flat faces as well. So we know the name of the shape is a cylinder. But do you know what? To identify the actual shape that we’re talking about, we need to use that unimportant clue. Only one of our cylinders is small.

To guess the name of a shape, we don’t need to know its color, its size, or what position it’s in. And so the clue that was not going to help us guess the name of the shape is “it is small.” But once we’ve worked out that the shape was going to be a cylinder, we used all three clues to find that the shape in the bag was the green cylinder.

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