Understand the idea.
State the rule
To sort solids, say the rule before placing them in groups. “Has only flat faces” and “has a curved surface” use shape features that you can check on every object.
Look around the whole solid, including hidden parts. Colour or size may help you recognise an object in a picture, but these do not tell whether its surfaces are flat or curved.
An object may fit two groups
Read the word only carefully. A cylinder has flat faces, but it does not have only flat surfaces: its side is curved. “Has flat faces” allows curved parts as well.
Groups can overlap when their rules allow it. A cylinder belongs to both “has flat faces” and “has a curved surface”. If every object must go into just one group, use rules that do not overlap.
See the method in action.
Only flat faces
Which pictured objects have only flat faces?
Worked solution
- 1
Check A and D against the rule.
The cube A and rectangular box D have flat faces all around, with no curved surface.
- 2
Check B and C as well.
The cylinder B has a curved side and the sphere C has a curved surface, so neither meets the word only.
- 3
List all the matching labels.
A and D are exactly the pictured objects that have only flat faces.
A and D.
Both features
Can a cylinder fit “has flat faces” and “has a curved surface”?
Worked solution
- 1
Check for flat faces.
The cylinder has two flat circular ends, so it meets the first rule.
- 2
Check for a curved surface.
Its side curves around between the circular ends, so it meets the second rule.
- 3
Allow membership in both groups.
Neither rule says only, so the same cylinder can satisfy both descriptions.
Yes: both descriptions are true.
Try it yourself.
Check your understanding
Do both a cube and a cylinder have flat faces?
Show answer & reasoning
Yes
Yes. A cube has six flat faces and a cylinder has two flat circular faces. The cylinder’s curved side does not remove its flat faces.
Keep practising
6 worksheet questions
Read the questions online
- List the labelled objects with a curved surface.
- Which labelled object has no flat faces?
- A cube and rectangular box are grouped together. Suggest a shape rule that fits both.
- A sphere and cylinder are grouped because they can roll. Is that a sensible rule? Explain.
- Give a feature that separates a sphere from a cylinder.
- Sort cube, cylinder and sphere into “only flat surfaces” and “has a curved surface”.
Read the worked solutions
- List the labelled objects with a curved surface.
The cylinder and sphere have curved surfaces.
B and C. - Which labelled object has no flat faces?
C is a sphere.
C. - A cube and rectangular box are grouped together. Suggest a shape rule that fits both.
Other correct shared-feature rules, such as both have corners, are acceptable.
Example: both have only flat faces. - A sphere and cylinder are grouped because they can roll. Is that a sensible rule? Explain.
Both have a curved surface.
Sharing a feature does not make the solids identical.
Yes, if the rule is rolling on a curved surface. - Give a feature that separates a sphere from a cylinder.
Both have curved surfaces, so that feature would not separate them.
The cylinder has flat faces; the sphere has none. - Sort cube, cylinder and sphere into “only flat surfaces” and “has a curved surface”.
This rule places each object in one group.
Only flat surfaces: cube. Has a curved surface: cylinder and sphere.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum
Solid-object features; comparing/classifying objects; turns; smaller/larger than a right angle.
Official curriculum source ↗