Acute and obtuse angles: what children learn
Year 4 pupils identify acute and obtuse angles and compare and order angles up to two right angles by size.
In this game children sort angles into acute, right angle and obtuse, using clock hands, shape corners and turns. An acute angle is less than a right angle. An obtuse angle is greater than a right angle but less than a straight line.
This topic covers the Year 4 statutory requirement in the National Curriculum in England (Geometry – properties of shapes): pupils should be taught to “identify acute and obtuse angles and compare and order angles up to 2 right angles by size”. Year 4 pupils are usually aged 8–9.
For families following US standards, the closest Common Core standard is 4.G.A.1, which is taught in Grade 4.
How to play
Sort the cards into the right groups. Tap a card, then its group. You can also drag a card, or press the number on its group. Each game has 3 rounds with no timer. Every card is checked as soon as you place it: a card in the wrong group comes back with a hint, and you get a point for each card you place right first time. For a quicker go, choose Quick quiz: 10 multiple-choice questions in 150 seconds (choose Relaxed for more time).
Step by step
- Start with a right angle. Compare each angle with a square corner.
- Acute. If the angle is smaller than a right angle, it is acute.
- Obtuse. If it is bigger than a right angle but less than a straight line, it is obtuse.
- Order angles. Put acute angles first, then right angles, then obtuse angles.
Worked examples
Example 1: The angle between clock hands at 2 o'clock
Working: Smaller than a right angle.
Answer: acute
Example 2: The angle between clock hands at 3 o'clock
Working: Exactly a square corner.
Answer: right angle
Example 3: The angle between clock hands at 4 o'clock
Working: Wider than a right angle, but less than a straight line.
Answer: obtuse
Common mistakes and how to fix them
- Mixing up the words acute and obtuse. Acute angles are 'a cute' little angles. Obtuse angles are the wide ones.
- Judging size by arm length. Draw two angles the same size with different arm lengths and compare.
- Calling a straight line an obtuse angle. A straight line is two right angles. Obtuse angles are less than that.
Activities to try at home
- Make angles with arms or with two pencils and name them.
- Spot acute and obtuse angles in the letters of a name.
- Use the hands of a clock to show an acute and an obtuse angle.
Ideas for the classroom
- Pupils order a set of drawn angles from smallest to largest.
- Give pupils a shape and ask them to label each angle as acute, right or obtuse.
- Make angles with geostrips and challenge a partner to name them.
Free printable game and worksheet
The free printable starts with a game sheet, such as a colour-by-answer picture, an answer maze or a secret-code joke, followed by a worksheet. Both use the same checked questions as the online game, and the answers are on the last page. Print it, or save it as a PDF from your browser’s print menu.
Open the free printable game and worksheet
Frequently asked questions
When do children measure angles?
Year 5 pupils measure angles in degrees with a protractor. Year 4 is about comparing and naming.
What is a reflex angle?
An angle greater than two right angles. Reflex angles are introduced in Year 5.
Can it be played with a keyboard?
Yes. Tab to a card, press Enter to pick it up, then choose the group it belongs in.