Mathematics K–6 · Years 3–4
Sharper or wider than a right angle: angle testers and turns
Measurement and space
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The idea
An angle is an amount of turn between two arms, and every angle can be compared with a right angle (a quarter turn) to name it acute, right, obtuse, straight or reflex.
What you need
- a sheet of scrap paper per child (folded twice to make a right-angle tester)
- two card strips 15 cm long joined at one end with a split pin (an angle maker)
- a set of 10 angle cards drawn in different sizes and orientations, including one with short arms and one with long arms
- a chalk circle on the playground or a masking-tape cross on the floor
How to do it
- Fold the paper in half and in half again so the folded corner is square. Fit the corner into corners around the room (book, door, tile) and sort them: smaller than, equal to, bigger than the tester.
- Stand on the cross facing one arm. Make a quarter turn, a half turn, a three-quarter turn and a full turn, saying how many right angles each turn is.
- Open the angle maker to match each angle card. Compare with the tester and name each angle acute, right, obtuse, straight or reflex.
- Compare the card with short arms and the card with long arms, which have the same opening. Use the tester to show they are the same angle.
- Draw one example of each angle name in your book and mark the turn with an arc.
What you should see
A quarter turn is one right angle, a half turn is two right angles (a straight angle) and a full turn is four. Book corners and door corners fit the tester exactly (right angles), a partly opened pair of scissors makes an acute angle, and the angle between the hands of a clock at 4 o'clock is obtuse. The short-armed and long-armed cards match the same opening on the angle maker, so arm length does not change the angle. The tester settles each card as smaller than, equal to or bigger than a right angle, so both partners can check the same name.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- An angle with longer arms is a bigger angle.
- A right angle has to have one arm pointing straight up.
Safety card
Hazards
- split pins have sharp ends
Controls
- an adult fits the split pins and folds the ends flat
Note
No chemicals or heat are used, so the NSW Department of Education Chemical Safety in Schools package is not needed; ordinary classroom supervision under the school's usual risk management.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
Sources
The pages the author read to write this activity.