Lesson 4 of 10
Measuring an angle
Show on a big screen"Bigger" and "smaller" are not enough. Maths needs a number for a turn.
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Step 1 of 4
- The protractor scale has many equal steps.
- A second arm is drawn at the angle you want to read.
- Find the point where it crosses the scale.
- That point gives the number: 55°.
The rule
A full turn has 360 equal steps. Each step is one degree (°). Half of 360 is 180, so a half turn is 180°. A quarter of 360 is 90, so a quarter turn is 90°.
A protractor reads an angle in degrees. Put its centre on the vertex. The centre is the mark in the middle of the straight edge. Put that straight edge, the base, on one arm. Read where the other arm crosses the scale. The scale is the curved row of numbers.
Watch out!
A protractor has two rows of numbers. Use the row that shows 0 on your base arm. The other row counts from the wrong end.
Try the lab to finish this lesson.