Discover Mathematics P4: Term 2, Unit 13, Lessons 5–8
GoalName the parts of a protractor and explain how its two scales work.
New words
protractor · a half-circle tool for measuring and drawing anglesbaseline · the straight bottom edge of the protractor; it goes along one arm of the anglecentre mark · the small mark or hole on the baseline; it goes on the vertex
Explanation
A protractor is a tool to measure angles in degrees. It is a half circle, so it measures from 0° to 180°. It has a straight edge called the baseline, a centre mark in the middle of the baseline, and a curved edge with a scale of numbers. Each small mark is 1°, and the numbers are written every 10°. There are two scales with the same marks, running in opposite directions: one starts from 0° at the right end of the baseline and the other starts from 0° at the left end. For example 40° on one scale is next to 140° on the other. To read an angle, use the scale that begins with 0° on the arm lying along the baseline. Then check your answer with an estimate: if the angle is smaller than a right angle, the number must be less than 90°.
Worked examples
Part
What it does
baseline
lies along one arm
centre mark
goes on the vertex
scale
gives the number of degrees
Parts of a protractor. The baseline is the straight bottom edge. The centre mark is in the middle of it. The curved edge carries the two scales, 0° to 180° from the right and 0° to 180° from the left.
Which scale do we read? Here the baseline lies along the arm on the right, so we use the scale that starts from 0° at the right end. The other arm crosses it at 65°, which is 6 tens and 5 more small marks. On the other scale the same place reads 115°, but that is the wrong scale here.
Check with an estimate. This angle looks smaller than a right angle, so it is acute. The two scales give 40° and 140°. An acute angle is less than 90°, so the answer must be 40°, not 140°.
Class activity
Know your protractor. Take a protractor from your geometry box. Find the baseline, the centre mark and both scales. Find 90° and 180° on each scale. Then lay it on the corner of a page: which number does the second edge of the page show? Do the same with a set square’s 45° corner and compare.
Practice
1
The baseline lies along the right arm of the angle. What angle does the other arm show?
50°. (The other scale would say 130°.)
2
Each small mark on a protractor is 1°. How many small marks are there from the 60° mark to the 90° mark?
30 marks, because 90° − 60° = 30°.
3
The two scales read 70° and 110° for an angle that is clearly wider than a square corner. Which is right?
110°, because the angle is obtuse (more than 90°).
4
Where do we place the centre mark of the protractor?
On the vertex of the angle.
5
Seif reads 130° for an angle that is smaller than a right angle. What mistake did he make?
He used the wrong scale. The right reading is 180° − 130° = 50°.
