Angles

CBC Grade 5 Mathematics — Angles

Lesson Objectives

By the end of this topic, a learner should be able to:

• Relate a turn (quarter, half, full) to its angle in degrees
• Read angles from a protractor, in both directions
• Use a protractor to measure angles accurately
• Identify angles in the everyday environment

Angles

An angle measures turning. It does not measure length, so a long line does not mean a big angle.

We measure turning in degrees. We write them with a small circle, like 90°.

a) Turning all the way round

Stand up and turn right round until you face the front again. That is a full turn.

A full turn is 360°.

b) Parts of a turn

Half a turn is 180°, because it is half of 360.

A quarter turn is 90°, because it is half of 180.

Three quarters of a turn is 270°.

A quarter turn is also called a right angle. It makes a square corner.

c) Right angles around you

The corner of this page is a right angle. So is the corner of a door, a window, a table and a book.

Check with the corner of a piece of card. If it fits exactly, the angle is a right angle.

d) Bigger and smaller than a right angle

An angle smaller than a right angle looks sharp and narrow.

An angle bigger than a right angle looks wide and open.

Compare with a square corner before you measure. Then you will know roughly what to expect.

e) Using a protractor

A protractor is a half circle marked from 0 to 180.

Put the middle of the flat edge exactly on the corner of the angle.

Line up one arm with the zero line.

Follow the other arm out to the curved edge and read the number.

f) The two scales

A protractor has two rows of numbers. One goes from the left, one from the right.

Use the row that starts at zero on your arm. The other row will give the wrong answer.

Check your reading. An angle narrower than a square corner should read under 90. A wider one should read over 90.

Reading 130 for a narrow angle means you used the wrong row.

g) Where this is used

A carpenter cuts a corner. A footballer turns to face the goal. A driver turns at a junction. A builder checks a wall corner is square.

:::checkpoint Check yourself

1. How many degrees are in a full turn?
2. How many degrees are in a quarter turn?
3. What is another name for a quarter turn?
4. Why does a protractor have two rows of numbers? :::

Definitions

• Angle — a measure of turning between two directions, in degrees.
• Degree (°) — the unit angles are measured in; a full turn is 360 degrees.
• Protractor — a tool used to measure angles.
• Vertex — the corner point where two lines meet to form an angle.

Must Remember

1. A full turn is always 360 degrees.
2. Line up the protractor's centre with the vertex first.
3. A protractor has two scales - read from the one starting at 0 on your arm.
4. Angles are measured in degrees.

Conclusion

Every angle is a portion of a full 360-degree turn.