Estimating and Measuring Angles
Estimate, construct and measure angles in degrees, using appropriate tools including a protractor, and relate these measures to angle names.
Read this first
Compare with a right angle first. Then use the protractor to measure.
Half a right angle is 45 degrees.
Words you need
- Degree
- The unit we measure angles in.
- Protractor
- A tool for measuring angles.
- Estimate
- A close guess before you measure.
Worked example
e.g. Measure an angle that looks a bit less than a right angle.
- Estimate first. It is less than 90, so guess about 70.
- Put the middle of the protractor on the corner.
- Line up one arm with the zero line.
- Read where the other arm crosses. It says 70 degrees.
- 70 is close to the estimate. The reading is sensible.
Estimate before you measure. Then a wrong reading stands out.
Practice
The pictures fade as you go. By the last few you are on your own.
1Estimate this angle.with a picture
AnswerAbout 45 degrees.
2Is this angle acute or obtuse?with a picture
AnswerAcute. It is much less than 90.
3A protractor shows 60 and 120. Which do you read?with a hint
Need a hint?
Is the angle bigger or smaller than a right angle?
Answer60, if the angle is acute.
4How many degrees in a straight angle?with a hint
Need a hint?
Two right angles.
Answer180.
5How many degrees in a full turn?on your own
Answer360.
6An angle measures 95 degrees. Name it.on your own
AnswerObtuse.
7Why estimate before measuring?on your own
AnswerSo you notice if you read the wrong scale.
Think about it
1
Why does a protractor have two number scales?
One good answerSo you can measure from either arm.
2
How can you tell you read the wrong scale?
One good answerThe answer will not match your estimate.
Common mistakes
Reading the wrong scale.
Estimate first. Then pick the number that fits.
Not lining up the centre.
The middle mark must sit exactly on the corner.