Measurement
Angles and the protractor
Name an angle as acute, right, obtuse or straight, and measure one with a protractor.
An angle measures a turn. Two arms meet at a point, and the angle is how far you would have to swing one arm to land on the other. Turns are measured in degrees, and a full turn all the way round is 360°.
The four names
A quarter turn is a right angle, 90°. Smaller than that is acute. Bigger than a right angle but less than a half turn is obtuse. Exactly a half turn, 180°, is a straight angle.
Worked example
What kind of angle is 110°?
Compare it with 90°. It is bigger.
The right angle is the fence every other angle is judged against.
Using a protractor
Put the protractor's centre on the point where the arms meet, and lay one arm along the zero line. Then read off the number the other arm passes through. If the arm does not start at zero, take the smaller reading away from the larger one.
Try it together
An angle is measured with a protractor. One arm reads 30 and the other reads 105. Let us work it out.
The arm was not on zero this time, so there is one small step before the answer.
1.Take the smaller reading away from the larger one. What do you get?
Have a go
Have a go on your own: an angle is measured from 15 to 100 on a protractor. How many degrees is it?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.