Lesson 18 — Activity 2: Measuring Angles

Getting Ready

In this activity, you will measure angles. A protractor is an instrument for measuring angles.  

All of the angles in L18 — A1 were shown on a protractor so that you could see the measurement of the angle.

You must have a protractor to complete this activity. If you don't have one, talk to your teacher.

Here's an example of using a protractor to measure a straight angle, which is 180° or a straight line.

You can see how the protractor is placed to measure the angle.

A protractor measures angles from 0 degrees to 180 degrees. This protractor has a scale of one degree, which means that each mark on the outside of the protractor is equal to one degree.

      Image 1

In order to use a protractor, you have to put the point where the two lines meet right in the middle of the base, where the line coming down from 90° meets the line going across from 0° to 180°.

This is called the vertex of the angle.

Image 2

Next, you have to line up the horizontal line with one of the lines of the angle so that the other line can be found in the area that you can measure angles with. Each line is called a ray.

Image 3

Starting at 0° and moving upwards (counterclockwise) along the protractor, we find that the next ray (line) of the angle crosses the protractor at 40° so this angle has a measure of 40°.

You can use the same steps in order to measure an obtuse angle as well.

Image 4

Looking at the angle above, you can see that we have an obtuse angle. It is slightly more than 130°, but how much more? In order to be more accurate, we need to move to the outside of the protractor where the individual degrees are located. Counting along here, we see that it is 2° more than 130°, so it is 132°.

You can also read the protractor the other way if necessary. Just follow the same steps as above but read the protractor clockwise.

Image 5

As you can see, the angle above is 65°.

Digging Deeper

Watch the following video to review measuring angles.

Try This:

Click here to practise measuring various angles. Be sure to rotate the protractor so that the horizontal black line of the crossbar lies directly on top of one of the rays.