In this activity, you will practise measuring angles.

As you know, angles are measured in units called degrees. To measure an angle, you can use a device called a protractor. A protractor has 180° marked on it.

Public Domain
                  

Think about This:

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

Courtesy of Public Domain

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

   

How to Measure Angles with a Protractor

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.

  
Courtesy of Public Domain

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.

Courtesy of Public Domain
  

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 the angle above has a measure of 40°.

When measuring acute angles, use the small numbers.

---

You can use the same steps in order to measure an obtuse angle as well. When measuring obtuse angles, use the large numbers.

Courtesy of Public Domain

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.

Courtesy of Public Domain

  Self-check!