Measuring and Estimating Angles

Est. Class Sessions: 1–2

Developing the Lesson

Part 2. Measuring Angles

Learn to Use A Protractor. In order to measure angles other than right angles, we use a protractor.

Ask:

Look at your protractor. Describe what you notice. (Students will probably notice the two sets of numbers from 0° to 180° on the protractor, one appearing clockwise and the other counterclockwise.)
Where is zero? (On the bottom or bottom line of the protractor. Some protractors do not have zero marked.)
What does the zero mean? (This is where the two rays start, before they open up.)
Are there two zeros? Why? (Yes. One for the angle to open from the left and one for the angle to open from the right.)

Use the straws-and-paper-clip model students used in Lesson 1. Place a clear protractor on the overhead projector. Place the straw model of the angle on the protractor with the vertex on the center and both rays at 0°. Show the angle starting at zero and opening to an acute angle measure. See Figure 2.

Say:

Describe the size of the angle I created with the straws.

Record a few descriptions on the board. Then ask the class to help you use the protractor to measure the size of the acute angle you created with the straws.

Ask:

Is the angle less than or greater than 90°? (smaller than 90°)
Is the angle closer to 0°, closer to 90°? or somewhere in-between?

Starting at zero, have students skip count by 10 degrees as you point to the protractor until you reach the other ray. Describe the two sets of numbers you see on the protractor (e.g., 130° or 50°) and help students decide which measurement makes sense.

Assign pairs to work on Questions 5–16 in the Student Guide. Students are asked to describe the size of the angles or choose a best estimate and then measure using the protractor. Circulate as students are working.

Ask:

How did you know which number to use on the protractor?
Is that angle a little larger than 130° or 140°?
Did you start at zero? Show me.
Did you compare your estimate to your measurement? Is your measurement reasonable?

More Work with Angle Sort Cards. Ask students to remove the Measuring Angles pages from their Student Activity Book. In Question 1, Maya shows how she taped the ∠F Angle Sort Card to the page and measured the angle with a protractor. After students have completed and discussed Question 1, ask each student to choose and measure a different angle from the Angle Sort Cards (Question 2).

Review Math Practices Expectations 3 and 6 on the Math Practices page in the Reference section of the Student Guide. Tell students that they are going to share with a partner their size description and measurement for Question 2. While they are sharing, they should check to see that their partner's work is labeled with a unit and that their partner checked for reasonableness.

Ask:

What should you do if your estimate or measurement does not seem reasonable? (Recheck your measurements and how you justified your estimate)
How would you know if your partner checked for a reasonable response? (The estimate and measurement should be close. If they are not, your partner should explain how he or she changed either the estimate or measurement.)

Ask volunteers to write an angle measurement for each Angle Sort Card on the board. Ask students to compare the measurements to the descriptions they recorded on the back of their cards.

Student Guide: 386 387 388 389

Answers