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This lesson is an introduction to using a protractor




4, 5, 6  


Title – Introduction to using a Protractor
By – Elaine
Primary Subject – Math
Grade Level – 4-6

Purpose: Identify and measure angles using a protractor.

Behavioral Objective: Students will be able to identify and measure angles using a protractor.


      M2 : Geometry and Measurement Concepts.


      c: Measure angles using appropriate units.

Prior knowledge: identify and name angles.

Prior vocabulary:



      -angle symbol:    <


      -three-letter procedure


      - A protractor to each student.


    - Chalkboard protractor.

New vocabulary: degrees, straight, acute, right and obtuse angles.

Focusing event:

      Say: “Today we will measure angles using a protractor.” Before giving one protractor to each student, explain that a protractor is an instrument of measurement much like a ruler when measures inches in a straight path. A protractor measures and constructs angles. Display the chalkboard protractor.


      Say: “Let’s name and label the parts of a protractor”.
      Distribute one protractor per student.


    Protractors come in several colors, therefore to discourage color picking, while giving them out say “You get what you get, don’t get upset.”

Demonstrating new material:

      Indicate the vertex point in the lower middle part of the protractor.


      Show the zero line for the outside scale and the zero line for the inside scale.


      Show the outside and the inside scale.


      Ask: How is each scale numbered? (Each scale is numbered by tens from 0 degree through 180 degree.) What do we call an angle that measures exactly 180 degrees? (a straight angle) What does it look like? (it looks like a straight line but it has one point on it which is identified as the vertex.) Draw the following on the chalkboard:


                        F                  G                  H
      Ask: “how can we verify the measurement of angle FGH?” (Place the protractor so that the center mark is on G, which is the vertex of the straight angle, and ray GH is on the 0 degree mark. This will be one side of the straight angle.)

      Ask “Through which numbers does ray GF pass? (0 and 180 ) Which is the measure of the angle? (180 ) What is a right angle?” (An angle which is exactly 90 , which is exactly half that of a straight angle.) Ask the students to identify examples of straight angles and right angles around the classroom.


      Draw several angles on the chalkboard, including one right angle.

      <———–o————-o               |___      <      >
                        A                  B                  C       D     E
      Say: “Which drawing represents a right angle?” Explain (Angle C appears to be a right angle because it looks like a half of a straight angle so it probably measures 90 .) How can we verify whether or not it is a right angle? (We can measure angle C using a protractor. ) To use the protractor correctly, place the vertex point of the protractor on the vertex of angle C and the 0 mark along one ray of the angle.” Ask a student to place the chalkboard protractor at the vertex of angle C. Say: “What measure does the other ray indicate on the protractor scale? (It crosses at the point marked 90 .) In what units do we measure angles? (In degrees.) How is the measure of angle C expressed? (Angle c measures 90 degrees ) How can angle C be described in another way. (Angle C is a right angle.)
      Let’s try measuring angle C another way. Again, place the vertex point at point C. This time, place the other 0 mark along the other ray of the angle. Read the protractor. What is the measure of the angle? (Again, the protractor shows the measure to be 90 .)”
      Say: “We can also use a protractor to construct a right angle.” Using the straight edge of the chalkboard protractor, draw ray AB.


      A               B
      Say: “We are going to construct a right angle with its vertex at point A. How should we place the center point of the protractor?” (Place the center point of the protractor on point A and the 0 line of the protractor on ray AB.”) Ask a student to demonstrate this at the board with the large protractor.





      C  o












          A               B
      Say, “Through which marking on the protractor scale should the other side of the right angle pass?” (90 ) Ask a student to place a point on the board at the 90 mark, label it point C, and construct a ray from point A through point C.
      Ask: “How can we be certain that we have constructed a right angle? (It measures 90 we can match against a known right angle, such as the corner of a page in a book.) What are the correct names for this angle?” (We can call it angle A, angle BAC, or angle CAB.)
    Ask a student to write the angle names on the board as the others write them in their notebooks. Repeat the procedure by constructing additional angles.

Summary: Ask students to complete the practice exercise on the board. Go over each example as a group, so that everyone understands how to name an angle.

Complete the worksheet.

Tomorrow we will measure and construct acute and obtuse angles.

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