Transcript Protractor Lesson

How to Use a Protractor
Protractor:
A tool to
measure angles
Two Major ideas:
1. How to correctly place the protractor
2. How to correctly read the protractor
Part of the Protractor
Outer
Scale
Inner Scale
Zero-Edge
Center Mark
Vocabulary Words
Zero Edge
Always the same level
as the 0 degree mark
Inner Scale
The numbers on the
inner edge of the
protractor; the scale
goes from right to left.
Center Mark
Always at the middle of
the zero-edge
Outer Scale
The numbers on the
outer edge of the
protractor; the scale
goes from left to right.
Measuring an Angle
1. Determine if the angle is acute or obtuse
before using the protractor.
Acute, Right or Obtuse?
• Right Angle – an angle
whose measure is 90˚.
• Acute Angle – an angle
whose measure is greater
than zero but less than 90˚.
• Obtuse Angle – an angle
whose measure is greater
than 90 but less than 180˚.
Measuring an Angle
2. Place the center mark of the protractor on
the vertex of the angle.
Measuring an Angle
3. Rotate the zero-edge of the protractor to line
up with one ray of the angle for the other ray
of the angle to cross the protractor’s scale.
Measuring an Angle
4. Read the measure of the angle of the ray that
crosses the protractor’s scale.
Measuring an Angle
Step 1
Determine if it’s acute
or obtuse.
Step 3
Line up zero-edge.
Step 2
Place center mark on
vertex of the angle.
Step 4
Read angle’s measure.
•If acute: use scale with
numbers less than 90
•If obtuse, use scale
with number greater
than 90
Common Mistakes
1. Using the incorrect scale.
2. The center mark is not on the vertex.
– This usually happens after rotating the
protractor. Always check your centering when
moving the protractor.
3. Counting the marks in the wrong direction.
– Pay attention to whether the numbers are
increasing or decreasing.
How would you measure these angles?
How about this angle?