Overview
This lecture explains how to accurately measure angles using a protractor, including steps for lining up the protractor and reading angle measurements.
Measuring Angles with a Protractor
- A ruler measures lengths, but a protractor is needed to measure angles between two rays.
- Angles are always measured in degrees (°).
- Place the protractor's bottom middle (T, dot, or hole) on the angle's vertex (where the rays meet).
- Align the protractor's bottom line with one of the rays.
Choosing the Correct Scale
- Always start measuring from zero degrees.
- Use the inner scale when the aligned ray points to zero on the inner numbers.
- Use the outer scale when the aligned ray points to zero on the outer numbers.
- The correct scale ensures an accurate measurement.
Reading the Angle
- Once aligned, the second ray points to the angle's degree mark on the protractor.
- Example: If the second ray hits 80°, the angle measures 80 degrees.
- Always write the degree symbol (e.g., 80°).
Measuring Angles Greater Than 180°
- Standard protractors measure up to 180°, but larger angles can be measured in two steps.
- First, measure 180° using the protractor.
- Without moving the vertex point, tilt the protractor to measure the remaining angle.
- Add 180° to the extra measured degrees for the total angle (e.g., 180° + 30° = 210°).
- Extend the ray if it does not clearly reach the scale.
Key Terms & Definitions
- Protractor — a tool used to measure angles in degrees.
- Vertex — the point where two rays meet to form an angle.
- Ray — part of a line with one endpoint, extending infinitely in one direction.
- Degrees (°) — the unit for measuring angles.
Action Items / Next Steps
- Download and complete the free worksheet and memo provided in the description.