Fourth Grade · Math · week 35

Measuring and classifying angles

Classify angles as acute, right, obtuse, or straight, and measure them in degrees with a protractor.

An angle measures how far two rays open apart from a shared point, called the vertex, measured in degrees. A right angle is exactly 90 degrees, like a square corner. An acute angle is smaller than 90 degrees, a narrow opening. An obtuse angle is bigger than 90 but smaller than 180 degrees. A straight angle is exactly 180 degrees, a straight line.

A protractor measures the exact number of degrees; lining up its base line correctly on one ray is the key to reading it right.

📋 For the teacher
Teach it (10–15 min):
  • Use the corner of an index card as a quick "right angle tester" to compare against any angle by eye first.
  • Practice lining up a real protractor base line on the ray, and check which of its two number rows starts at 0 on that ray.
  • Look for angles around the house or yard and classify them by eye before checking with a protractor.
  • Keep reminding them that classifying is a fast estimate, while measuring gives the exact degree count.
Talk about it:
  • What is the exact degree measure of a right angle?
  • How can you tell an obtuse angle from an acute one without measuring?
  • Why might a protractor be confusing at first, and what helps you read it correctly?

Mastery looks like: They classify angles as acute, right, obtuse, or straight and measure angles accurately using a protractor.

Common stumbles: Reading the wrong scale on a two-scale protractor, getting an answer off by its "supplement" (reading 60 instead of 120); misclassifying an angle close to 90 degrees without checking carefully.

Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Math for this learner.
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