Grade 4 (age 9-10)

grade 4 Angles

Angles

An angle is the opening between two rays that start at one point. Opening scissors or a door makes that same opening in everyday life. A protractor measures it in degrees: 90 is a right angle, a smaller one is acute, and a larger one is obtuse. Move the slider to open the angle, and the mark on the protractor and the turn of the clock hand change together.

What is an angle?

📐 Two lines meet → an angle

①Open scissors → an angle forms

②Open them more → angle gets bigger

③Measure size in degrees (°)

④A protractor measures it precisely

Measure with a Protractor

Angle45°

💡 Types of Angles

①Acute: greater than 0° and less than 90°

②Right: exactly 90° (L-shape, square corner)

③Obtuse: greater than 90° and less than 180°

④Straight: 180° (a line)

⑤0° is not acute — the angle has not opened yet

Angles in Daily Life

🔄 Angles and Rotation

①One full clock turn = 360°

②Half turn = 180° (straight)

③Quarter turn = 90° (right)

④The three-angle sum of a triangle is on Types of Triangles. This page uses a protractor and a clock

Summary

Right Angle

90° = right angle

Square corner, L-shape

Types of Angles

Acute < 90° < Obtuse < 180° (Straight)

<90° = acute, >90° = obtuse

One Turn

360° = one full rotation

Angle of one full clock turn

🎯 Key Points

①Angle = how far two lines spread

②Unit is degree (°)

③Right = 90°, straight = 180°

④Acute < Right < Obtuse < Straight

⑤The pictures here are a protractor and a clock. Triangle angle sum is the next lesson

Examples and Unit Test

Example 1

Is a 70° angle acute, right, or obtuse?

An acute angle is greater than 0° and less than 90°.

70° is less than 90°, so it is acute.

▸ Acute

Less than 90° = acute, exactly 90° = right, more than 90° = obtuse.

Example 2

How many degrees is a half turn of a clock hand?

One full clock turn is 360°.

A half turn is half of that, 180°, the same as a straight angle.

▸ 180°

A half clock turn = 180° = a straight angle.

Unit Test

How many degrees is one full turn of a clock hand?

①90°

②180°

③270°

④360°

▸ ④ 360°

One full turn is one complete rotation.

A half turn is 180°, and a full turn is twice that: 360°.

← Previous

Estimation — Number Range

Types of Triangles