Lesson 17: Measuring Angles with a Protractor

New Concept

Angles are measured in units called degrees. Key benchmarks, based on a full circle, are used to classify and measure them.

A full circle measures $360^{\circ}$.

A half circle measures $180^{\circ}$.

Property

Angles are measured in units called degrees ($\circ$). A full circle measures $360^{\circ}$, a half circle (straight angle) measures $180^{\circ}$, and a quarter circle (right angle) measures $90^{\circ}$.

Examples

A full rotation is $360^{\circ}$, which is the same as a full circle.
A straight line represents a straight angle, which measures exactly $180^{\circ}$.
A perfect corner, like the corner of a square, is a right angle measuring $90^{\circ}$.

Explanation

Think of degrees as tiny, equal slices of a circular pizza! A full circle gets 360 slices, or degrees. So a perfect corner, a right angle, is a quarter of the pizza and has 90 degrees. An angle’s measure in degrees tells you exactly how wide it opens, from a tiny sliver to a wide-open jaw.

Property

Angles are classified by their size in degrees:

• Zero Angle: Exactly 0°. The two sides completely overlap and point in the same direction.
• Acute Angle: Greater than 0° but less than 90°.
• Right Angle: Exactly 90°. It forms a perfect square corner.
• Obtuse Angle: Greater than 90° but less than 180°.
• Straight Angle: Exactly 180°. It forms a completely flat, straight line.

Examples

• An angle measuring 45° is an acute angle.
• An angle measuring 110° is an obtuse angle.
• Two clock hands pointing exactly at 12:00 form a zero angle (0°), while hands pointing at 12:00 and 6:00 form a straight angle (180°).

Explanation

Angles have personalities just like people, and we classify them by their size! A zero angle has no opening at all. An "acute" angle is a cute, small one under 90 degrees. A right angle is a perfect corner. An obtuse angle is big and wide, while a straight angle is a completely flat line.