Primary 3 Angles
Learn what an angle is, then compare it with a right angle. An acute angle is smaller than a right angle; an obtuse angle is greater.
Keywords
- Angle
- The opening between two lines that meet at a point.
- Line
- One of the two straight lines that form an angle.
- Vertex
- The point where the two lines meet.
- Right angle
- An angle that measures exactly 90°, formed where two perpendicular lines meet. It is marked with a small square at the vertex.
- Acute angle
- An angle smaller than a right angle.
- Obtuse angle
- An angle greater than a right angle but smaller than a straight angle.
Concept of an angle
An angle is an opening between two lines that meet at a point called the vertex. A wider opening means a greater angle. Drawing the lines longer does not change the angle.
Watch the opening change
See worked example
A door swings from closed to partly open. Where is the angle, and what changes as the door opens wider?
- Find the hinge, where the door and frame meet. That is the vertex.
- Use an edge of the frame and an edge of the door as the two lines.
- As the door turns farther from the frame, the opening and the angle become greater.
Answer: The vertex is at the hinge. The angle becomes greater as the door opens wider.
Try together
Two clock hands move farther apart while their lengths stay the same. What happens to the angle between them?
Reveal the answer
The angle becomes greater because the opening increases.
Right angles and angles greater or smaller than a right angle
A right angle measures exactly 90°, is formed when two perpendicular lines meet, and is marked by a small square at the vertex in diagrams. An acute angle is smaller than a right angle. An obtuse angle is greater than a right angle but smaller than a straight angle.
Compare with a square corner
See worked example
A pair of scissors is opened less widely than the square corner of a book. What kind of angle is at its pivot?
- Place the book's square corner at the scissors' pivot.
- Line up one book edge with one blade.
- The other blade lies inside the square corner, so the opening is smaller than a right angle.
Answer: It is an acute angle.
Try together
A door is opened wider than a square corner but not as far as a straight line. What kind of angle is at the hinge?
Reveal the answer
It is an obtuse angle because it is greater than a right angle but smaller than a straight angle.