Angles

In Primary 1, you learned to name shapes like the square. A square has 4 straight sides and 4 corners. Today, we will learn what an angle is, and compare angles with a right angle. Mei opens a book stand on a table. Its two arms make angle p. Is angle p smaller than, equal to or greater than a right angle? The book stand has two straight arms. The two arms meet at one point, the hinge. Angle p is the amount of turning from one arm to the other. Long or short arms do not change the angle. A right angle is a quarter turn, like the corner of a square. Put the corner of the square on the hinge, with one edge along the lower arm. The upper arm lies exactly along the other edge. So angle p fits the corner of a square. Angle p is equal to a right angle. The stand is tilted, but tilting it does not change angle p.

Siti opens a pair of scissors. Its two blades make angle q. Is angle q smaller than, equal to or greater than a right angle? Try it first. Pause if you like. The two blades are the arms of angle q. Put the corner of the square on the pivot, with one edge along the lower blade. The upper blade is inside the corner of the square. So angle q is narrower than the corner of a square. Angle q is smaller than a right angle. Long blades do not make the angle bigger.

Bala opens his laptop. The screen and the keyboard make angle r. Is angle r smaller than, equal to or greater than a right angle? Try it first. Pause if you like. The screen and the keyboard are the arms of angle r. Put the corner of the square on the hinge, with one edge along the keyboard. The screen leans outside the corner of the square. So angle r is wider than the corner of a square. Angle r is greater than a right angle. Short arms do not make the angle smaller. Use a square's corner to compare an angle with a right angle. Long arms, short arms or a tilt do not change the angle.