Page 130 - ICSE Math 5

Straight angle

An angle that measures 180° is called a straight angle. Some examples of straight angles that we
see in our surroundings are shown below.

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Measure of an Angle

The measure of an angle means the amount of rotation between its two arms. It increases
or decreases depending on the increase or decrease in the gap between its two arms. Let's
consider an example of a hand fan to see how it rotates about its vertex through different
angles.

A closed
hand fan

Observe that the space or the region between the two arms of the hand fan slowly
increases on opening the fan. In other words, the measure of the angle increases as the gap
between the two arms of the hand fan increases and vice versa. However, if we increase
or decrease the size or length of the arms of the fan, the measure of the angle will not
change.
As there are standard units used for measuring length, weight, time, etc., similarly, degree is the
standard unit used for measuring angles. The symbol for degree is ‘ ° ’ . It is written after the
number in superscript, for example, 1° and it is read as 1 degree.

Measuring an Angle with a Protractor

Angles are measured in degrees with the help of
Inner
an instrument called protractor. Look at the figure scale
of the protractor given alongside. It is typically Outer
a flat semicircle or a half circle with two sets of scale
numbers marked in opposite directions along its
curved edge. The two sets of numbers written on
it are called scales. There is an inner and an outer Base Centre
scale marked on the protractor for measuring line point
angles from 0° to 180°.

• The inner scale of the protractor is marked from 0° to 180° in an anticlockwise direction.
• The outer scale of the protractor is marked from 0° to 180° in a clockwise direction.

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