Measuring an Angle by a Protractor

In measuring an angle by a protractor, first we need to know what a protractor is.


Protractor:

We measure the size of an angle by using protractor. The protractor is in the shape of a semicircle. The semicircle is divided into 180 equal parts which show angle measurements from 0° to 180°.

The protractor has two sets of measurements. The marking on the inner circle is 0° to 180° in the opposite direction to the markings on the outer circle.

It is an instrument for measuring or constructing an angle of a given measure. It is a circular or semicircular piece of metal or plastic.

Measuring an Angle by a Protractor



It is a circular protractor which is marked in degrees from 0° to 360° from left as shown in adjoining figure.



Measuring an Angle by a Protractor



It is a semicircular protractor which is marked in degrees from 0° to 180° from left as shown in adjoining figure.

To measure an angle ABC, we place the mid point of the protractor on the vertex O of the angle. The base OA arm falls along with the base line of the protractor as shown in the figure below.

Measuring Angles

The angle is measures on the protractor counting from 0 up to the point where the arm OB lies. In the above figure ∠AOB = 60°. 

The angle which is formed outside is known as the exterior angle. 

Interior and Exterior Angles

In the above figure ∠2 is the exterior angle and ∠1 is known as interior angle.


For Example:

1. Use your protractor to draw 60°.

Use your protractor to draw 60°

The centre O of the piece is also the midpoint of its base line. In order to measure ∠AOB, place the protractor in such a way that its centre is exactly on the vertex O of the angle, the base line lies along the arm OA. We need to read the mark through which the arm OB passes, starting from O on the side A, as we observe in the above figure.

Thus we find ∠ AOB = 60°.



2. Use your protractor to draw 90°.

Use your protractor to draw 90°

The centre O of the piece is also the midpoint of its base line. In order to measure ∠AOB, place the protractor in such a way that its centre is exactly on the vertex O of the angle, the base line lies along the arm OA. We need to read the mark through which the arm OB passes, starting from O on the side A, as we observe in the above figure.

Thus we find ∠ AOB = 90°.



● Angle.

Interior and Exterior of an Angle.

Measuring an Angle by a Protractor.

Types of Angles.

Pairs of Angles.

Bisecting an angle.

Construction of Angles by using Compass.

Worksheet on Angles.

Geometry Practice Test on angles.

 






5th Grade Geometry Page 

5th Grade Math Problems 

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