Circle Math

In circle math the terms related to the circle are discussed here.

We have learned to draw a circle, by tracing or moving a pencil around the outlines of objects like a bangle or a bottle cap.

We shall learn now how to draw a circle using a compass.

Compass is a handy drawing tool available in the geometry box. A compass has two arms which can be pulled apart easily to adjust to the size of the circle we want to draw.

Fix a sharp pencil into the holder on the compass.

Fix the needle of the compass on the point where the center of the circle would be.

Open and adjust the other arm which is holding the pencil and rotate the compass to draw a circle.

The point O where the needle of the compass is placed in the centre of the circle. The distance of any point on the circle from the centre O is the radius of the circle. In the given figure OX is the radius.

A line segment which joins any two points on the circle is called a chord. MN is a chord in the given figure.

The line passing through the centre which joins two points on the circle is the diameter. Diameter of the circle is twice its radius. YZ is the diameter of the circle in the given figure.

The length of the circle is called its circumference.

A circle is such a closed curve whose every point is equidistant from a fixed point called its center.

The symbol of circle is O.

What are the terms related to the circle?

The terms related to the circle math are:

(i) Center:

The center of a circle is a fixed point within the circle from which all the points of the closing curve are equidistant. 

The point O inside the circle is the center of the circle.

(ii) Circumference:

The curve which closes a circle is called its circumference. The length of the circumference is called the length of the circle.

The length of a circle (distance around) is called the circumference of the circle.

(iii) Radius:

The distance from the centre to any point on the circumference of a circle is called the radius of the circle. The symbol of the radius is r.

A line segment drawn from the center to any point on the circle is called a radius of the circle. In the figure, the line segment OA is a radius of the circle. Line segment OB is also a radius of the circle.

(iv) Diameter:

The line-segment passing through the centre and meeting the points on the circumference is called the diameter of the circle. Diameter is denoted by 'D'.

The line segment drawn through the center that joins two points on the circle is called a diameter of the circle. In the figure, line segment AB is a diameter of the circle.

AB is a diameter of the circle.

Diameter is twice the length of the radius concerned.

Thus, D = 2r [Diameter = 2radius]

or

r = D/2 [Radius = Diameter/2]

(v) Arc:

Any part of the circumference is called an arc of the circle.

(vi) Chord:

The line-segment joining the two ends of an arc is known as a chord. A diameter is the longest chord of a circle.

A line segment that joins two points on a circle is called a chord of the circle. In the figure, line segment PQ and DE are chords of the circle.

REMEMBER: Diameter id the longest chord of a circle.

Now observe the figure below:

Here,

O is the centre.

OP is one radius.

AB is a diameter.

MN is a chord. (line-segment)

OA and OB are also radii.

Drawing a Circle

We can also draw a circle using a pair of compasses. To do so, we proceed as follows:

Step I: Fix a sharpened pencil in the compass. Step II: Open the compasses to any suitable length. Step III: Make a point on a piece of paper. Step IV: Put the metal end of the compasses at the point O Step V: Let the pencil-point touch the paper. Step VI: Move the pencil around, keeping the metal point fixed at the point O. Remove the compasses. The figure you get is a circle.

Worksheet on Circle Math:

1. Look at the adjoining figure of a circle. Measure its radius OA and diameter BC. Find the relation between radius and diameter.

OA = __________ cm BC = __________ cm BC = __________ OA

2. Complete the following table regarding circles:

Answer:

2. 

Related Concepts on Geometry - Simple Shapes & Circle

● Simple Closed Curves

● Polygon

● Different Types of Polygons

● Angle

● Triangle

● Quadrilateral

● Circle Math

● Line Symmetry

● Symmetrical Shapes

4th Grade Math Activities

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