The comparison of an unknown quantity with a known fixed quantity is termed as measurement. It basically consists of two things:

- Units: The known fixed quantity used for measurement is termed as unit.
- Number: The number of units an object contains.

## Methods of Measurement

Since ancient times, people have been inquisitive about measuring the distance. They developed many ways of measuring. Some of them are enlisted below.

### Conventional System of Measurements

These methods predict the approximate measure of an object. One of the biggest demerits of this method is it varies from person to person. Some common units used in such kind of measurement include:

### Standard System of Measurements:

This system includes standard units of measurement. These units are consistent and do not vary from person to person. This system of units now is better known as International System of Units (SI unit). SI unit of distance is meter (m).

## Points to consider while taking measurements

- Select right measurement device for taking any measurement. For example, you cannot measure the length of a curvy line with the help of measuring scale.
- Place the scale along the length you are measuring. Place the initial point accordingly with the value on a scale equal to 0.
- In some scales you may be unable to see the zero mark select the initial and final notations on the scale. The actual measurement is given by:

**Length = final value on the scale – initial value on the scale** - Correct position of the eye is also important. You can obtain different value for different positioning of your eye as per the scale. Make sure your eye is exactly in front of the scale reading.

## Motion of bodies

A body is said to be in motion if it changes its position over a period of time. Different types of motion are discussed below.

### Rectilinear Motion

The motion in a straight line is termed as rectilinear motion. For example: motion of sprinters in a race, march past of soldiers in a parade etc.

### Circular motion

Motion of objects such that they always remain at a same distance from a fixed point is termed as circular motion. For example motion of blades of fan. This motion is also termed as the rotational motion if objects execute circular motion on their own axis. For example blades of windmill.

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### Periodic motion

The motion in which particle attains its natural position after a fixed interval of time is termed as periodic motion. For example: motion of a pendulum.

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