**SI Unit**

For having uniform measurements across the world, scientists, engineers, mathematicians, etc all over the world adopted a common set of units to express measurements. This system is called SI units or the International System of units.

### SI Units for common things

- Lenght – Metre
- Mass – KG (Kilogram)
- Time – Second
- Area of Surface –
**m**^{2} - Volume of Soild –
*m*^{3}

**Lenght**

The distance between one end and the other end is called length. The standard unit of length is ‘Metre’ which is represented by ‘m’.

Very small lengths can be measured in mm(millimetre) and cm(centimetre).

Longer length can be measured by kilometre(m).

- 1km = 1000m
- 1m=100cm
- 1cm=10mm

## Volume

### Volume of Solids

Length is a fundamental quantity and it cannot be expressed in any other quantities.

By using length, one can find Area and volume.

The area is measured using two lengths. The area of the square is l x l as the length of both the sides are equal and the Area of the rectangle is l x b, as length and breadth are different and always length is greater than the breath.

**Volume**

The volume of Cube = *m*^{3} (As Length, breath and Height are equal)

Volume of Cuboid =*w x h x l*

## Volume of Liquid

A liquid takes the shape of the container, it is easy to measure the volume of the liquid. To measure the volume of liquid, the liquid is poured into the container such as cylinder, beaker, burettes, pipettes, etc.

Nowadays, Graduated cylinders, beakers, pipettes, burettes are available with exact measurements to measure the volume of the liquid.

The volume of the liquids is usually measured in litres.

## Volume of Gas

Unlike liquids, gases expand to fill the container. Also, gases get compressed when filling the container. As a result, it is not easy to measure the volume of the gas like measuring the volume of liquid.

Therefore the pressure at which the gases are kept in the container is taken into consideration. But the gases are measured in a cubic metre *m*^{3}.

1 *m*^{3} = 1 kilolitre (kl or kL)

1 c*m*^{3} = 1 millilitre (ml or mL)

1 *mm*^{3} = 1 microlitre (µl or µL)

## Mass and Weight

Mass is the measure of the amount of matter in an object. Whereas Weight is the gravitational pull experienced by the mass of the object.

SI Unit of Mass is kilogram and it’s represented by ‘KG”.

- 1000 milligram = 1 gram
- 1000 gram = 1 kilogram
- 1000 kilogram = 1 tonne

Instruments used to measure the Mass are Beam Balance and Electronic Balance.

Beam balance works by comparing test objects with standard mass. Electronic balance works using different sensors calibrated to provide the accurate mass of a test object.

## Time

Clocks are used to measure time. In earlier days people used instruments such as Sand clocks to measure time.

- 1 Seconds = 1000
- 1 Minutes = 60 seconds
- 1 Hour = 60 Minutes or 3600 Seconds
- 1 Day = 24 Hours
- 1 Year = 52.1786 Weeks / 12 Months / 365.25 Days / 366 Days every 4 years

Atomic clocks provide the most accurate time and frequency standards are used as basic standards for international time distribution services.

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