**Unit**

- A unit is the standard quantity with which unknown quantities are compared.
- For example Kilogram is the unit of measuring Weight.

**SI system of Units**

**Derived Quantities and their Units**

**Fundamental units of SI system**

**1.Length**

- Length is defined as the distance between two points.
- The SI unit of length is meter.
- One meter is the distance traveled by light through vacuum in 1/29,97,92,458 second.

**Light year:** It is the distance traveled by light in one year in vacuum and it is equal to 9.46 × 1015 m.

**Astronomical unit (AU)**: It is the mean distance of the center of the Sun from the center of the earth. 1 AU = 1.496 × 10 11 m.

**Parsec:** Parsec is the unit of distance used to measure astronomical objects outside the solar system.

1 parsec = 3.26 light year.

**2.Mass**

- Mass is the quantity of matter contained in a body.
- The SI unit of mass is kilogram.

1 quintal = 100 kg

1 metric tonne = 1000 kg = 10 quintal

1 solar mass = 2 × 10 30 kg

- Mass of a proton, neutron and electron can be determined using atomic mass unit.
- 1 amu = 1/12th of the mass of carbon-12 atom.

**3.Time**

- Time is a measure of duration of events and the intervals between them.
- The SI unit of time is second.
- One second is the time required for the light to propagate 29,97,92,458 metres through vacuum.
- It is also defined as 1/86, 400th part of a mean solar day.
- A chronon is a proposed quantum of time, that is, a discrete and indivisible “unit” of time as part of a hypothesis that proposes that time is not continuous.

**4.Temperature**

- Temperature is the measure of hotness. SI unit of temperature is kelvin(K).
- One kelvin is the fraction of 1/273.16 of the thermodynamic temperature of the triple point of water .
- Zero kelvin (0 K) is commonly known as absolute zero.
- The other units for measuring temperature are degree Celsius and Fahrenheit .
- The formula for converting Celsius to Fahrenheit is (0
**°C**× 9/5) + 32 = 32**°F**