# Measurement and Measuring Instruments

## 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 ## 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