Not a blog

Back

A physical quantity is a property of a material or system that can be quantified by measurement. For example, a book of English literature would have a mass, and three dimensions (length, breadth and thickness). The mass is a physical quantity which can be measured.

A unit is a measure of a physical quantity. For example, a pack of milk is 1 litre in volume; here the unit used is litre.

Physical Quantity	Unit	Dimension
Mass	1 kilogram	M
Force	1 newton	MLT-2
Current	1 ampere	A

A physical quantity, say X can be represented as follows,

X = n * u

u = unit; n = variable

For a particular physical quantity (X), if unit (u) is large then variable (n) is small and if u is small then n is large.

E.g. 1

A Cricket Pitch is 22.00 yard long, which in meters is 20.12 m.

1 meter = 1.093 yard

1 yard = 0.914 m

Units can be classified in two types:

• Fundamental and Supplementary Units
• Derived Units

Fundamental Units

Fundamental units are assigned to physical quantities like mass, current, time, etc.. Fundamental units are independent of each other i.e. changing the unit of one physical quantity does not affect others.

In SI (Système Internationale d’ Unites - French for International System of Units), their are seven fundamental quantities (and units) and two supplementary units.

Table of fundamental units

Quantity	Unit	Dimension
Mass	1 kilogram	M
Length	1 metre	L
Time	1 second	T
Temperature	1 kelvin	K
Amount of substance	1 mole	mol
Current	1 ampere	A
Luminous Intensity	1 candela	Cd

Supplementary units

Physical Quantity	Unit
Plane Angle	1 radian
Solid Angle	1 st-radian

Derived units

 Derived units are assigned to derived quantities like Force, Area, Charge etc.

 Derived quantities / units are made of two or more fundamental quantities for e.g.  

1 m² = 1m * 1m

1 newton = 1 kg-m/s² = (1kg*1m)/1s²

1 coulomb = 1 ampere-second

Scientific Prefixes

1 cm = 10^-2m       1 Dm = 10m

1 mm = 10^-3m     1 Gm = 10⁹m

1 dm = 10^-1m      1 nm = 10^-9m

1 Mm = 10⁶m     1 pm = 10^-12m

1 µm = 10^-6m      1 fm = 10^-15m

System of Units

• MKS System – metre, kilogram, second
• CGS System – centimeter, gram, second
• FPS System – foot, pound, second
• SI

E.g.2

Convert 1m² into cm².

1m = 100 cm

1m² = 10⁴ cm²

E.g.3

Convert 1g² into kg².

1g = 10^-3 kg

1g² = 10^-6 kg²

E.g.4

Convert 1 newton (MKS unit of force) to 1 dyne (CGS unit of force).

1 newton = 1 kg-ms^-2 = 1 kg * 1m/1s² = 10³g * 10²cm/1s² = 10⁵g-cms^-2 = 10⁵dyne

Q1. Convert 10^-7 erg (CGS unit of energy) to joule.

Q2. Light year is a unit of

a) time b) distance c) mass d) energy

Q3. Which of the following is not represented in correct unit ?

a) stress = 1N/m²

b) surface tension = 1N/m

c) energy = 1kgm/s

d) pressure = 1kgm^-1s^-2

Q4. Choose a suitable unit for gravitational constant `G’.

a) 1 Nm²kg^-2 b) 1 Nmkg^-2 c) 1 Nm²kg^-1 d) 1 Nmkg^-1

This site is open source. Improve this page.