Physics is the study of nature and its laws. The word physics has come from the Greek word fusis which means nature. Physics can be explained with the help of following theories.
✦ Theory of relativity
✦ Quantum mechanics
Physical Quantities, Standards and Units
Measurement is the fundamental of physics. Measurement is the process of comparison of an unknown quantity with a standard quantity of the same kind. Without measurement there is no law of physics. Laws of physics are expressed in terms of physical quantities such as time, force, temperature, density and numerous others.
Physical quantities are often divided into fundamental and derived quantities. Derived quantities are those whose definitions are based on other physical quantities, i.e., fundamental quantities, e.g., speed, area, density, etc.
Fundamental quantities are not defined in terms of other physical quantities, e.g., length, mass and time. Physical quantities in general are divided into two classes; (1) Scalar quantities, and (2) Vector quantities.
A scalar quantity is one which has only magnitude.
A vector quantity has both magnitude and direction.
Unit of Length
The SI unit of length is the metre (m). Various other metric units used for measuring lengths are related to metre by either multiples or submultiples of 10. Thus,
1 kilometre (km) = 1000 (or 103)m
1 centimetre (cm) = 1/1000 (or 10-2)m
1 millimetre (mm) = 1/1000 (or 10-3)m
Very small distances are measured in micrometres or microns (μm), angstroms (A), nanometers (nm) and femtometres (fm)
1μm = 10-6 m
1nm = 10-9 m
1A = 10-10 m
1 fm = 10-15 m
The light year is a unit of length and is equal to the distance travelled by light in one year.
1 light year = 9.46 x 1015 m
= 9.46 x 1012 km
An astronomical unit (A.U) is the mean distance from the centre of the earth to the centre of the sun.
1 A.U = 1.495 x 1011m
A parsec is the distance at which an astronomical unit subtends an angle of one second of arc.
1 parsec = 3. 084 x 1016m
Unit of Mass
The SI unit of mass is the kilogram (kg). In practice, multiples and sub – multiples of kilogram are used:
1 tonne (t) = 103kg (1000 kg)
1 gram (g) = 10-3 (1/1000 kg)
1 milligram (mg) = 10-6 kg
Unit of Time
The SI unit of time is the second(s). In the earlier days people used different system of units for measuring length, mass and time.
FPS (Foot, Pound and Second) system
CGS (Centimetre, Gram and Second) system
MKS ( Metre, Kilogram and Second) system
British (or FPS) system of units
In this system, the unit of length is foot (ft), the unit of mass is pound (lb) and the unit of time is second (s).
1 foot = 0.3048 m
1 pound = 0.4536 kg
The unit of time is same in both the S.I and the F.P.S systems.
The CGS System
It is another system of measure used in earlier days. In this system unit of length is Centrimetre (cm), Gram (gm) and Second (s).
The SI system
There are 7 basic quantities in the SI system and 2 supplementary quantities.
Length - Metre (m)
Mass - Kilogram (kg)
Time - Second (s)
Electric Current - Ampere (A)
Temperature - Kelvin (K)
Luminous Intensity - Candela
Amount of substance - Mole (mol)
Plane Angle – Radian
Solid Angle - Steradian
Plane angle and solid angle are supplementary quantities till 1995. They were derived quantities now.
Quantities got by the multiplication or division of fundamental physical quantities like length, mass and time are called derived quantities. Apart from these above mentioned derived quantities, there are some other derived quantities like area, volume, density, etc.
Important derived units
Physical quantity definition SI unit
1)Area Length square m2
2)Velocity Displacement – per unit time ms-1
3)force mass X acceleration kgms-2