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Physical Quantity.

A quantity which can be measured and by which various physical happenings can be explained and expressed in form of laws is called a physical quantity. For example length, mass, time, force etc.

Types of Physical Quantity.

(1) Ratio (numerical value only): When a physical quantity is a ratio of two similar quantities, it has no unit.

(2) Scalar (Magnitude only): These quantities do not have any direction e.g. Length, time, work, energy etc.

(3) Vector (magnitude and direction) : e.g. displacement, velocity, acceleration, force etc.

Fundamental and Derived Quantities.

(1) Fundamental quantities: These quantities are called fundamental or base quantities, as all other quantities are based upon and can be expressed in terms of these quantities.

(2) Derived quantities : All other physical quantities can be derived by suitable multiplication or division of different powers of fundamental quantities.

Fundamental and Derived Units.

Arbitrarily the physical quantities mass, length and time are choosen for this purpose. So any unit of mass, length and time in mechanics is called a fundamental, absolute or base unit. Other units which can be expressed in terms of fundamental units, are called derived units.

System of units : A complete set of units, both fundamental and derived for all kinds of physical quantities is called system of units. The common systems are given below –

· CGS system

· MKS system

· FPS system

· S. I. system