**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**