Unit and Dimension

Unit : The chosen standard used for measuring a physical quantity is called unit.

1. well defined

2. easy to reproduce

3. easy to compare

4. internationally accepted

5. independent of changes in physical conditions

Units are of two types :

1. Fundamental Unit and

2. Derived Unit

System of Units: Units depend on choice. Each choice of units leads to a new system (set) of units. The internationally accepted systems are 1. CGS system 2. MKS System 3. FPS System 4. SI Units.

In SI Units, there are seven fundamental units given in the following table :

Besides these seven fundamental units, two supplementary units are also defined, viz., radian [rad] for plane angle and steradian (sr) for solid angle.

⇒ All the units which are defined/expressed in terms of fundamental units are called derived units.

⇒ Astronomical distances are measured in light year because speed of light is always the same.

Dimension of a physical quantity are the powers through which different fundamental quantities are raised to define the quantity.

The expression by which a physical quantity is defined in terms of fundamental quantities is called dimensional formula of the quantity.

For example : Dimensional formula of momentum is MLT-¹

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