**Physical Quantity:**The quantities by means of which we describe the laws of physics are called Physical Quantities. Measurement: The comparison of any physical quantity with its standard unit is called measurement. Unit: The ‘Standard’ of measurement of a physical quantity is called the unit. The magnitude of the physical quantity:

**Magnitude = (Numerical value of the measure of the quantity) * (Unit of the quantity)**

Systems of Units: A system of units is the complete set of units, both fundamental and derived, for all kinds of physical quantities.

- CGS System: In this system, the unit of length is centimetre, the unit of mass is gram and the unit of time is second.
- FPS System: In this system, the unit of length is foot, the unit of mass is pound and the unit of time is second.
- MKS System: In this system, the unit of length is metre, the unit of mass is kilogram and the unit of time is second.
- SI System: This system contains seven fundamental units and two supplementary fundamental units.

**Fundamental Units:** Those physical quantities which are independent of each other are called fundamental quantities and their units are called fundamental units. Fundamental quantities: length, mass, time, electric current, temperature, luminous intensity, and

**Fundamental quantities:** length, mass, time, electric current, temperature, luminous intensity, and amount of substance. Radian and steradian are two supplementary fundamental units. It measures

Radian and steradian are two supplementary fundamental units. It measures the plane angle and solid angle respectively. Derived Units: The units of other quantities which are derived from mass, length and time are called derived units.

**Derived Units:** The units of other quantities which are derived from mass, length and time are called derived units.

**SI Units**

**SI prefixes**

**Dimensions** of a physical quantity are the powers to which the fundamental quantities must be raised to represent the given physical quantity.

In mechanics, all physical quantities can be expressed in terms of mass (M), length (L) and time (T).

**Homogeneity Principle:** If the dimensions of the left-hand side of an equation are equal to the dimensions of the right-hand side of the equation, then the equation is dimensionally correct.

**Applications of Dimensions:**

- To check the accuracy of physical equations.
- To change a physical quantity from one system of units to another system of units.
- To obtain a relation between different physical quantities

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