# Physical Quantity

**Physical Quantity.**

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

**Measurement** is a process in which a physical quantity is compared with its unit.

Measurement is a comparison process.

Measurement is necessary to determine magnitude of a physical quantity, to compare two similar physical quantities and to prove physical laws or equations.

** Representation: **A physical quantity is represented completely by its magnitude and unit. For example, 100

*metre*means a length which is hundred times the unit of length 1

*kg*. Here 100 represents the numerical value of the given quantity and

*metre*represents the unit of quantity under consideration. Thus in expressing a physical quantity we choose a unit and then find that how many times that unit is contained in the given physical quantity,

*i.e.*

Physical quantity (*Q*) = Magnitude × Unit = *n *× *u*

*n* represents the numerical value and *u* represents the unit. Thus while expressing definite amount of physical quantity.

Note: as the unit(*u*) changes, the magnitude(*n*) will also change but product ‘*nu*’ will remain same.

* i.e.* ** n u = constant, ** or

therefore

i.e. Larger the unit, smaller will be the magnitude.

**Types of Physical Quantity.**

(1)** Scalar** : These quantities which have only magnitude and do not have any direction *e.g*. Length, tim e, work, energy *etc*.

Magnitude of a physical quantity can be negative. Scalar quantities can be added or subtracted with the help of following ordinary laws of addition or subtraction.

(2) **Vector **: These are those quantities which have both magnitude and direction. *e.g.* displacement, velocity, acceleration, force *etc*.

Vector physical quantities can be added or subtracted according to vector laws of addition. These laws are different from laws of ordinary addition.

**Note : ** There are certain physical quantities which behave neither as scalar nor as vector. For example, **moment of inertia** is not a vector as by changing the sense of rotation its value is not changed. It is also not a scalar as it has different values in different directions (*i.e.* about different axes). Such physical quantities are called **Tensors.**