# What are Physical Quantities?

Anything that is measurable in this physical world is called a physical quantity. For example, the length of a table can be measured. Here, length is a physical quantity. The mass of your body can be measured, mass is a physical quantity. The time during which you are reading this book can be measured, time is a physical quantity. If you apply force to lift some thing, that force can be measured. So force is a physical quantity. There are many such quantities in this physical world. Among these, it is seen that there are a few quantities which can be measured without any help from any other quantities. These quantities are ‘fundamental quantities’.

For instance, to measure the length of a table, you need to measure only the length. To measure this length, there is no need of measuring any other quantity. So, length is a fundamental quantity. On the other hand, measurement of some quantities need the help of other quantities. For example, to measure the density of copper bar it is necessary to measure the mass and volume of a piece of copper bar and then mass is to be divided by the volume. Again, to measure the volume, the length, the breadth and the height are to be measured, that is, lengths are to be measured three times in three directions. So, it is seen that, there are certain quantities which are fundamental. They do not depend on other quantities. These are called fundamental quantities.

So, the physical quantities which are independent or neutral that is, they do not depend on other quantity, rather other quantities depend on them, are called fundamental quantities. Scientists have identified seven such quantities as fundamental quantities which are used in all branches of science for measurement.

These are (1) length (2) mass (3) time (4) temperature (5) electric current (6) luminous intensity and (7) amount of substance.

All other quantities may be derived from fundamental quantities that means, these are obtained from the product or quotient of one or more fundamental quantities. These are called derived quantities or compound quantities. So, the quantities depend on fundamental quantities or obtain from fundamental quantities are called derived quantities. Velocity, acceleration, force, work, heat, electric current etc. are derived quantities since these are obtain from fundamental quantities.

For instance, force = mass * acceleration

then, mass * (velocity/time) [here, acceleration = velocity/time]

then,  mass * (distance/time) * (1/time)  [here, velocity = distance/time]

so, mass * distance/time2

Hence, force is a derived quantity.