Introduction

Any quantity that can be measured directly or indirectly is said to be physical quantity. For example: length of a book can be measured by using a measuring scale (directly) and volume or an irregular stone can be calculated by using liquid displacement method (indirectly). However, such quantities can be dependent or independent over other quantities. For example: when you decide to measure the area of a square, you will need to know its side length or length of diagonal. What did you observe? You needed to measure length before calculating area. Thus, there is the classification of physical quantities into fundamental physical quantities and derived physical quantities. The units associated with such quantities are called fundamental units and derived units, respectively.

Fundamental Physical Quantity

Any physical quantity that can be measured directly and is independent of any other quantity is called fundamental physical quantity. It has its own identity. For example: length, mass, time, etc. In SI system of measurement, there are seven fundamental physical quantities.

Derived Physical Quantity

Any physical quantity that cannot be measured directly and is dependent on fundamental physical quantities is called derived physical quantity. It does not have its own identity. For example: area, velocity, volume, etc. In SI system of measurement, there are hundreds and thousands of derived physical quantities.

Unit

Recall the moment when you go to buy vegetables in the stores. They weigh the vegetables as much as you desire to buy. If you asked for 1 kilogram of vegetables, they use a weighing balance and put your vegetables on one side of the balance. Then, they keep a standard reference measurement of 1 kilogram on the other side. When they equal each other, then we get assured that the vegetable is now 1 kilogram.

Yes, that standard reference quantity used in measurement is said to be unit. A unit is a measured quantity that acts as a standard for doing measurement.

When you say 2 kilograms of vegetables, you just mean that it is 2 times 1 kilogram. When you say half a kilo of vegetables, you mean that it is 1/2 of 1 kilogram. So, 1 kilogram has remained the standard go-to reference quantity during measurement.

Units are introduced to bring uniformity in measurement across the world. Just imagine, what does 1 kilogram feel like if there were no certain standard to compare it? We could be assuming 10 kilograms to be 1 kilograms, too. Or, as little as 100 gram could be said 1 kilogram. So, it would hamper the trust in the marketplace. Hence, units are essential in measurement. Therefore, never forget to mention units after you find your answer to any Numerical Problem.

Fundamental Units

The units of fundamental physical quantities is called fundamental units. Example: meter for length, second for time, kilogram for mass, etc. In SI system of measurement, there are 7 fundamental units.

Derived Units

The units of derived physical quantities is called derived units. Example: Pascal for pressure, Newton for Force, etc. In SI system of mesurement, there are hundreds and thousands of derived units.