**Class XI Chapter 2**

__Units and Measurements__

** **Our measurements usually look like these:

mass 2 kg

length 5 metre

time 10 seconds

Each measurement contains a numerical value (number) and a unit.

** ***Q: What is a unit?*

*Answer:* *A unit is an internationally accepted standard value for a quantity (like mass, length, time etc).*

** **** **In physics, we have hundreds of different quantities like mass, length, time, speed, momentum, energy etc. Each quantity has its own unit.

** **Out of these hundreds of quantities, the scientists had decided to use only 7 quantities as **'Fundamental' quantities (or 'base' quantities**).

These 7 quantities are Mass, Length, Time, Temperature, ‘Electric current’, ‘Luminous intensity’ and ‘Quantity of matter’. Their units are called **'base units'**.

e.g. 'kilogram' (kg) is a base unit.

The other quantities are written in terms of these 7 base quantities. So, they are called **'Derived' quantities**.

Their units are called **'derived units'**.

e.g. Speed is a derived quantity whose unit is m/s (metre per second).

** ***A complete set of base and derived units is called a 'System of Units'.*

In earlier years, scientists of different countries used different system of units. Three such systems are CGS, FPS (or British system) and MKS.

But now an internationally accepted system of units is used, which is called '**SI**' system of units.

(to be continued ....... )