Physical quantity

when we want to study about a natural phenomenon, we may want make a hypothesis and to search for a mathematical relation. The mathematical relation contains many parameters concerning the phenomenon. The various parameters appearing in the mathematical relation are called physical quantities. For examples,to study the motion of a planet around the sun, various parameters to be studied may be the time period for one revolution, the angular velocity of the planet, the distance between the planet and the sun, the mass of of the planet etc.
The most common physical quantities in our daily life are mass, volume, area,length,time etc.In fact a standard unit should have the following characteristics:

1. The standard unit should be portable size.
2. It should be well defined.
3. It should be universal;i.e. it should be accepted by all the con tries in the world.
4. It should be easily available everywhere in the world.
5. It should be independent of time and place.
6. It should be independent of physical condition such as temperature,pressure etc.
7. It should be cheaply available.