**Principle of Measurements**

We know, measurement is the process to know a quantity accurately. Without measurement it is not possible to have accurate knowledge about a quantity. Actually, basic principle of physics is to take measurement of different quantities. For this reason, physics is called Measurement Science. To learn about a quantity we have two ways- one is qualitative and the other one is quantitative. Characteristics of matter and energy can be felt by our instinct and can be expressed in language. This is our qualitative knowledge about matter and energy. But in order to know them quantitatively measurement is needed and for this measurement some yardsticks are needed

To measure a physical quantity a convenient and a fixed part or portion of that is considered as standard and the quantity is measured with reference to that standard and everywhere that fixed portion or part is used. This standard of measurement is called unit or yardstick of that quantity. If it is said that the length of a room is 20 m, then we understand that a fixed length called a metre has been taken as standard and the room is 20 times longer than the standard. Again, if we say that mass of a body is 10 kg, then we understand that a unit called kilogramme has been taken as standard and the body is 10 times heavier than the standard. So, as many times the number of unit is found in a quantity, that number will be the index of measurement of that quantity and in order to take measurement of a quantity two things are needed. One is number and the other one is unit. One is meaningless without the other.

For example- there is 10 kg of rice in a ration bag. Here mass is a quantity and ’10’ is a number and kilogramme is unit. But if it is said that the mass of rice in the ration bag is 10, then it has no meaning. A quantity cannot be expressed only by number, unit is also needed to mention.

**So, Measurement of a quantity = number x unit**

**This is the principle of Measurement**