# Units and Measurements, Class 11 Physics ( based on CBSE Revised syllabus )

## Need for measurement:

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The study of physics completely depends upon the quantitative knowledge of physical quantities. To gain  this quantitative knowledge or data measurement is necessary.

To understand the importance and needs for measurement, we can start with certain examples : As we know that to move a body force is necessary. Now if a force is applied on a body, the body gets accelerated and the value of acceleration depends upon the force applied on it. So if we want to measure the acceleration of a body we shall have to measure the magnitude of force applied on the body and mass of the body.

Similarly, if we throw a stone along the horizontal and after covering some distance, it comes to rest. Now if we want to know about the distance travelled by the  stone then we will have to measure the initial speed of the stone, time taken by the stone to travel that distance and acceleration of the stone during the motion.

Therefore, to know or study  about any natural phenomenon aur physical situation, we must measure some  quantities related to the situation.

The quantities which can be measured and used to explain any physical phenomenon are known as physical quantities.

### Units of measurement:

If we are about to measure any physical quantities we should have a base or a standard value of the same kind, so that we can compare that quantity from that standard value.

The standard value of the same kind, chosen as a reference in order to measure a physical quantity is called Unit of that physical quantity.

The meaning of the measurement of physical quantity is to compare that physical quantity with its standard value or unit. That is to find out that the given value of physical quantity is how many times of its unit. This we can express the process of measurement of physical quantities in 2 steps as

• The selection of unit and

• to find out  the number of times that unit is contained in the physical quantity.

For example, if we want to measure the length of a wall then first we have to select an unit or standard value. Suppose that we use metre as the unit. Suppose We use a stick or metal rod of 1 metre and place it successively along the length of the wall and find out how many times we had to use that metre rod successively. Consider we used it 5 times. Then 5 is the numerical value of the length of the wall if we use metre as the standard value or unit.

Hence length of the wall is = 5 × 1m

= 5 m.

It is to be noted here that we cannot mention the value of length by writing only its numerical value. we must mention the unit along with that numerical value, as we have mentioned it here. In length 5m, 5 is the numerical value and metre ( m ) is its unit. If we mention the length by writing only 5 then it will be wrong information about length.

### System of Units:

Common systems of units are

• cgs system

• fps system

• mks system

• SI system

cgs system:

This was used mainly in France. In this system of unit centimetre, gram and second are used as the fundamental units of length, mass and time respectively.

fps system :

This is also known as British System of units. In this system of units foot, pound and second are used as the fundamental units of length, mass and time respectively.

mks system :

This system was also set up in France. In this system of units metre, kilogram and second are used as the fundamental units of mass, length and time respectively.

### SI system or SI units :

The units of mass, length and time can be used to obtain the units of physical quantities in mechanics only. These 3 units are not sufficient to derive all the physical quantities to study different branches of Physics. A new system of units was introduced in 1960, and is named as Systeme Internationale d' Unites and it is abbreviated as SI Units.

In this system of units, there are 7 basic and 2 supplementary units. These are called SI units and are sufficient to obtain units of all physical quantities in different branches of physics. These are shown in figure. Fig : SI system of units