**Science|Class 11th|Notes|Physics|Unit- 1. Physical World And Measurements Chapter-1 Physical World part -3**

**Measurement**

In the nineteenth century Lord Kelvin stated that physics is the science of measurement "so it is necessary to express any feeling in terms of numbers. For example one of the three utensils considered cold water. in the second there is warm water and in the third there is warm water. If we put our finger in cold water first and then put it in light hot water, then the sense of our touch - power (sense) Will experience hotness, but if we put our finger in more hot water first, then in warm hot water, our sense of touch - the power of touch will experience coldness.

The measurement from is unreliable. It is clear from how hot the water is "a number is needed to express it.

Knowledge of an observation can be said to be satisfactory only when it can be revealed by a number.

**Physical Quantity**

All those quantities which can be expressed by a number and can be measured directly or indirectly, are called physical quantities.**For example, length, mass, temperature, speed, force, time etc. **are physical quantities.

**Unit **- To measure a physical amount, a certain quantity of that amount is considered as '**standard**' and gives this standard a name.

This standard is called the** 'unit**' of that physical zodiac.

Measurement is the comparison of a given unknown amount to the chosen standard.

To measure a physical amount, it is necessary to know the following two things:**(ii) Numerical value -** which shows how often its unit is included in that amount.

(i) **Unit** - in which that physical amount is to be measured.

**For example****:-**

If it is said that the mass of a wheat sack is __100__ kg (kg), it means that the unit of mass is done and it is included __100__ times the mass of the wheat sack.

Similarly, if the length of a rod is 12 meters, it means that the length is the meter to measure and it is included 12 times the length of the rod.

If we take a small unit of length centimeter (cm), the length of the same rod will be __1200__ cm.

Now if we take a smaller unit of length and millimeter (mm), the length of the rod will be 12,__000__ mm.

It is clear from this example that the smaller the unit of measurement of a physical amount, the higher its numerical value means that in the measurement of a given physical amount, its numerical value and the product of its corresponding unit always remain constant.

If the numerical values of a physical zodiac are different units u1,u2, u3 ...

In n1, n2, n3 respectively

If n1 [u1] = n2 [u2] = n3 [u3] = constant

ie**n [u] = constant**

Apparently, **the numerical value of a physical amount is inversely**

**proportional to its unit.**

**Standard -** A chosen standard must have the following properties:

**(1)****Well Defined -** The standard must be clearly defined.

**(ii) Accessibility -** The standard should be readily available.

**(iii) Invariability -** The standard should be constant so that weather etc. is not affected by it.

In ancient times, special care was taken for the availability of physical standards.**For example****:-** standard yards, feet and inches of length were obtained by measuring the measurement of the person's arm, leg and thumb respectively.

Accurate measurement is required nowadays, so such flawed standards have no place because the constancy of physical standards has special significance.

Some physical quantities (such as distances between stars and the Earth) cannot be measured directly, so they have to be used by Indirect methods.

**Fundamental Units**

The units of different physical quantities are different, but they are not necessarily independent of each other;

Because many of these quantities are interrelated.**For example****:- ** speed, distance and time are related to the formula **speed = (distance / time)**, so the unit of speed can be expressed in units of** distance (length)** and** time**.

In fact, all the physical quantities in mechanics are expressed in units of **length, mass **and** time.**The units of these three zodiac signs are completely independent from each other and none of these units can be converted or related to any other unit.

Therefore, in mechanics the units of these quantities are called basic units.

The measurement of physical quantities - weighed mainly in three countries in three methods, which are named after the basic units:

1.

**Centimeter-gram-second system**

**(**

**C.G.S.System**

**)**- In this method, length, mass and time unit are centimeters, grams and

**2. Foot-pound-second system (F.P.S. System)**- In this method the units of length, mass and time are taken in feet, feet and seconds respectively.

This method is also called the British method.

**3. Meter - Kilogram Secondary System**

**(**

**M.K.S.System**

**)**- In this method, units of length, mass and time have been considered as meters, kilograms and seconds respectively.

Basic signs are dead signs that are completely independent of each other and cannot be related to each other.

**(Fundamental units are those units which are entirely independent of one another and can not be related with one another.)**

The basic and complementary unit of international unit system

Type of Unit Amount Unit symbol

(Fundamental Units)

**1.Length**** - meter - m**

**2****.****Mass**** **** ****- mass - kg **

**3.****Time**** - second - s **** **

**4.****Electric current****- ampere - A**

**5.Thermo-dynomic temperature - kelvin - cd**

**6.Luminous intensity of a source - mole - mol**

**7.Amount of substance **** **

**(S**upplementary unit)

**1,plane angle - radian - rad**

2. **solid angle - steradian - sr**

**Derived Units**

In addition to the basic quantities** (length, mass, time, electrical current, volume of matter, temperature, light intensity of the source and quantity of matter**), the units of all other physical quantities are obtained by applying appropriate powers to one or more basic units.Can go

Such units are called **derived units**.** **

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