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Units and Measurements in Physics

Hi viewers:

As you may know, that units and measurements in physics is very important topic.  It is because all the other topics in physics is very much related to this topic.  So before jumping into the ocean of physics one should grasp thoroughly this topic units and measurement with utmost passion.

Physical Quantity

Anything that can be measured is called a physical quantity.  Eg.  Length and Breadth

Quantities are two types scalar and vector.


Scalar Quantity

It is that quantity to express which magnitude is known only and no direction is known is called scalar quantity.  Eg.  Speed, time, length and mass

Vector Quantity 

It is that quantity to express which both magnitude and direction are required is known as a vector quantity.  Eg.  Weight, velocity, acceleration, force.



It is a standard convenient and internationally accepted portion of a physical quantity by which we can measure quantities of similar type.

The utility of a unit is to express the measure of a physical quantity accurately.  For example when we say the length of a field is 100, it makes no senses and no clear expression.  But when we say the length of a field is 100 m then it becomes clear.  Therefore with the application of the unit ‘m’ the statement carries a clear idea.  This is the utility of a unit.

Fundamental unit

Unit of mass, unit of length and unit of time are taken as fundamental unit as they are independent of each other.


Derived unit

Derived units are the units, which are derived from one or more fundamental units.  Eg. Unit of speed – Km/hr,  Density- cubic centimeter.


System of units

French System – C.G.S. system – cm, gm and sec

British System – F.G.S. system – foot, pound and sec

M.K.S. System – meter, kilogram and sec

S.I. System – LS-2 unit of acceleration



It is the unit of the length in C.G.S. System.  At O°C the distance between two lines of a

platinum – Iridium bar kept at the international bureau of weights and measures at Paris in France is taken as a meter.  The one hundredth part of a meter is a centimeter.



It is the unit of length in F.P.S.  At 32°F  the distance between two transverse line on a bronze bar kept at the British Exchequer at London is taken as a yard.  One third of a yard is called a foot.

1 mile = 1760 yards,  1 yard =  3 feet,  1 foot = 12 inches



It is the unit of mass in the C.G.S. system.  The mass of certain cylindrical piece of platinum-iridium alloy kept in Paris is called a standard kilogram, which is known as the international prototype Kilogram.  One thousandth part of Kg. is a gram.



It is the standard unit of mass in the F.P.S system.  The mass of a certain piece of platinum kept at the standard department of the Board of Trade in England is taken as a pound and is known as the prototype pound.


Unit of Volume

The space occupied by a substance is known as its volume.  If the length breadth and height of a cube is each one cm, then the volume of the cube is taken as the unit of volume in C.G.S. system.  It is called one cubic centimeter or one c.c.


Unit of Density

Density at a temperature may be defined as mass per unit volume at that specific temperature. Eg. Density of water is 1gm/c.c. at 4°C.

Density is dependent on temperature with the increase of temperature the mass remains constant while the volume generally increases and so density as a whole decrease.


Unit of Time

Earth possesses two kinds of motion.  The rotational motion from west to east on the geographical axis is called the diurnal motion, which causes day and night.  When the sun attains the highest position in the celestial sphere, it is said to be on the meridian.  The interval of time between any two successive passage of the sun across the meridian is called an apparent solar day.  On measuring the length of the apparent solar day for some consecutive days, we see it is not constant, but it varies slowly.  The average taken over a year is known as a mean solar day.  A mean solar day has 24 hours.  1/24 th part of a day is one hour, 1/24 th part of a hour is a minute and 1/24 th part of a part of a minute is second.


How to write the SI units correctly


1.  Names of the units are to be written in a lower case.  Eg. metre, kilogram, second

2.  Units named after scientists are to be written in lower case.  Eg. kelvin, newton, joule

3.  The symbols for the units derived from the names of scientists are written in capital letters.
Eg. °C, °F, °K, N (newton), J(joule)

4.  Symbols are followed by any full stop Eg. 75cm and not 75cm.

5.  But symbol occurs at the end of a sentence can be followed by a full stop indicating the end of the sentence

6.  Units are never written in plural.


Advantage of C.G.S. over F.P.S.

CGS is accepted internationally by all but FPS is confined to a particular country England

Conversion of units of CGS is much easier than FPS simply sifting the decimal point to the right or left we can change the units

In CGS the mass, volume and density are conveniently related to one another but FPS the mass, volume, and density are not co-related

Density of water in FPS = 62.5 lbs/cubic foot at 62°F

Density of iron in FPS = 62.5x7lbs/cubic foot at 62°F


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See you in my next blog post.



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Hi viewers My name is Partha Pratim Goswami, (PpG) I am a teacher by profession. I have been teaching biology, chemistry and physics, sometimes mathematics also for last sixteen years. I would like to share my entire experience with all the viewers across the world through YouTube, which I think is an amazing platform to share all my experiences.

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