Dr.
Homi Bhabha Competition Test Series
UNITS FOR MEASUREMENTS
When you go to buy sugar, you do not say that "I want to buy a Apple", you will say "I want to
buy 1kg of Apple". The shopkeeper then weighs the Apple, against STANDARD weights and
gives you 1 kg of Apple. Similarly, when you measure with a ruler, say length of a pencil, the
markings on the ruler are STANDARD lengths. Take another physical quantity, say time. A
second, a minute and an hour on your watch has been set to a STANDARD. Now you may ask,
why this is so? The answer to this question is that anywhere on the earth, your measurements
should match anybody else's measurements.
When you say that you have bought 10 kg of Onion, whether you buy them in Mumbai, London
or New York, 10 kg of Onions have to be 10 kg of Onions and nothing else. Therefore, whenever
you measure any physical quantity, they are relative to or compared against
some STANDARD measurements. The international community from time to time announces
new, revised standards and all nations have to comply with these standards.
The basic quantity of the standard is known as a unit. The amount of a physical quantity that is
used as a reference for the measurement of that quantity is called the unit of that quantity.
CGS SYSTEM:
The CGS system was proposed in 1874 with Unit for length as Centimeter, Mass as Gram and
time as Second.
The sizes of many CGS units turned out to be inconvenient for practical purposes. For example,
many everyday objects are hundreds or thousands of centimetres long, such as humans, rooms
and buildings. Thus the CGS system never gained wide general use outside the field of science
MKS SYSTEM:
MKS is the system of units based on measuring lengths in meters, mass in kilograms, and time
in seconds. MKS is generally used in engineering and beginning physics,
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SI SYSTEM:
Fundamental or Basic Units of Measurement
In our universe there are large numbers of human beings and majority of them have different
names (though many have common names). Similarly, there are large numbers of the physical
quantities, and each of them should have the units of measurement. However, it is not
necessary to define separate unit for each and every physical quantity. Out of so many physical
quantities, there are only seven that do not depend on any other physical quantity for their
measurement. These are called as the fundamental quantities.
The seven fundamental quantities are: length, mass, time, temperature, electric current,
luminous intensity, and the amount of substance. Since these are totally independent
quantities, it is necessary to define the units of measurement for these units only. The units of
measurement defined for the fundamental physical quantities are called as the fundamental or
the basic units of measurement.
Derived Units of Measurement
Apart from the seven fundamental quantities, all the other physical quantities depend on the
fundamental quantities for their measurement. These quantities are derived from the
fundamental quantities, hence they are called as the derived physical quantities and the units
of measurement of these quantities are called as the derived units.
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In our above example, the length of the rod is the fundamental physical quantity. The physical
quantity, "length," does not depend on any other quantity. Now let is consider the example of
the speed of the car, which is let us say 100 miles per hour. In this case the physical quantity
speed depends on the miles (length) and hour (time). Thus speed is the unit derived from
fundamental quantities length and time and its units of measurement is derived from the units
of measurement of length and time.
Comparing all
SI MKS CGS
Length Meter(M) Meter(M) Centimeter(cm)
Mass Kilogram(Kg) Kilogram(Kg) gram(g)
Time Second (S) Second (S) Second (S)
Acceleration ms-2 ms-2 Galileo(G0
Electric charge coulomb( C ) coulomb( C ) Franklin (fr)
Electric current Ampere(A) Ampere(A) Biot(bi)
Energy, work Joule(J) Joule(J) erg
Force Newton(N) Newton(N) Dyne(dyn)
Pressure pascal(Pa) pascal(Pa) barye(ba)
-1 -1
Velocity, Speed ms ms cms-1
Temperature Kelvin(K) Kelvin(K) Centigrade (oC)
Area m2 m2 cm2
Cubic
Volume Cubic Meter (m3) Cubic Meter (m3)
Centimeter
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