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As pointed out earlier, the basic units should be correlated to our immediate context. For example, a meter represents a length that we are able to correlate and visualize with the physical entities in our world. For example, we say that height of the room is 2.1 m – not something like 2.1 X 10 - 11 m.

Nature presents a kind of continuum, which ranges from very small to very large. Consider the dimensions of a nucleus (∼ 10 - 15 m) and distance of the sun (∼ 10 11 m). The system of units, therefore, needs to have a scheme to express wide variations often seen in physical quantities.

Finally, the advancement in scientific studies has expanded scope of studies much beyond human physical existence. We study atoms at one hand and galaxies on the other. The quantities involved are ether so small or so big that the physical comparison with a real time measuring device may not be possible. For example, we can not think of going inside an atom and measure its radius with a scale. Inferred (indirect) measurements are, therefore, allowed and accepted in such situations.

Features of fundamental units

Following are the features/ characteristics of fundamental units :

  • They are not deducible from each other.
  • They are invariant in time and place (in classical context).
  • They can be accurately reproduced.
  • They describe human physical world.

International system of units

Its short name is SI system, which is an abbreviated form of French equivalent “Systeme Internationale d’ units”. As study of science became more and more definitive and universal, it was felt to have a system of units, which can be refereed internationally. The rationales for adopting SI system as international system are two fold. First, this system is based on the powers of 10. Second, there is a well structured prefixes to represent range of measurements associated with a physical quantity.

The “power of 10” makes it easy to change smaller to bigger unit and vice versa. A mere shift of “decimal” does the job.

12.0 mm (smaller) = 1.20 cm (bigger)

Equivalently, we multiply the given quantity with 10 raised to positive integer to obtain the measurement in terms of smaller unit; and divide it with 10 raised to positive integer to obtain the measurement in terms of bigger quantity.

Finally, SI system has a set of prefixes for a given unit to represent smaller or bigger quantities. This set of “prefixed” represents a predefined factor in terms of the power of 10. We should remind ourselves that all of these prefixes are applicable uniformly to all quantities.

Prefix factors

The factors are powers of 10.

Femto( 10 - 15 ), pico( 10 - 12 ), nano( 10 - 9 ), micro( 10 - 6 ), milli( 10 - 3 ), centi( 10 - 2 ), deci ( 10 - 1 ), deka ( 10 1 ), hector( 10 2 ), kilo( 10 3 ), mega( 10 6 ), giga( 10 9 ),tera( 10 12 ),peta( 10 15 )

Note that except for few prefixes in the middle, the powers of the factor differs by “3” or “-3”.

Basic units

The seven basic quantities included in SI system of measurement are :

  1. Length
  2. Mass
  3. Time
  4. Current
  5. Temperature
  6. Amount of substance (mole)
  7. Luminous intensity

The corresponding seven basic units with their symbols are defined here (as officially defined):

1: meter (m) : It is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

2: kilogram (kg) : It is equal to the mass of the international prototype of the kilogram. The prototype is a platinum-iridium cylinder kept at International Bureau of Weights and Measures, at Severes, near Paris, France.

3: time (t) : It is the duration of 9, 192, 631, 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium - 133 atom.

4: ampere (A) : It is is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 X 10 7 newton per meter of length.

5: kelvin (K) : It is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

6: mole (mol) : It is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12.

7: candela (cd) : It is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 X 10 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian (measure of solid angle).

Mks system of units

MKS is an abbreviation of M eter, K ilogram and S econd. These three quantities form the basic set of units in MKS system. Clearly, it is a subset of SI system of units. As we can realize, mechanics - a branch of physics - involves only length, mass and time. Therefore, MKS system is adequate to represent quantities used in mechanics.

This distinction between mechanics and rest of physics is hardly made in recent time. We can, therefore, completely do away with MKS nomenclature in favor of SI system.

Conversion of units

Despite endeavor on world level for adoption of SI unit, there are, as a matter of fact, wide spread variation in the selection of unit system. Engineering world is full of inconsistencies with respect to the use of unit system. We often need to have skill to convert one unit into another. We take a simple example here to illustrate how it is done for the case of basic quantity like mass.

Let us consider a mass of 10 kg, which is required to be converted into gram - the mass unit in cgs unit (Gaussian system). Let the measurements in two systems are “ n 1 u 1 ” and “ n 2 u 2 ” respectively. But, the quantity, “Q”, is “10 kg” and is same irrespective of the system of units employed. As such,

Q = n 1 u 1 = n 2 u 2

n 2 = u 1 u 2 n 1

n 2 = 1 kg 1 gm n 1

n 2 = 10 3 gm 1 gm 10

n 2 = 10 4

Q = n 2 u 2 = 10 4 gm

The process of conversion with respect to basic quantities is straight forward. The conversion of derived quantities, however, would involve dimensions of the derived quantities. We shall discuss conversion of derived quantities in a separate module.

Questions & Answers

A stone propelled from a catapult with a speed of 50ms-1 attains a height of 100m. Calculate the time of flight, calculate the angle of projection, calculate the range attained
Samson Reply
water boil at 100 and why
isaac Reply
what is upper limit of speed
Riya Reply
what temperature is 0 k
Riya
0k is the lower limit of the themordynamic scale which is equalt to -273 In celcius scale
Mustapha
How MKS system is the subset of SI system?
Clash Reply
which colour has the shortest wavelength in the white light spectrum
Mustapha Reply
how do we add
Jennifer Reply
if x=a-b, a=5.8cm b=3.22 cm find percentage error in x
Abhyanshu Reply
x=5.8-3.22 x=2.58
sajjad
what is the definition of resolution of forces
Atinuke Reply
what is energy?
James Reply
Ability of doing work is called energy energy neither be create nor destryoed but change in one form to an other form
Abdul
motion
Mustapha
highlights of atomic physics
Benjamin
can anyone tell who founded equations of motion !?
Ztechy Reply
n=a+b/T² find the linear express
Donsmart Reply
أوك
عباس
Quiklyyy
Sultan Reply
Moment of inertia of a bar in terms of perpendicular axis theorem
Sultan Reply
How should i know when to add/subtract the velocities and when to use the Pythagoras theorem?
Yara Reply
Centre of mass of two uniform rods of same length but made of different materials and kept at L-shape meeting point is origin of coordinate
Rama Reply

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Source:  OpenStax, Physics for k-12. OpenStax CNX. Sep 07, 2009 Download for free at http://cnx.org/content/col10322/1.175
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