Physical Quantities
All the quantities in terms of which laws of physics are described, and whose measurement is necessary are called physical quantities.
Units
A definite amount of a physical quantity is taken as its standard unit.
The standard unit should be easily reproducible, internationally accepted.
Fundamental Units
Those physical quantities which are independent to each other are called fundamental quantities and their units are called fundamental units.
Fundamental Quantities Fundamental Units Symbol
1. Length metre m
2. Mass. kilogram kg
3. Time. second S
4. Temperature. kelvin k
5 Electric current. ampere A
6 Luminous intensity. candela cd
7 Amount of substance. mole mol
Supplementary Fundamental Units
Radian and steradian are two supplementary fundamental units. It measures plane angle and solid angle respectively.
Supplementary Fundamental Quantities and their Units
1 Plane angle radian (rad)
2 Solid angle steradian (Sr)
Derived Units
Those physical quantities which are derived from fundamental quantities are called derived quantities and their units are called derived units.
e.g., velocity, acceleration, force, work etc.
Definitions of Fundamental Units
The seven fundamental units of SI have been defined as under.
1. 1 kilogram A cylindrical prototype mass made of platinum and iridium alloys of height 39 mm and diameter 39 mm. It is mass of 5.0188 x 1025 atoms of carbon-12.
2. 1 metre 1 metre is the distance that contains 1650763.73 wavelength of orange-red light of Kr-86.
3. 1 second 1 second is the time in which cesium atom vibrates 9192631770 times in an atomic clock.
4. 1 kelvin 1 kelvin is the (1/273.16) part of the thermodynamics temperature of the triple point of water.
5. 1 candela 1 candela is (1/60) luminous intensity of an ideal source by an area of cm’ when source is at melting point of platinum (1760°C).
6. 1 ampere 1 ampere is the electric current which it maintained in two straight parallel conductor of infinite length and of negligible cross-section area placed one metre apart in vacuum will produce between them a force 2 x 10-7 N per metre length.
7. 1 mole 1 mole is the amount of substance of a system which contains a many elementary entities (may be atoms, molecules, ions, electrons or group of particles, as this and atoms
12 in0.012kgofcarbonisotope6C .
Systems of Units
A system of units is the complete set of units, both fundamental and derived, for all kinds of physical quantities. The common system of units which is used in mechanics are given below:
1. CGS System In this system, the unit of length is centimetre, the unit of mass is gram and the unit of time is second.
2. FPS System In this system, the unit of length is foot, the unit of mass is pound and the unit of time is second.
3. MKS System In this system, the unit of length is metre, the unit of mass is kilogram and the unit of time is second.
4. SI System This system contain seven fundamental units and two supplementary fundamental units.
Relationship between Some Mechanical SI Unit and Commonly Used Units
S.No. Physical Quantity Unit
1 metric ton = 10^3 kg
1 micrometre = 10^-6 m
1 angstrom =10^-10 m
1 pound = 0.4537 kg
1 amu = 1.66 x10^-23 kg
1 litre = 10^-32 m3
1 dyne = 10^-5 N
1 kgf = 9.81 N
1 kgfm^2 = 9.81Nm^-2
1 mm of Hg = 133 Nm^-2
1 pascal = 1 Nm^-2
1 atmosphere pressure = 76 cm of Hg = 1.01 x 10^5 pascal
1 erg =10^-7 J
1 kgf-m = 9.81 J
1 kWh = 3.6 x 10^6 J
1 eV = 1.6 x 10^-19 J
1 kgf- ms-1 = 9.81W
1 horse power = 746 W
1 fermi =10^-15 m
1 X-ray unit = 10^-13 m
1 astronomical unit = 1.49 x 10^11 m (average distance between sun and earth)
1lightyear=9.46x10^15 m
1 parsec = 3.08 x 10^16 m = 3.26 light year
Some Approximate Masses of Objects in Kilogram
Our galaxy 2 x 10^41
Sun 2 x 10^30
Moon 7 x 10^22
Asteroid Eros 5 x 10^15
Note: Here ^ means - "to the power"
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