# Important Physics Questions

Unit: – The chosen standard used for measuring a physical quantity is called unit.

Unit should be:
(i)                 Well defined
(ii)               easy to reproduce
(iii)              Easy to compare
(iv)            internationally accepted
(v)              Independent of changes in physical conditions

Units are two types
(I)                Fundamental Unit
(II)              Derived Unit
Systems of Units depend on choice Each Choice of units leads to a new system (set) of units. The internationally accepted systems are (i) CGS system (ii) MKS System (iii) FPS System (iv) SI Units.

In SI Units, there are seven fundamental units given in the following table:

Physical Quantity     SI Unit      Symbol      Physical Quantity         SI Unit      Symbol
Length                        metre         m              Temperature                Kelvin      K
Mass                          Kilogram     Kg            Luminous intensity        candela  Cd
Time                          second         s             Amount of substance    mole    mol
Electric Current       ampere        A
Besides these seven fundamental units, two supplementary units are also defined viz radian ( rad ) for plane angle and steradian (sr) for the solid angle. All the units which are defined/expressed in terms of fundamental units are called derived units.

Some important derived units.
S.No. Physical Quantity   CGS units         SI unit          Relation
1.         Force                       dyne              Newton      1 Newton=10^5   dyne

2.         Work                        erg                  joule           1 joule= 10 ^7 erg

Some practical units of length mass and time.
Light                                                          Mass                                          Time
Light year =   distance               1 quintol = 10^2  kg                          1 solar day = 86400 sec.
Travelled by light in one            1 metric ton = 10^3 kg                     1 year=365 ½ solar days.
Year in vaccum.                        1 atomic mass unit (amu)                1 lunar month
1 astronomical unit(A.U)    or    Dalton = 1.66 X 10^-27 kg                 = 27.3 solar days.
=1.5 x10^11 m                           1 slug = 14.59 kg                         Tropical year = it is the
1 parsec = 3.26                          1 pound = 0.4537 kg                  year in which total solar
=3.08×10^16 m                                                                               eclipse occurs.
1 nautical mile or seamile    1Chandrashekhar limit =              Leap year = it is the year
=6020 ft.                                 1.4 times the mass of sun=         in which the month of
1 micron = 1 um=10^-6 m        2.8 x 10 kg                                      February is of 29 days.

1 Angstron (A) =10^-15m

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