Fundamental Unit & Measurement-Physics-Part 1- RRB JE, All Government Exams

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Any physical quantity has a definite magnitude of a quantity that is called unit. Units are two types: fundamental unit and derived unit.

S.I Unit & Symbol

Length Metre(m)
Mass Kilogram(Kg)
Time Second(s)
Temperature Kelvin(k)
Electric Current Ampere(A)
Luminous Intensity Candela(cd)
Amount of Substance Mole(mol)

• The unit of long distance or astronomical distance is Light Year (प्रकाश वर्ष).
1 Light Year = 9.46 x 1015 metre
• Parsec is the highest unit for distance measurement.
1 Parsec= 3.26 light year = 3.08 x 1016 metre
• The S.I unit of Force is Newton (N).
• C.G.S unit of Force is Dyne.
1 N = 105 Dyne
• S.I unit of Work is Joule
• C.G.S unit of Work is Erg
1 Joule= 107 Erg

There are two types of Physical Quantity:

1. Scalar Quantity: Which quantity has only magnitude and that doesn’t have direction, i.e. called scalar quantity.
Ex: – Distance, speed, current, Temperature, Pressure etc.
1. Vector Quantity: Which quantity has magnitude with direction, i.e. called vector quantity.
Ex: – Displacement, velocity, force, acceleration etc.
• Distance is always +ve. But Displacement is +ve, -ve or zero.
• Speed= distance/time, I unit= metre/second or m/s.
• Velocity (v) = dx/dt = rate of change of displacement with respect to time
S.I unit= m/s
• Acceleration (a) = dv/dt= rate of change of velocity with respect to time.
S.I unit= m/s2
[If the velocity of a body is reduced with time i.e. called retardation or –ve acceleration.]
• Force(F)= ma
(m= mass of object, a= acceleration)(S.I unit= Newton (N) or Kg m/s2)

Energy has two types:

• Potential Energy (P)= mgh ; m= mass, g=gravity, h=height
[ S.I unit= Joule(J)]
• Kinetic Energy (K.E)= (½)mv2 ; v=velocity, m= mass;
[S.I unit= Joule]   {Joule(J)= Kg m2/s2}

• Momentum(P)= Mass x velocity  [ S.I unit= Kg m/s]
• Density(d)=mass/volume(m/v)  [S.I unit= Kg/m3 ]
• Pressure: It is defined as a Force of one Newton per square metre. S.I unit of Pressure is Pascal(Pa).

The conversion between atm, Pa, and torr is:-
1 atm = 101325 Pa = 760 torr.