Introduction
Physics is about the study of energy and forces. In order to test and measure physical quantities we need to define some standard measures which we everyone can agree on. These standards can never perfectly accurate because they are rooted in the physical world but every endevour is made to make them as precise as possible. The internationally recognised authority for the definition of these standards is the Conference Generale des Poids et Measures (CGPM).
Units
We must ensure that the result we use in our calculations are in the correct units. The consequence of getting it wrong can be very expensive as with the loss of the NASA Mars Climate Orbiter spacecraft in 1999. It spun out of control because part of the software assumed Imperial units and another part assumed metric units.
The units by which we now measure physical quantities is called the S.I. (System International) system established in 1960. Within this system, the most commonly used set of units in physics are M.K.S (Metres, Kilograms, Seconds) system
Base Units
The basic units are shown in Table. 1.
Quantity , Unit
Length , Metre (m)
Mass , Kilogram (kg)
Time , Second (s)
Electric Charge, Ampere (A)
Temperature , Kelvin (K)
Luminenscent Intensity ,Candela (Cd)
Table 1. Base Units
Derived Units
Multiplication of physical quantities creates new units. When you calculate the area, the unit becomes multiplied by itself to become, m2. The unit of area is an example of a derived unit. Other derived units occur so often they are named after illustrious scientists, in honour of their work. Table 2, list derived units and their special names.
Name , Symbol , Quantity , Expression in terms of other units
Hertz , Hz ,Frequency , 1/s
Newton , N ,Force , m.kg.s-2
Joule, J ,Work, N.m
Watt , W, Power, , J/s
Pascal , Pa, Pressure , N/m2
Lumen , lm Luminous,flux , 1/cd
lux ,lx, Illuminance, 1/(m2.cd)
Coulomb , C, Electric charge , A.s
Volt , V ,potential difference ,W/A = J/C
Ohm , Ω, Electric resistance, V/A
Farad, F, Electric capacitance ,C/V
Weber, Wb, Magnetic flux , J/A
Tesla, T, magnetic induction, V.s.m-2 = Wbm-2 kg.s-2.A-1
Henry , H ,Inductance , V.s/A=Wb/A
Siemens , S, Electrical conductance, Ω-1
Becquerel Bq Radioactivity s-1
Gray, Gy, Absorbed dose , J/kg
Sievert, Sv ,Equivalent dose, J/kg
Katal , kat ,Catalytic activity ,mol/s s-1.mol
Celsius , °C ,temperature , TC=TK-273.15
Table 2. Derived units and their special names.
Other Derived Units
Name , Symbol , Quantity
square metre , m2 , area
cubic metre , m3 , volume
metre per second , m•s−1 , speed, velocity
metre per second squared ,•s−2 , acceleration
metre per second cubed , m•s−3 , jerk
radian per second , rad•s−1 , angular velocity
Newton second , N•s , momentum
newton metre second , N•m•s , angular momentum
newton metre , N•m , torque, moment of force
reciprocal metre , m−1 , wavenumber
kilogram per cubic metre ,kg•m−3 , density, mass density
cubic metre per kilogram ,g−1•m3 , specific volume
mole per cubic metre , m−3•mol , amount (-of-substance) concentration
cubic metre per mole , m3•mol−1 ,molar volume
joule per kelvin , J•K−1 , entropy
Suplementary Units
These angular units and solid-angle unit are often used but are actually dimensionless.
Name , Unit
radian , rad
steridan , rad2
