Measurement facts
Physics is a science of measurement. Lord Kelvin (18241907), one of the outstanding physicists have emphasised the importance of measurement.
Laws of physics are expressed in terms of physical quantity such as timing, force, temperature, density and numerous others. Physical quantities are often divided into fundamental and derived quantities. Derived quantities those whose definitions are based on other physical quantities e.g speed, area, intensity, etc. Fundamental quantities are not defined in the terms of other physical quantities, e.g. Length, mass and time etc.
Physical quantities may, in general, be divided into two categories
 Scaler quantities: A scaler quantities is one which has only magnitude. Mass, length, time, volume, speed, energy, work etc are examples of scaler quantities.
 Vector quantities : A vector quantity has both magnitude and direction. Velocity, momentum, force, acceleration etc are examples of vector quantities.
Measurement is the comparison of an unknown quantity with known fixed quantity.
Measurement involves comparison of the quantity to be measured with a reference standard. The reference standard of the measurement is what we call a unit.
for example unit of density is
Density = Mass/Volume = Mass/(Length)^{ 3 } = Kg/m^{3}
SI Base Quantities And Units
Base quantities  Units  Symbol is  Definitions 

Length  meter  m  The meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. (1983) 
Mass  kilogram  kg  The kilogram is equal to the mass of international prototype of the kilogram (a platinumirridium alloy cylinder) at international bureau of weights and measures, at Severs, near Paris, France. (1889) 
Time  second  s  The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state to the Cesium133 atom. (1967) 
Electric current  ampere  A  The ampere is that constant current, if maintained into straight parallel conductors of in finite length, of negligible circular crosssection, and placed 1 m apart in a vacuum, would produce between these conductors of force equal to 2×10^{7} newton per metre of length.(1948) 
Thermodynamic temperature  Kelvin  K  The kelvin, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. (1967) 
Amount of substance  Mole  mol  The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon12.(1971) 
Luminous intensity  candela  cd  The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x 1012 Hertz and that has a radiant intensity in the direction of 1/683 watt per steradian. (1979) 




