SI - The International System of Units

							SI - The International System
           of Units




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             Overview
 Units and Dimensions
 The SI Sysytem
 Summary




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            Dimensions
 Dimension: A physical property, such
  as mass, length, time, temperature or
  a combination thereof, regarded as a
  fundamental measure or as one of a set
  of fundamental measures of a physical
  quantity.




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             Dimensions
 Fundamental Dimensions
  – Mass, Length, Time (M, L, T) or Force,
    Length, Time (F,L,T)
  – Temperature (q)
  – Electric Current
  – Amount of Substance
  – Luminous Intensity



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              Dimensions
 Derived Dimensions
  – velocity (L/T), acceleration (L/T2)
  – momentum (MV)
  – work, energy (FL) or (ML/T2)
  – pressure, stress (F/L2)




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                   Units
 Unit: A precisely specified quantity in
  terms of which the magnitudes of other
  quantities of the same kind can be
  stated.
 Examples
  – grams, pounds
  – meters, inches, furlongs
  – hours, seconds, years


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                Units
 Base, Supplementary, Derived




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                     Units
 Base Units: Units regarded as
  dimensionally independent
 Examples:
  – meters, inches
  – grams, pounds
  – seconds




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                         Units
 Derived Units: Units formed by combining
  base units, supplementary units and other
  derived units according to algebraic relations
  linking the corresponding quantities.
 Examples
  – velocity (m/s)
  – area (ft2)
  – pressure (lbs/in2)




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                      Units
 Supplementary Units: In the SI System,
  two special derived units - the radian
  and the steradian
 Example
  – radian: the unit of plane angle
    measurement calculated as the ratio of the
    circular arc length subtended divided by
    the radius of the circle.
     o a length divided by a length


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            Unit Systems
 Unit Systems
  – lb-ft-sec
  – kilogram-meter-sec
  – dyne-centimeter-sec
  – many more ….




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           Unit Systems
 force = mass x acceleration

            accel

force
            mass




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             Unit Systems
 Coherent System of Units: A system of units
  in which one unit of mass is accelerated one
  unit of acceleration by one unit of force.
 Example:                    1 N = 1 kg x 1 m/s2
                1 m/s2
                                   or
  1 Newton                    1 N = 1 kg m/s2
                1 kg




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           Unit Systems
 Coherent or Consistent Systems (cont.):
 Example:
                         1 lb = 1 slug x 1 ft/s2
             1 ft/s2
                               or
 1 lb                    1 lb = 1 slug ft/s2
             1 slug




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             Unit Systems
 Incoherent or Inconsistent Systems: A
  system of units which is not coherent or
  not consistent.
 Example:    32.2 ft/s2
                           1 lbf = 1 lbm x 32.2 ft/s2
                                 or
 1 lbf                     1 lbf = 32.2 lbm ft/s2
              1 lbm




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             Unit Systems
 Incoherent or Inconsistent Systems
  (cont.):
 Example:
                          1 kgf = 1 kgm x 9.81 m/s2
              9.81 m/s2
                               or
 1 kgf                    1 kgf = 9.81 kgm m/s2
              1 kgm




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             The SI System
 SI Units
  – Base
  – Supplementary
  – Derived




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              The SI System
 The SI System is a coherent system of
  units based on seven base units
  –   length: metre, m
  –   mass: kilogram, kg
  –   time: second, s
  –   electric current: ampere, A
  –   thermodynamic temperature: kelvin, K
  –   amount of susbstance: mole, mol
  –   luminous intensity: candela, cd



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              The SI System
 SI System supplementary units:
  – radian (plane angle)
  – steradian (solid angle)
 Example
  – radian: the plane angle given as the ratio
    of the circular arc length subtended to the
    radius of the circle.
     o a length divided by a length
     q =s/r          r
                         q   s

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           The SI System
 SI System derived units
  – force: Newton, N (kg-m/s2)
  – pressure, or stress: Pascal, Pa (N/m2)
  – energy, work: joule, J (N-m)
  – power: watt, W (J/s)
  – Celsius temperature: Celsuis, C
           tC = TK - 273.15



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            The SI System
 SI Prefixes
  – 1 000 000 000 = 109    giga
  –     1 000 000 = 106    mega
  –         1 000 = 103    kilo
  –           0.1 = 10-1   deci
  –          0.01 = 10-2   centi
  –         0.001 = 10-3   milli
  –     0.000 001 = 10-6   micro
  – 0.000 000 001 = 10-9   nano



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               Summary
 Dimensions
  – fundamental, derived
 Units
  – base, derived, supplemental
 Unit Systems
  – coherent, incoherent
 SI Unit System
  – coherent, metric


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Where to get more information

 ASTM 380-93, “Standard Practice for
  Use of the International System of Units
  (SI)”




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