SI - The International System of Units

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```							SI - The International System
of Units

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1
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
 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
 Example
– radian: the unit of plane angle
measurement calculated as the ratio of the
circular arc length subtended divided by
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:
 Example
– radian: the plane angle given as the ratio
of the circular arc length subtended to the
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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 ASTM 380-93, “Standard Practice for
Use of the International System of Units
(SI)”

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