SI The International System of Units

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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.
• 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

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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 or 1 N 1 kg m/s2
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Unit Systems

• Coherent or Consistent Systems (cont.)
• Example

1 lb 1 slug x 1 ft/s2 or 1 lb 1 slug
ft/s2
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Unit Systems

• Incoherent or Inconsistent Systems A system of
  units which is not coherent or not consistent.
• Example

1 lbf 1 lbm x 32.2 ft/s2 or 1 lbf
32.2 lbm ft/s2
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Unit Systems

• Incoherent or Inconsistent Systems (cont.)
• Example

1 kgf 1 kgm x 9.81 m/s2 or 1 kgf
9.81 kgm m/s2
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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.
• a length divided by a length

s/r
s
r
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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)