ENGR 1402 Concepts of Engineering

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ENGR 1402 Concepts of Engineering

• Chapter 6
• Dimensions, Units, and Conversions

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Dimensions Used to describe physical quantities.

• Independent of units
• Divided into 2 areas Fundamental and Derived
• Derived dimensions are a combination of
  fundamental dimensions (i.e. velocity,
  acceleration, etc.)

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• Dimensional System the smallest number of
  fundamental dimensions which will form a
  consistent and complete set for a field of
  science.
• Examples
• 1) Absolute system Dimensions not affected by
  gravity (length, time, mass)
• 2) Gravitational System Some dimensions
  affected by gravity (length, time, force)

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Units Predetermined reference amounts

• Two primary systems used SI and Engineering
  System
• Engineering System sometimes called simply U.S.
  or, perhaps more accurately, U.S. Inconsistent

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S.I. System of Units

• 3 classes of units base, supplementary,
  derived
• 7 base units shown described on p. 205. Well
  use m, kg, s, A, K most often
• Supplementary units shown on p. 206. Well often
  use rad.
• Derived units shown on p. 207.

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Prefixes

• Used to better represent quantities and
  significant figures
• Rule of thumb express values between 0.1 and
  1000. Use prefix to make it work.
• Most common prefixes in engineering are m, µ, c,
  k, M, G. Commit all to memory!
• Precision implied by the way number is presented
  (p. 210 examples)
• Read section 6.6 Rules for using SI units

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Consistency of Units - gc

• gc is known as the gravitational constant
• It is a constant of proportionality which reminds
  us that units must be and are consistent with all
  systems.
• SI gc 1 kg-m/N-s2
• U.S. gc 32.2 ft-lbm/lbf-s2 1 slug-ft/ lbf-s2
  
• Confused? Read section 6.6.4 in text

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U.S. System (Engineering System)

• Fundamental Units ft, s, lbf
• Derived Units slug, lbm
• Note 1.0 slug 32.174 lbm

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Unit Conversions

• ALWAYS write them out
• Appendix D of your text has many useful
  conversions
• ALWAYS write them out
• Dont try to commit lots of trivial information
  to memory
• ALWAYS write out unit conversions
• 1 in. 2.54 cm