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Units and Conversions

Units and Conversions. What is. Ratio of an igloo’s circumference to its diameter? Eskimo π Time between slipping on a peel and smacking the pavement? 1 bannosecond 1000 aches? 1 killohurtz. Quantity (Q) = Numerical Value * Units (U)

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Units and Conversions

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  1. Units and Conversions

  2. What is • Ratio of an igloo’s circumference to its diameter? • Eskimo π • Time between slipping on a peel and smacking the pavement? • 1 bannosecond • 1000 aches? • 1 killohurtz

  3. Quantity (Q) = Numerical Value * Units (U) Without the units to value has no meaning except as a ratio.

  4. Types of dimensions • Fundamental or base, what system is built on • Derived, the ones that are built on the base dimensions • For mechanical systems we have length, time, and mass or force as base

  5. SI System of Units (International System) • An absolute system (not gravity dependent) • Comes from the “metric system” or MKS • Base units: • Length: meter, m • Time: second, s • Mass: kilogram, kg Derived unit: Force, newton, N or kg m/s2

  6. SI System • Base units: • Length, meter • Time, second • Mass, kilogram • Electrical current, ampere • Temperature, kelvin • Amount of substance, mole • Luminous intensity, candela

  7. SI System • Supplemental Units: • Plane angle: radian, rad • Solid angle: steradian, sr • Derived Units: • See tables 6.4 and 6.5 on pg 207 • See figure 6.2 on pg 208 for relationships

  8. SI System • SI System is a base 10 system • Use prefix to identify size of unit • 109 giga G • 106 mega M • 103 kilo k • 10-3 milli m • 10-6 micro μ • 10-9 nano η

  9. Rules for Using SI Units • Look at section 6.6, pg 210 • Symbols and names • Multiplication and division • Numbers

  10. Engineering Notation • Scientific notation: number between 0 and 1.0 times 10 to a power, example 2.334 * 105m • Engineering notation: number between 0.1 and 1000 times 10 to a power of 3, 6, 9, -3, -6, -9, etc., example 233.4 km or 233.4 * 103 m

  11. Coherent and Noncoherent Systems Coherent systems: SI, US Customary No additional conversion factors needed Noncoherent systems: American Engineering System Additional conversion factor needed 1 lbf = 32.174 lbms2/ft

  12. US Customary System • Base units: • Length, foot, ft • Time, second, s • Force, pound, lb • Derived unit: • Mass, slug

  13. American Engineering System • Base units: • Length, ft • Time, s • Force, lbf • Derived unit: • Mass, lbm, 32.174 lbm = 1 slug

  14. Effect • For equation F = ma • SI system N = 1 kg m/s2 • USC system lb = 1 slug ft/s2 • Am Engr System lbf = 32.174 lbm ft/s2

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