University:
California State University, Northridge
Course:
ME 101/L | Introduction to Mechanical Engineering and Lab
Academic year:

2024
Views:

294

Pages:

17
Author:

Genevieve Wright

Chapter 16 -Units and Conversions 1 Lecture Objectives and Activities • Introduce you to systems of units • Teach you how to accurately use units and perform unit conversions • Activities: unit conversion exercises 2 16.1 History of Units • A common denomination of units is essential for the development of trade and economics around the world • Majority of nations (include US) in the world today operate on the International System (SI) of Units because of its simplicity • In the US, some sectors still use US Customary System (USCS) 3 Base Units in the SI System 4 Derived Units in the SI System 5 Base Units in the USCS 6 Derived Units in USCS 7 Prefixes for SI Units Examples: • 1 kW = 1000 W = 103 W • 1 kpsi = 1000 psi • 2 GHz = 109 Hz =1,000,000,000 Hz 8 Conversion factors (1) • Example Conversion factors: 1in =1 25.4 mm 1L =1 0.2642 gal 9 Conversion factors (2) • Example Conversion factors: 1lbm =1 0.45359 kg 1 hp =1 0.7457 kW • Conversion factors are also called unity factors 10 Unit Conversion • Accurately using units and converting them is an essential engineering skill • Accidents related to unit errors • Total loss of $125-million NASA Mars Climate Orbiter spacecraft due to errors in impulse (I = Force * time) coding • The spring constant (k = F/x) error that bankrupted a company 11 Unit Conversion Method • To convert from one unit to another 1. Identify unity/conversion factors 2. Write down starting information, and multiply by unity/conversion factors until the desired units are obtained 12 Unit Conversion Example 1 • Convert 10 ft to inches 1. Unity/conversion factor: 2. 12 inches =1 1 ft  12 inches   =120 inches 10 ft   1 ft  13 Unit Conversion Example 2 • Convert 25 cents/minute to $/hour 1. Unity factors: 2.  1$    ,  100 cents   60 minutes    =  1 hour   25 cents  1$  60 minutes  15 $     =  1 minute  100 cents  1 hour  hour 14 Unit Conversion Example 3 • Convert 25 lb/in2 to kilogram/cm2  25 lb  .45359 kg  1in  1in  1.76 kg      = 2  2 1 in 1 lb 2 . 54 cm 2 . 54 cm cm      • “lb” in this case is lbm: Must be very careful in clarifying the unit(s) 15 Unit Conversion Example 4 • Convert 5 M ft-lb/s to KW  1 kW ft ⋅ lb  1W   5 000000 s  7376 ft ⋅ lb / s  1000 W   = .678kW  16 Pair and Share Exercises 1. 2. 3. 4. 5. 6. 7. 8. Convert 2.5$/gal to cent/mL Convert 5 m/s to ft/min Convert 5 J to ft-lb Convert 100 kW to ft-lb/s Convert 200 horse powers to (N-m)/s Convert 5 Btu to kg-m2 / s2 Convert 1 kpsi to g/(m-s2) Convert 10 gal/min to L/s 17

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