Chapter 16 -Units and
Conversions
ME101
Dr. Nhut Tan Ho
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 
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Unit Conversion Example 2
• Convert 25 cents/minute to $/hour
1.
2.
 25 cents
 25 cents
 1$ 1$ 60 minutes
 60 minutes
 15 $ 15 $

  
 
   
Unity factors:
,
100 cents
 1 minute
 1 minute
 100cents
 1hour
 1 hour
 hour
 hour
 25 cents  1$  60 minutes  15 $



 
 1 minute  100 cents  1 hour  hour
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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)
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Unit Conversion Example 4
• Convert 5 M ft-lb/s to KW
 1 kW
ft  lb 
1W


5 000000
s  7376 ft  lb / s  1000W

  .678kW

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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
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Lecture recap and assignment
• Engineers work with SI and USCS units
• Accurately using and converting units is an essential
engineering skill
• Assignments: 16.1,16.7,16.11,16.12,16.21,16.22
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