1 Chapter 16 -Units and Conversions ME101 Dr. Nhut Tan Ho.
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Transcript of 1 Chapter 16 -Units and Conversions ME101 Dr. Nhut Tan Ho.
1
Chapter 16 -Units and Conversions
ME101Dr. Nhut Tan Ho
2
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
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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)
4
Base Units in the SI System
5
Derived Units in the SI System
6
Base Units in the USCS
7
Derived Units in USCS
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Prefixes for SI Units
Examples: 1 kW = 1000 W
= 103 W
1 kpsi = 1000 psi
2 GHz = 109 Hz =1,000,000,000
Hz
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Conversion factors (1) Example
Conversion factors:
12642.0
1
14.25
1
gal
L
mm
in
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Conversion factors (2) Example Conversion
factors:
Conversion factors are also called unity factors
17457.0
1
145359.0
1
kW
hp
kg
lbm
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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
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Unit Conversion Method
To convert from one unit to another
1. Identify unity/conversion factors2. Write down starting information,
and multiply by unity/conversion factors until the desired units are obtained
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Unit Conversion Example 1
Convert 10 ft to inches
1. Unity/conversion factor:
2.
11
12
ft
inches
inchesft
inchesft 120
1
1210
14
Unit Conversion Example 2 Convert 25 cents/minute to $/hour
1. Unity factors: ,
2. hourhour
minutes
centsminute
cents $15
1
60
100
$1
1
25
hourhour
minutes
centsminute
cents $15
1
60
100
$1
1
25
hourhour
minutes
centsminute
cents $15
1
60
100
$1
1
25
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Unit Conversion Example 3 Convert 25 lb/in2 to kilogram/cm2
“lb” in this case is lbm: Must be very careful in clarifying the unit(s)
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76.1
54.2
1
54.2
1
1
45359.
1
25
cm
kg
cm
in
cm
in
lb
kg
in
lb
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Unit Conversion Example 4 Convert 5 M ft-lb/s to KW
kWW
kW
slbft
W
s
lbft678.
1000
1
/7376
10000005
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Pair and Share Exercises
1. Convert 2.5$/gal to cent/mL 2. Convert 5 m/s to ft/min3. Convert 5 J to ft-lb4. Convert 100 kW to ft-lb/s5. Convert 200 horse powers to (N-m)/s6. Convert 5 Btu to kg-m2 / s2
7. Convert 1 kpsi to g/(m-s2)8. 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