机械工程导论(双语)课件2

TABLE 2.3 Base Units in the SI
Quantity Length Force SI Base Unit meter kilogram Abbreviation m kg
Time
Electric current Thermodynamic temperature Amount of substance Luminous intensity
Why does the United States stand out in retaining the USCS?
Conversion away from the existing system would be a significant and expensive burden.
The dimensions of countless existing structures, factories, machines, and spare parts have already been specified and built in terms of the USCS.
Btu
hp ˚F
1 Btu = 778.2 ft· lb
1 hp = 550 ft· lb/s ˚F = ˚R - 460
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2 WHAT IS ENGINEERING?---International System of Units
The International System of units (or SI) is the measurement standard based in part on the quantities of meters, kilograms, and seconds.
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1 OVERVIEW----the important of unit
Each quantity in mechanical engineering has two components: a numerical value and a unit.
How important a unit is?
The United States Customary System of units is a historical and traditional system, and its origin traces back to the ancient Roman Empire. The reasons are both logistical and cultural: There is already a vast continent-sized infrastructure within the United States that is based on the USCS.
mile
U.S. gallon slug ounce
mi
gal slug oz
1 mi = 5280 ft
1 gal = 0.1337 ft3 1 slug = 1 lb· s2/ft 1 oz = 0.0625 lb
Force
Moment of a force Pressure or stress Energy, work, or heat
LOGO
An
introduction
to
mechaniFra Baidu bibliotekal engineering
Lecturer: Liu Ju-rong
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CHAPTER2 Problem-Solving Skills
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OVERVIEW
UNIT SYSTEMS AND CONVERSIONS DIMENSIONAL CONSISTENCY
What is derived units? Derived units, as their name implies, are constructed as combinations of base units.
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2 WHAT IS ENGINEERING?---United States Customary System
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2 WHAT IS ENGINEERING?---United States Customary System
TABLE 2.1 Base Units in the USCS
Quantity
Length Force Time
USCS Base Unit
foot pound second
Abbreviation
TABLE 2.2 Certain Derived in the USCS. Although a change in temperature of 1 Rankine also equals a change of 1 degree Fahrenheit, the absolute values are converted using the formula.
ft lb s
Electric current
Thermodynamic temperature Amount of substance Luminous intensity
ampere
Rankine mole candela
A
R mol cd
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2 WHAT IS ENGINEERING?---United States Customary System
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After study, what should you be able to have?
2 UNIT SYSTEMS AND CONVERSIONS- Unit System
what is USCS and SI?
Engineers specify physical quantities in two different, but conventional, systems of units: the United States Customary System (USCS) and the International System of Units. Practicing mechanical engineers must be conversant with both unit systems.
second
ampere Kelvin mole candela
s
A K mol cd
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2 WHAT IS ENGINEERING?---International System of Units
TABLE 2.4 Certain Derived in the SI. Although a change in temperature of 1 Kelvin also equals a change of 1 degree Celsius, the absolute values are converted using the formula. Quantity Length Volume Force Moment of a force Pressure or stress SI Derived Unit micron liter newton N· m pascal Abbreviation μm L N N· m Pa definition 1 μm = 10-6 m 1 L =0.001 m3 1 N = 1kg· m/s2 ----------1 Pa = 1 N/m2
ton
ft· lb psi ft· lb
ton
ft· lb psi ft· lb
1 ton = 2000lb
------------1 psi = 1 lb/in2 -------------
Energy, work, or heat
Power Temperature
Btu
horsepower degree Fahrenheit
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1 OVERVIEW--- The ability to be available in this chapter
Report both a numerical value and its unit in each calculation that you perform. List the base units in the United States Customary System and the System International d‘ Units, and state some of the derived units used in mechanical engineering. Understand the need for proper bookkeeping of units when making engineering calculations and the implications of not doing so. Convert numerical quantities from the United States Customary System to the System International d‘ Units, and vice versa. Check your equations and calculations to verify that they are dimensionally consistent. Understand how to perform order-of-magnitude approximations.
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SIGNIFICANT DIGITS
AN ERROR OF UNITS ON THE WAY TO MAR APPROXIMATION IN ENGINEERING PROBLEM-SOLVING METHODOLOGY
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Vocabulary
Torque 扭矩 thermal conductivity 热导率 shear stress 剪应力 fluid viscosity 流体的粘度 elastic modulus 弹性模量 kinetic energy 动能 Reynolds number 雷诺数
Quantity
Length Length Length
USCS Derived Unit
mil inch yard
Abbreviation
mil in. yd
Definition
1 mil = 0.001 in. 1 in. = 0.0833ft 1 yd = 3 ft
Length
Volume Mass Force
Boeing 767 How much fuel should be added ? Total fuel pounds (lb); kilograms 1.77 kg/L 1.77 lb/L
9000 liters 16,000 liters
22,300 kilograms (kg)
Fuel in tanks 7682 liters (L)
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2 UNIT SYSTEMS AND CONVERSIONS- an example
Keeping Track of Units on Fright 143 In July of 1983, Air Canada Flight 143 was en route from Montreal to Edmonton. The Boeing 767 had three fuel tanks, one in each wing and one in the fuselage, which supplied the plane's two jet engines.
volume (L), weight (lb), mass (kg)
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2 UNIT SYSTEMS AND CONVERSIONS- Base and Derived Units what is base units?
A base unit is a fundamental quantity that cannot be further broken down or expressed in terms of any simpler elements. Base units are the core building blocks of any unit system.
One is simply meaningless without the other, and practicing engineers pay as close and careful attention to the units in a calculation as they do to the numbers.