3 CHAPTER 1 study of thermodynamics that does not require a knowledge of the behavior of individual particles is called classical thermodynamics.It provides a direct and easy way to the solution of engineering problems.A more elabo- rate approach,based on the average behavior of large groups of individual particles,is called statistical thermodynamics.This microscopic approach is rather involved and is used in this text only in the supporting role. Application Areas of Thermodynamics All activities in nature involve some interaction between energy and matter; thus,it is hard to imagine an area that does not relate to thermodynam- ics in some manner.Therefore,developing a good understanding of basic principles of thermodynamics has long been an essential part of engineering Solar education. collectors Thermodynamics is commonly encountered in many engineering systems and other aspects of life,and one does not need to go very far to see some application areas of it.In fact,one does not need to go anywhere.The heart is constantly pumping blood to all parts of the human body,various energy Shower conversions occur in trillions of body cells,and the body heat generated is Hot constantly rejected to the environment.The human comfort is closely tied to water the rate of this metabolic heat rejection.We try to control this heat transfer Cold- Hot wa ter tank rate by adjusting our clothing to the environmental conditions. water Other applications of thermodynamics are right where one lives.An ordi- Heat exchanger Pump nary house is,in some respects,an exhibition hall filled with wonders of thermodynamics (Fig.1-4).Many ordinary household utensils and appli- FIGURE 1-4 ances are designed,in whole or in part,by using the principles of thermo- The design of many engineering dynamics.Some examples include the electric or gas range,the heating systems,such as this solar hot water and air-conditioning systems,the refrigerator,the humidifier,the pressure system,involves thermodynamics. cooker,the water heater,the shower,the iron,and even the computer and the TV.On a larger scale,thermodynamics plays a major part in the design and analysis of automotive engines,rockets,jet engines,and conventional or nuclear power plants,solar collectors,and the design of vehicles from ordi- nary cars to airplanes (Fig.1-5).The energy-efficient home that you may be living in,for example,is designed on the basis of minimizing heat loss in winter and heat gain in summer.The size,location,and the power input of the fan of your computer is also selected after an analysis that involves thermodynamics. 1-2.IMPORTANCE OF DIMENSIONS AND UNITS Any physical quantity can be characterized by dimensions.The magnitudes assigned to the dimensions are called units.Some basic dimensions such as mass m,length L,time t,and temperature T are selected as primary or fundamental dimensions,while others such as velocity V energy E,and volume Vare expressed in terms of the primary dimensions and are called secondary dimensions,or derived dimensions. A number of unit systems have been developed over the years.Despite strong efforts in the scientific and engineering community to unify the world with a single unit system,two sets of units are still in common use today:the English system,which is also known as the United States
3 CHAPTER 1 study of thermodynamics that does not require a knowledge of the behavior of individual particles is called classical thermodynamics. It provides a direct and easy way to the solution of engineering problems. A more elaborate approach, based on the average behavior of large groups of individual particles, is called statistical thermodynamics. This microscopic approach is rather involved and is used in this text only in the supporting role. Application Areas of Thermodynamics All activities in nature involve some interaction between energy and matter; thus, it is hard to imagine an area that does not relate to thermodynamics in some manner. Therefore, developing a good understanding of basic principles of thermodynamics has long been an essential part of engineering education. Thermodynamics is commonly encountered in many engineering systems and other aspects of life, and one does not need to go very far to see some application areas of it. In fact, one does not need to go anywhere. The heart is constantly pumping blood to all parts of the human body, various energy conversions occur in trillions of body cells, and the body heat generated is constantly rejected to the environment. The human comfort is closely tied to the rate of this metabolic heat rejection. We try to control this heat transfer rate by adjusting our clothing to the environmental conditions. Other applications of thermodynamics are right where one lives. An ordinary house is, in some respects, an exhibition hall filled with wonders of thermodynamics (Fig. 1–4). Many ordinary household utensils and appliances are designed, in whole or in part, by using the principles of thermodynamics. Some examples include the electric or gas range, the heating and air-conditioning systems, the refrigerator, the humidifier, the pressure cooker, the water heater, the shower, the iron, and even the computer and the TV. On a larger scale, thermodynamics plays a major part in the design and analysis of automotive engines, rockets, jet engines, and conventional or nuclear power plants, solar collectors, and the design of vehicles from ordinary cars to airplanes (Fig. 1–5). The energy-efficient home that you may be living in, for example, is designed on the basis of minimizing heat loss in winter and heat gain in summer. The size, location, and the power input of the fan of your computer is also selected after an analysis that involves thermodynamics. 1–2 ■ IMPORTANCE OF DIMENSIONS AND UNITS Any physical quantity can be characterized by dimensions. The magnitudes assigned to the dimensions are called units. Some basic dimensions such as mass m, length L, time t, and temperature T are selected as primary or fundamental dimensions, while others such as velocity V, energy E, and volume V are expressed in terms of the primary dimensions and are called secondary dimensions, or derived dimensions. A number of unit systems have been developed over the years. Despite strong efforts in the scientific and engineering community to unify the world with a single unit system, two sets of units are still in common use today: the English system, which is also known as the United States FIGURE 1–4 The design of many engineering systems, such as this solar hot water system, involves thermodynamics. Solar collectors Hot water Heat exchanger Pump Shower Cold water Hot water tank cen98179_ch01_001-050.indd 3 11/28/13 3:14 PM
4 INTRODUCTION AND BASIC CONCEPTS Refrigerator Boats Aircraft and spacecraft McGraw-Hill Education,Jill Braaten Doug Menuez/Getty Images RF PhotoLink/Getty Images RF Power plants Human body Cars Malcolm Fife/Getty Images RF Ryan McVary/Getty Images RF Mark Evans/Getty Images RF Wind turbines Food processing A piping network in an industrial facility. F.Schussler/PhotoLink/Getty Glow Images RF Courtesy of UMDE Engineering Contracting Images RF and Trading.Used by permission FIGURE 1-5 Some application areas of thermodynamics. Customary System (USCS),and the metric SI (from Le Systeme Interna- tional d'Unites),which is also known as the International System.The SI is a simple and logical system based on a decimal relationship between the various units,and it is being used for scientific and engineering work in most of the industrialized nations,including England.The English sys- tem,however,has no apparent systematic numerical base,and various units in this system are related to each other rather arbitrarily (12 in I ft, 1 mile 5280 ft,4 gt 1 gal,etc.),which makes it confusing and difficult to learn.The United States is the only industrialized country that has not yet fully converted to the metric system. The systematic efforts to develop a universally acceptable system of units dates back to 1790 when the French National Assembly charged the French Academy of Sciences to come up with such a unit system.An early version of the metric system was soon developed in France,but it did not
4 INTRODUCTION AND BASIC CONCEPTS Customary System (USCS), and the metric SI (from Le Système International d’ Unités), which is also known as the International System. The SI is a simple and logical system based on a decimal relationship between the various units, and it is being used for scientific and engineering work in most of the industrialized nations, including England. The English system, however, has no apparent systematic numerical base, and various units in this system are related to each other rather arbitrarily (12 in 5 1 ft, 1 mile 5 5280 ft, 4 qt 5 1 gal, etc.), which makes it confusing and difficult to learn. The United States is the only industrialized country that has not yet fully converted to the metric system. The systematic efforts to develop a universally acceptable system of units dates back to 1790 when the French National Assembly charged the French Academy of Sciences to come up with such a unit system. An early version of the metric system was soon developed in France, but it did not FIGURE 1–5 Some application areas of thermodynamics. Cars © Mark Evans/Getty Images RF Power plants © Malcolm Fife/Getty Images RF Human body © Ryan McVay/Getty Images RF Aircraft and spacecraft © PhotoLink/Getty Images RF Refrigerator © McGraw-Hill Education, Jill Braaten Boats © Doug Menuez/Getty Images RF A piping network in an industrial facility. Courtesy of UMDE Engineering Contracting and Trading. Used by permission Wind turbines © F. Schussler/PhotoLink/Getty Images RF Food processing Glow Images RF cen98179_ch01_001-050.indd 4 11/28/13 3:14 PM
5 CHAPTER 1 find universal acceptance until 1875 when The Metric Convention Treaty TABLE 1-1 was prepared and signed by 17 nations,including the United States.In this international treaty,meter and gram were established as the metric units The seven fundamental (or primary) for length and mass,respectively,and a General Conference of Weights dimensions and their units in SI and Measures (CGPM)was established that was to meet every six years Dimension Unit In 1960,the CGPM produced the SI,which was based on six fundamental Length meter (m) quantities,and their units were adopted in 1954 at the Tenth General Con- Mass kilogram (kg) ference of Weights and Measures:meter (m)for length,kilogram (kg)for Time second (s) mass,second (s)for time,ampere (A)for electric current,degree Kelvin Temperature kelvin(K) (K)for temperature,and candela (cd)for luminous intensity (amount of Electric current ampere (A) light).In 1971,the CGPM added a seventh fundamental quantity and unit: Amount of light candela(cd) mole (mol)for the amount of matter. Amount of matter mole (mol) Based on the notational scheme introduced in 1967,the degree symbol was officially dropped from the absolute temperature unit,and all unit names were to be written without capitalization even if they were derived from proper names (Table 1-1).However,the abbreviation of a unit was TABLE 1-2 to be capitalized if the unit was derived from a proper name.For example, the SI unit of force,which is named after Sir Isaac Newton (1647-1723), Standard prefixes in SI units is newton(not Newton),and it is abbreviated as N.Also,the full name of a Multiple Prefix unit may be pluralized,but its abbreviation cannot.For example,the length 1024 yotta,Y of an object can be 5 m or 5 meters,not 5 ms or 5 meter.Finally,no period 1021 zetta,Z is to be used in unit abbreviations unless they appear at the end of a sen- 108 exa,E tence.For example,the proper abbreviation of meter is m(not m.). 1015 peta,P The recent move toward the metric system in the United States seems to 1012 tera,T have started in 1968 when Congress,in response to what was happening 109 giga,G in the rest of the world,passed a Metric Study Act.Congress continued 105 mega,M to promote a voluntary switch to the metric system by passing the Metric 103 kilo,k Conversion Act in 1975.A trade bill passed by Congress in 1988 set a 102 hecto,h 101 September 1992 deadline for all federal agencies to convert to the metric deka,da 10-1 deci,d system.However,the deadlines were relaxed later with no clear plans for 10-2 centi,c the future. 10-3 milli,m The industries that are heavily involved in international trade (such as the 10-6 micro,.μ automotive,soft drink,and liquor industries)have been quick in convert- 10-9 nano,n ing to the metric system for economic reasons (having a single worldwide 10-12 pico,p design,fewer sizes,smaller inventories,etc.).Today,nearly all the cars 10-15 femto.f manufactured in the United States are metric.Most car owners probably do 10-18 atto,a not realize this until they try an English socket wrench on a metric bolt. 10-21 zepto,z Most industries,however,resisted the change,thus slowing down the con- 10-24 yocto,y version process. Presently the United States is a dual-system society,and it will stay that way until the transition to the metric system is completed.This puts an extra burden on today's engineering students,since they are expected to retain their understanding of the English system while learning,thinking,and working in terms of the SI.Given the position of the engineers in the transi- 1M2 tion period,both unit systems are used in this text,with particular emphasis 200ml (02L) (103g) on SI units. (1062) As pointed out,the SI is based on a decimal relationship between units. The prefixes used to express the multiples of the various units are listed in FIGURE 1-6 Table 1-2.They are standard for all units,and the student is encouraged to The SI unit prefixes are used in all memorize them because of their widespread use (Fig.1-6) branches of engineering
5 CHAPTER 1 find universal acceptance until 1875 when The Metric Convention Treaty was prepared and signed by 17 nations, including the United States. In this international treaty, meter and gram were established as the metric units for length and mass, respectively, and a General Conference of Weights and Measures (CGPM) was established that was to meet every six years. In 1960, the CGPM produced the SI, which was based on six fundamental quantities, and their units were adopted in 1954 at the Tenth General Conference of Weights and Measures: meter (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, degree Kelvin (°K) for temperature, and candela (cd) for luminous intensity (amount of light). In 1971, the CGPM added a seventh fundamental quantity and unit: mole (mol) for the amount of matter. Based on the notational scheme introduced in 1967, the degree symbol was officially dropped from the absolute temperature unit, and all unit names were to be written without capitalization even if they were derived from proper names (Table 1–1). However, the abbreviation of a unit was to be capitalized if the unit was derived from a proper name. For example, the SI unit of force, which is named after Sir Isaac Newton (1647–1723), is newton (not Newton), and it is abbreviated as N. Also, the full name of a unit may be pluralized, but its abbreviation cannot. For example, the length of an object can be 5 m or 5 meters, not 5 ms or 5 meter. Finally, no period is to be used in unit abbreviations unless they appear at the end of a sentence. For example, the proper abbreviation of meter is m (not m.). The recent move toward the metric system in the United States seems to have started in 1968 when Congress, in response to what was happening in the rest of the world, passed a Metric Study Act. Congress continued to promote a voluntary switch to the metric system by passing the Metric Conversion Act in 1975. A trade bill passed by Congress in 1988 set a September 1992 deadline for all federal agencies to convert to the metric system. However, the deadlines were relaxed later with no clear plans for the future. The industries that are heavily involved in international trade (such as the automotive, soft drink, and liquor industries) have been quick in converting to the metric system for economic reasons (having a single worldwide design, fewer sizes, smaller inventories, etc.). Today, nearly all the cars manufactured in the United States are metric. Most car owners probably do not realize this until they try an English socket wrench on a metric bolt. Most industries, however, resisted the change, thus slowing down the conversion process. Presently the United States is a dual-system society, and it will stay that way until the transition to the metric system is completed. This puts an extra burden on today’s engineering students, since they are expected to retain their understanding of the English system while learning, thinking, and working in terms of the SI. Given the position of the engineers in the transition period, both unit systems are used in this text, with particular emphasis on SI units. As pointed out, the SI is based on a decimal relationship between units. The prefixes used to express the multiples of the various units are listed in Table 1–2. They are standard for all units, and the student is encouraged to memorize them because of their widespread use (Fig. 1–6). TABLE 1–1 The seven fundamental (or primary) dimensions and their units in SI Dimension Unit Length meter (m) Mass kilogram (kg) Time second (s) Temperature kelvin (K) Electric current ampere (A) Amount of light candela (cd) Amount of matter mole (mol) TABLE 1–2 Standard prefixes in SI units Multiple Prefix 1024 yotta, Y 1021 zetta, Z 1018 exa, E 1015 peta, P 1012 tera, T 109 giga, G 106 mega, M 103 kilo, k 102 hecto, h 101 deka, da 1021 deci, d 1022 centi, c 1023 milli, m 1026 micro, m 1029 nano, n 10212 pico, p 10215 femto, f 10218 atto, a 10221 zepto, z 10224 yocto, y FIGURE 1–6 The SI unit prefixes are used in all branches of engineering. 200 mL 1 kg (0.2 L) (103 g) 1 M (106 ) cen98179_ch01_001-050.indd 5 11/28/13 3:14 PM
6 INTRODUCTION AND BASIC CONCEPTS Some SI and English Units In SI,the units of mass,length,and time are the kilogram (kg),meter (m), and second (s),respectively.The respective units in the English system are the pound-mass (lbm),foot (ft),and second (s).The pound symbol lb is actually the abbreviation of libra,which was the ancient Roman unit of weight.The English retained this symbol even after the end of the Roman occupation of Britain in 410.The mass and length units in the two systems are related to each other by 11bm=0.45359kg 1ft=0.3048m In the English system,force is usually considered to be one of the primary dimensions and is assigned a nonderived unit.This is a source of confusion and error that necessitates the use of a dimensional constant (ge)in many formulas.To avoid this nuisance,we consider a=1 m/s2 force to be a secondary dimension whose unit is derived from Newton's m =I kg F=1N second law,that is, Force =(Mass)(Acceleration) a I ft/s2 m=32.1741bm F=1 Ibf or F=ma (1-1) FIGURE 1-7 The definition of the force units. In SI,the force unit is the newton (N),and it is defined as the force required to accelerate a mass of I kg at a rate of I m/s2.In the English system,the force unit is the pound-force (lbf)and is defined as the force required to 1 kgf accelerate a mass of 32.174 Ibm (I slug)at a rate of I fi/s2 (Fig.1-7).That is, 1N 1 kg-m/s2 10 apples m=1 kgc 11bf=32.1741bmf/s2 4 apples 1 apple m=ilbm A force of I N is roughly equivalent to the weight of a small apple m=102 g o 器 (m 102 g),whereas a force of 1 Ibf is roughly equivalent to the weight of four medium apples (mtotal =454 g),as shown in Fig.1-8.Another force unit in common use in many European countries is the kilogram-force (kgf), IN 1 Ibf which is the weight of I kg mass at sea level (1 kgf =9.807 N). The term weight is often incorrectly used to express mass,particularly by the "weight watchers."Unlike mass,weight W is a force.It is the gravi- tational force applied to a body,and its magnitude is determined from Newton's second law, W=mg (N) (1-2) where m is the mass of the body,and g is the local gravitational acceleration (g is 9.807 m/s2 or 32.174 ft/s2 at sea level and 45 latitude).An ordinary FIGURE 1-8 bathroom scale measures the gravitational force acting on a body. The relative magnitudes of the force The mass of a body remains the same regardless of its location in the units newton(N),kilogram-force(kgf),universe.Its weight,however,changes with a change in gravitational and pound-force (Ibf). acceleration.A body weighs less on top of a mountain since g decreases
6 INTRODUCTION AND BASIC CONCEPTS Some SI and English Units In SI, the units of mass, length, and time are the kilogram (kg), meter (m), and second (s), respectively. The respective units in the English system are the pound-mass (lbm), foot (ft), and second (s). The pound symbol lb is actually the abbreviation of libra, which was the ancient Roman unit of weight. The English retained this symbol even after the end of the Roman occupation of Britain in 410. The mass and length units in the two systems are related to each other by 1 lbm 5 0.45359 kg 1 ft 5 0.3048 m In the English system, force is usually considered to be one of the primary dimensions and is assigned a nonderived unit. This is a source of confusion and error that necessitates the use of a dimensional constant (gc) in many formulas. To avoid this nuisance, we consider force to be a secondary dimension whose unit is derived from Newton’s second law, that is, Force 5 (Mass)(Acceleration) or F 5 ma (1–1) In SI, the force unit is the newton (N), and it is defined as the force required to accelerate a mass of 1 kg at a rate of 1 m/s2 . In the English system, the force unit is the pound-force (lbf) and is defined as the force required to accelerate a mass of 32.174 lbm (1 slug) at a rate of 1 ft/s2 (Fig. 1–7). That is, 1 N 5 1 kg·m/s2 1 lbf 5 32.174 lbm·ft/s2 A force of 1 N is roughly equivalent to the weight of a small apple (m 5 102 g), whereas a force of 1 lbf is roughly equivalent to the weight of four medium apples (mtotal 5 454 g), as shown in Fig. 1–8. Another force unit in common use in many European countries is the kilogram-force (kgf), which is the weight of 1 kg mass at sea level (1 kgf 5 9.807 N). The term weight is often incorrectly used to express mass, particularly by the “weight watchers.” Unlike mass, weight W is a force. It is the gravitational force applied to a body, and its magnitude is determined from Newton’s second law, W 5 mg (N) (1–2) where m is the mass of the body, and g is the local gravitational acceleration (g is 9.807 m/s2 or 32.174 ft/s2 at sea level and 45° latitude). An ordinary bathroom scale measures the gravitational force acting on a body. The mass of a body remains the same regardless of its location in the universe. Its weight, however, changes with a change in gravitational acceleration. A body weighs less on top of a mountain since g decreases FIGURE 1–7 The definition of the force units. m = 1 kg m = 32.174 lbm a = 1 m/s2 a = 1 ft/s2 F = 1 lbf F = 1 N FIGURE 1–8 The relative magnitudes of the force units newton (N), kilogram-force (kgf), and pound-force (lbf). 1 kgf 10 apples m 1 kg 4 apples m 1 lbm 1 lbf 1 apple m 102 g 1 N cen98179_ch01_001-050.indd 6 11/28/13 3:14 PM
CHAPTER 1 with altitude.On the surface of the moon,an astronaut weighs about one- sixth of what she or he normally weighs on earth(Fig.1-9). WOW! At sea level a mass of I kg weighs 9.807 N,as illustrated in Fig.1-10.A mass of I lbm,however,weighs 1 lbf,which misleads people to believe that pound-mass and pound-force can be used interchangeably as pound (Ib), which is a major source of error in the English system. It should be noted that the gravity force acting on a mass is due to the attraction between the masses,and thus it is proportional to the mag- nitudes of the masses and inversely proportional to the square of the dis- tance between them.Therefore,the gravitational acceleration g at a location depends on the local density of the earth's crust,the distance to the center of the earth,and to a lesser extent,the positions of the moon and the sun. The value of g varies with location from 9.832 m/s2 at the poles (9.789 at the equator)to 7.322 m/s2 at 1000 km above sea level.However,at altitudes up to 30 km,the variation of g from the sea-level value of 9.807 m/s2 is less than 1 percent.Therefore,for most practical purposes,the gravitational acceleration can be assumed to be constant at 9.807 m/s2,often rounded to 9.81 m/s2.It is interesting to note that at locations below sea level,the value FIGURE 1-9 of g increases with distance from the sea level,reaches a maximum at about A body weighing 150 Ibf on earth will 4500 m,and then starts decreasing.(What do you think the value of g is at weigh only 25 Ibf on the moon. the center of the earth?) The primary cause of confusion between mass and weight is that mass is usually measured indirectly by measuring the gravity force it exerts.This approach also assumes that the forces exerted by other effects such as air buoyancy and fluid motion are negligible.This is like measuring the dis- tance to a star by measuring its red shift,or measuring the altitude of an 8=9.807m/s2 8=32.174/s2 airplane by measuring barometric pressure.Both of these are also indirect W=9.807kgm/s2 W=32.174bmf/s2 measurements.The correct direct way of measuring mass is to compare it =9.807N =1 Ibf to a known mass.This is cumbersome,however,and it is mostly used for =1 kgf calibration and measuring precious metals. Work,which is a form of energy,can simply be defined as force times FIGURE 1-10 distance;therefore,it has the unit "newton-meter(N-m),"which is called a The weight of a unit mass at sea level. joule (J).That is, 1J 1N.m (1-3) A more common unit for energy in SI is the kilojoule (1 kJ 103 J).In the English system,the energy unit is the Btu(British thermal unit),which is defined as the energy required to raise the temperature of I Ibm of water at 68F by 1F.In the metric system,the amount of energy needed to raise the temperature of I g of water at 14.5C by 1C is defined as I calorie (cal), and 1 cal =4.1868 J.The magnitudes of the kilojoule and Btu are almost identical (1 Btu 1.0551 kJ).Here is a good way to get a feel for these units:If you light a typical match and let it burn itself out,it yields approxi- mately one Btu(or one kJ)of energy (Fig.1-11). The unit for time rate of energy is joule per second (J/s),which is called a watt (W).In the case of work,the time rate of energy is called power. A commonly used unit of power is horsepower (hp),which is equivalent FIGURE 1-11 to 746 W.Electrical energy typically is expressed in the unit kilowatt-hour A typical match yields about one Btu (or (kWh),which is equivalent to 3600 kJ.An electric appliance with a rated one kJ)of energy if completely burned. power of I kW consumes I kWh of electricity when running continuously Photo by John M.Cimbala
7 CHAPTER 1 with altitude. On the surface of the moon, an astronaut weighs about onesixth of what she or he normally weighs on earth (Fig. 1–9). At sea level a mass of 1 kg weighs 9.807 N, as illustrated in Fig. 1–10. A mass of 1 lbm, however, weighs 1 lbf, which misleads people to believe that pound-mass and pound-force can be used interchangeably as pound (lb), which is a major source of error in the English system. It should be noted that the gravity force acting on a mass is due to the attraction between the masses, and thus it is proportional to the magnitudes of the masses and inversely proportional to the square of the distance between them. Therefore, the gravitational acceleration g at a location depends on the local density of the earth’s crust, the distance to the center of the earth, and to a lesser extent, the positions of the moon and the sun. The value of g varies with location from 9.832 m/s2 at the poles (9.789 at the equator) to 7.322 m/s2 at 1000 km above sea level. However, at altitudes up to 30 km, the variation of g from the sea-level value of 9.807 m/s2 is less than 1 percent. Therefore, for most practical purposes, the gravitational acceleration can be assumed to be constant at 9.807 m/s2 , often rounded to 9.81 m/s2 . It is interesting to note that at locations below sea level, the value of g increases with distance from the sea level, reaches a maximum at about 4500 m, and then starts decreasing. (What do you think the value of g is at the center of the earth?) The primary cause of confusion between mass and weight is that mass is usually measured indirectly by measuring the gravity force it exerts. This approach also assumes that the forces exerted by other effects such as air buoyancy and fluid motion are negligible. This is like measuring the distance to a star by measuring its red shift, or measuring the altitude of an airplane by measuring barometric pressure. Both of these are also indirect measurements. The correct direct way of measuring mass is to compare it to a known mass. This is cumbersome, however, and it is mostly used for calibration and measuring precious metals. Work, which is a form of energy, can simply be defined as force times distance; therefore, it has the unit “newton-meter (N·m),” which is called a joule (J). That is, 1 J 5 1 N·m (1–3) A more common unit for energy in SI is the kilojoule (1 kJ 5 103 J). In the English system, the energy unit is the Btu (British thermal unit), which is defined as the energy required to raise the temperature of 1 lbm of water at 68°F by 1°F. In the metric system, the amount of energy needed to raise the temperature of 1 g of water at 14.5°C by 1°C is defined as 1 calorie (cal), and 1 cal 5 4.1868 J. The magnitudes of the kilojoule and Btu are almost identical (1 Btu 5 1.0551 kJ). Here is a good way to get a feel for these units: If you light a typical match and let it burn itself out, it yields approximately one Btu (or one kJ) of energy (Fig. 1–11). The unit for time rate of energy is joule per second (J/s), which is called a watt (W). In the case of work, the time rate of energy is called power. A commonly used unit of power is horsepower (hp), which is equivalent to 746 W. Electrical energy typically is expressed in the unit kilowatt-hour (kWh), which is equivalent to 3600 kJ. An electric appliance with a rated power of 1 kW consumes 1 kWh of electricity when running continuously FIGURE 1–9 A body weighing 150 lbf on earth will weigh only 25 lbf on the moon. FIGURE 1–10 The weight of a unit mass at sea level. g = 9.807 m/s2 W = 9.807 kg·m/s2 = 9.807 N = 1 kgf W = 32.174 lbm·ft/s2 = 1 lbf g = 32.174 ft/s2 kg lbm FIGURE 1–11 A typical match yields about one Btu (or one kJ) of energy if completely burned. Photo by John M. Cimbala cen98179_ch01_001-050.indd 7 11/28/13 3:14 PM