Chapter 5: Work, Power & Energy

Transcription

1 Chapter 5: Work, Power & Energy Abition is like a vector; it needs agnitude and direction. Otherwise, it s just energy. Grace Lindsay Objectives 1. Define work and calculate the work done by a force.. Apply relationships between work, net force, displaceent, velocity and kinetic energy to solve a variety of probles. 3. Calculate the power of a syste. 4. Identify, describe, and calculate the potential energy of a syste. 5. Apply conservation of energy to analyze energy transitions and transforations in a syste. 6. Analyze the relationship between work done on or by a syste, and the energy gained or lost by that syste. 7. Use Hooke s Law to deterine the elastic force on an object. 8. Calculate a syste s elastic potential energy. Chapter 5: Work, Power & Energy 105

2 Work, energy and power are highly inter-related concepts that coe up regularly in everyday life. You do work on an object when you ove it. The rate at which you do the work is your power output. When you do work on an object, you transfer energy fro one object to another. In this chapter you ll explore how energy is transferred and transfored, how doing work on an object changes its energy, and how quickly work can be done. Work Soeties you work hard. Soeties you re a slacker. But, right now, are you doing work? And what is eant by the word work? In physics ters, work is the process of oving an object by applying a force, or, ore forally, work is the energy transferred by an external force exerted on an object or syste that oves the object or syste. I sure you can think up countless exaples of work being done, but a few that spring to ind include pushing a snowblower to clear the driveway, pulling a sled up a hill with a rope, stacking boxes of books fro the floor onto a shelf, and throwing a baseball fro the pitcher s ound to hoe plate. Let s take a look at a few scenarios and investigate what work is being done. In the first scenario, a onkey in a jet pack blasts through the atosphere, accelerating to higher and higher speeds. In this case, the jet pack is applying a force causing it to ove. But what is doing the work? Hot expanding gases are pushed backward out of the jet pack. Using Newton s 3rd Law, you observe the reactionary force of the gas pushing the jet pack forward, causing a displaceent. Therefore, the expanding exhaust gas is doing work on the jet pack. In the second scenario, a girl struggles to push her stalled car, but can t ake it ove. Even though she s expending significant effort, no work is being done on the car because it isn t oving. In the final scenario, a child in a ghost costue carries a bag of Halloween candy across the yard. In this situation, the child applies a force upward on the bag, but the bag oves horizontally. Fro this perspective, the forces of the child s ars on the bag don t cause the displaceent, therefore no work is being done by the child. It s iportant to note that when calculating work, only the force applied in the direction of the object s displaceent counts! This eans that if the force and displaceent vectors aren t in exactly the sae direction, you need to take the coponent of force in the direction of the object s displaceent. 106 Chapter 5: Work, Power & Energy

3 F θ Fcosθ Δr To do this, line up the force and displaceent vectors tail-to-tail and easure the angle between the. Since this coponent of force can be calculated by ultiplying the force by the cosine of the angle between the force and displaceent vectors, you can write the work equation as: W = ΔE = F r = Fr cosθ W is the work done, F is the force applied in newtons, r is the object s displaceent in eters, and theta is the angle between F and r. The equation as a whole states that the work done is equal to the energy transferred, which is equal to the coponent of force parallel to the displaceent ultiplied by the displaceent, which is equal to the agnitude of the force, ultiplied by the agnitude of the displaceent, ultiplied by the cosine of the angle between the. When the work done on an object is independent of the object s path, the force doing the work is known as a conservative force. The units of work can be found by perforing unit analysis on the work forula. If work is force ultiplied by distance, the units ust be the units of force ultiplied by the units of distance, or newtons ultiplied by eters. A newton-eter is also known as a Joule (J) Q: An appliance salesan pushes a refrigerator eters across the floor by applying a force of 00N. Find the work done A: Since the force and displaceent are in the sae direction, the angle between the is 0. W = Fr = Frcos θ = (00 N)( )cos0= 400J 5.0 Q: A friend s car is stuck on the ice. You push down on the car to provide ore friction for the tires (by way of increasing the noral force), allowing the car s tires to propel it forward 5 onto less slippery ground. How uch work did you do? 5.0 A: You applied a downward force, yet the car s displaceent was sideways. Therefore, the angle between the force and displaceent vectors is 90. W = Fr= Frcosθ = Fr cos90 = 0 Chapter 5: Work, Power & Energy 107

4 5.03 Q: You push a crate up a rap with a force of 10N. Despite your pushing, however, the crate slides down the rap a distance of 4. How uch work did you do? 5.03 A: Since the direction of the force you applied is opposite the direction of the crate s displaceent, the angle between the two vectors is 180. W = Fr = Frcos θ = (10 N)(4 )cos180 = 40J 5.04 Q: How uch work is done in lifting an 8-kg box fro the floor to a height of above the floor? 5.04 A: It s easy to see the displaceent is, and the force ust be applied in the direction of the displaceent, but what is the force? To lift the box you ust atch and overcoe the force of gravity on the box. Therefore, the force applied is equal to the gravitational force, or weight, of the box, g=(8kg)(9.8/s )=78.4N. W = Fr = Frcos θ = (78.4 N)( )cos0= 157J 5.05 Q: Barry and Sidney pull a 30-kg wagon with a force of 500N a distance of 0. The force acts at a 30 angle to the θ=30 horizontal. Calculate the work done. Fcosθ F=500N A: W = Fr= Frcos θ = (500 N)(0 )cos30 = 8660J 5.06 Q: The work done in lifting an apple one eter near Earth s surface is approxiately (A) 1 J (B) 0.01 J (C) 100 J (D) 1000 J 5.06 A: (A) The trick in this proble is recalling the approxiate weight of an apple. With an order-of-agnitude estiate, you can say an apple has a ass of 0.1 kg, or a weight of 1 N. Given this inforation, the work done is: W = Fr = Frcos θ = (1 N)(1 )cos0 = 1J 108 Chapter 5: Work, Power & Energy

5 5.07 Q: As shown in the diagra, a child applies a constant 0-newton force along the handle of a wagon which akes a 5 angle with the horizontal. How uch work does the child do in oving the wagon a horizontal distance of 4.0 eters? (A) 5.0 J (B) 34 J (C) 73 J (D) 80. J 5.07 A: (D) W = Fr = Frcos θ = (0 N)(4 )cos5 = 73J The area under a force vs. displaceent graph is the work done by the force. Consider the situation of a block being pulled across a table with a constant force of 5 Newtons over a displaceent of 5 eters, then the force gradually tapers off over the next 5 eters. The work done by the force oving the block can be calculated by taking the area under the force vs. displaceent graph (a cobination of a rectangle and triangle) as follows: Work = Area + Area rectangle triangle 1 Work = lw + bh 1 Work = (5 )(5 N) + (5 )(5 N) Work = 37.5J Chapter 5: Work, Power & Energy 109

6 5.08 Q: A boy pushes his wagon at constant speed along a level sidewalk. The graph below represents the relationship between the horizontal force exerted by the boy and the distance the wagon oves. What is the total work done by the boy in pushing the wagon 4.0 eters? (A) 5.0 J (B) 7.5 J (C) 10 J (D) 180 J 5.08 A: (C) 10 J Work = Area = lw = (4 )(30 N) = 10J rectangle 5.09 Q: A box is wheeled to the right with a varying horizontal force. The graph below represents the relationship between the applied force and the distance the box oves. What is the total work done in oving the box 6 eters? (A) 9.0 J (B) 18 J (C) 7 J (D) 36 J 110 Chapter 5: Work, Power & Energy

7 5.09 A: (C) Work = Area + Area 1 Work = lw + bh 1 Work = (3 )(6 N) + (3 )(6 N) Work = 7J rectangle triangle 5.10 Q: An 80-kg wooden box is pulled 10 eters horizontally across a wood floor at a constant velocity by a 50-newton force at an angle of 37 degrees above the horizontal. If the coefficient of kinetic friction between the floor and the box is 0.315, find the work done by friction A: First, draw a diagra of the situation. 50 N 80 kg Next, create a FBD and pseudo-fbd detailing the forces on the box. f N 37 F app f F app sin N F app cos g g Then, write a Newton s nd Law equation in the x-direction and solve for the force of friction, recognizing that the acceleration of the box ust be zero since it oves at constant velocity. a=0 F netx = F app cosθ f = a x 50N cos(37 ) f = 0 f = 00N Finally, solve for the work done by friction. W = F rcos = (00 N)(10 )cos(180 ) = 000J f f θ Chapter 5: Work, Power & Energy 111

8 5.11 Q: Four carts, initially at rest on a flat surface, are subjected to varying forces as the carts ove to the right a set distance, depicted in the diagra below. A kg 0 kg F=0N B F=30N 60 C d=8 F=5N 30 D 15 kg 1 kg d=1 F=15N d=8 d=6 Rank the four carts fro least to greatest in ters of I) work done by the applied force on the carts II) inertia III) noral force applied by the surface to the carts 5.11 A: I) D, A, C, B II) A, D, C, B III) A, D, C, B Power Power is a ter used quite regularly in all aspects of life. People talk about how powerful the new boat otor is, the power of positive thinking, and even the power copany s latest bill. All of these uses of the ter power relate to how uch work can be done in soe aount of tie, or the rate at which energy is transferred. In physics, work can be defined in two ways. Work is the process of oving an object by applying a force. The rate at which the force does work is known as power (P). Work is also the transfer of energy, so power is also the rate at which energy is transferred into, out of, or within a syste. The units of power are the units of work divided by tie, or Joules per second, known as a Watt (W). P = W t = ΔE t Since power is the rate at which work is done, it is possible to have the sae aount of work done but with a different supplied power, if the tie is different. 11 Chapter 5: Work, Power & Energy

9 5.1 Q: Rob and Peter ove a sofa 3 eters across the floor by applying a cobined force of 00N horizontally. If it takes the 6 seconds to ove the sofa, what aount of power did they supply? 5.1 A: P W Fr cos θ (00 N)(3 ) = = = = 100W t t 6s 5.13 Q: Kevin then pushes the sae sofa 3 eters across the floor by applying a force of 00N. Kevin, however, takes 1 seconds to push the sofa. What aount of power did Kevin supply? 5.13 A: P W Fr cos θ (00 N)(3 ) = = = = 50W t t 1s As you can see, although Kevin did the sae aount of work as Rob and Peter in pushing the sofa (600J), Rob and Peter supplied twice the power of Kevin because they did the sae work in half the tie! There s ore to the story, however. Since power is defined as work over tie, and because work is equal to force (in the direction parallel to the displaceent) ultiplied by displaceent, you can replace work in the equation with F r cos(θ): P W Fr cosθ = = t t Looking carefully at this equation, you can observe a displaceent divided by tie. Since displaceent divided by tie is the definition of average velocity, you can replace Δr/t with v in the equation and, assuing the force is in the direction of the displaceent (cosθ=1) you obtain: P W Fr cosθ = = = Fv t t So, not only is power equal to work done divided by the tie required, it s also equal to the force applied (in the direction of the displaceent) ultiplied by the average velocity of the object Q: Motor A lifts a 5000N steel crossbar upward at a constant /s. Motor B lifts a 4000N steel support upward at a constant 3 /s. Which otor is supplying ore power? Chapter 5: Work, Power & Energy 113

10 5.14 A: Motor B supplies ore power than Motor A. P = Fv = ( 5000N)( ) = 10000W MotorA P = Fv = ( 4000N)( 3 ) = 1000W MotorB s s 5.15 Q: A 70-kilogra cyclist develops 10 watts of power while pedaling at a constant velocity of 7 eters per second east. What average force is exerted eastward on the bicycle to aintain this constant speed? (A) 490 N (B) 30 N (C) 3.0 N (D) 0 N 5.15 A: (B) P = Fv P 10W F = = = 30N v 7 s 5.16 Q: Alien A lifts a 500-newton child fro the floor to a height of 0.40 eters in seconds. Alien B lifts a 400-newton student fro the floor to a height of 0.50 eters in 1 second. Copared to Alien A, Alien B does (A) the sae work but develops ore power (B) the sae work but develops less power (C) ore work but develops less power (D) less work but develops ore power 5.16 A: (A) the sae work but develops ore power Q: A 110-kilogra bodybuilder and his 55-kilogra friend run up identical flights of stairs. The bodybuilder reaches the top in 4.0 seconds while his friend takes.0 seconds. Copared to the power developed by the bodybuilder while running up the stairs, the power developed by his friend is (A) the sae (B) twice as uch (C) half as uch (D) four ties as uch 5.17 A: (A) the sae. 114 Chapter 5: Work, Power & Energy

11 5.18 Q: Mary holds a 5-kg irror against the wall 1.5 eters above the ground for 0 seconds while Bob nails it in place. What is Mary s power output during that tie period? (A).45 Watts (B) 3.68 Watts (C) 66.7 Watts (D) None of the above 5.18 A: (D) There is no power output because the work done is zero. There is no displaceent of the irror Q: Which of the following are appropriate units for power? Choose all that apply. (A) J s kg (C) s N (B) s kg (D) 3 s 5.19 A: (A) and (D) are appropriate units for power. 5.0 Q: A box of ass is pushed up a rap at constant velocity v to a axiu height h in tie t by force F. The rap akes an angle of θ with the horizontal as shown in the diagra below. F h What is the power supplied by the force? Choose all that apply. (A) gh t (C) Fh t sinθ (B) gh t sinθ (D) Fv 5.0 A: (C) and (D) are both correct expressions for the power supplied by the force. Chapter 5: Work, Power & Energy 115

12 Energy We ve all had days where we ve had varying aounts of energy. You ve gotten up in the orning, had to drag yourself out of bed, force yourself to get ready for school, and once you finally get to class, you don t have the energy to do uch work. Other days, when you ve had ore energy, you ay have woken up before the alar clock, hustled to get ready for the day while a bunch of thoughts bounced around in your head, and hurried on to begin your activities. Then, throughout the day, the ore work you do, the ore energy you lose... What s the difference in these days? In physics, energy is the ability or capacity to do work. And as entioned previously, work is the process of oving an object. So, if you cobine the definitions, energy is the ability or capacity to ove an object. So far you ve exained kinetic energy, or energy of otion, and therefore kinetic energy ust be the ability or capacity of a oving object to ove another object! Matheatically, kinetic energy is calculated using the forula: K = 1 Of course, there are ore types of energy than just kinetic. Energy coes in any fors, which you can classify as kinetic (energy of otion) or potential (stored) to various degrees. This includes solar energy, theral energy, gravitational potential energy, nuclear energy, cheical potential energy, sound energy, electrical energy, elastic potential energy, light energy, and so on. In all cases, energy can be transfored fro one type to another and you can transfer energy fro one object to another by doing work. v 116 Chapter 5: Work, Power & Energy

13 The units of energy are the sae as the units of work, joules (J). Through diensional analysis, observe that the units of KE (kg /s ) ust be equal to the units of work (N ): kg = N = J s Gravitational Potential Energy Potential energy is energy an object possesses due to its position or condition. In order for an object to have potential energy, it ust interact with another object or syste. An object in isolation can have only kinetic energy. Gravitational potential energy is the energy an object possesses because of its position in a gravitational field (height), where another object or syste is providing the gravitational field. Assue a 10-kilogra box sits on the floor. You can arbitrarily call its current potential energy zero, just to give a reference point. If you do work to lift the box one eter off the floor, you need to overcoe the force of gravity on the box (its weight) over a distance of one eter. Therefore, the work you do on the box can be obtained fro: W = F r = (g)δy = (10kg)(9.8 s )(1) = 98J So, to raise the box to a height of 1, you ust do 98 Joules of work on the box. The work done in lifting the box is equal to the change in the potential energy of the box, so the box s gravitational potential energy ust increase by 98 Joules. When you perfored work on the box, you transferred soe of your stored energy to the box. Along the way, it just so happens that you derived the forula for the change in gravitational potential energy of an object. In a constant gravitational field, the change in the object s gravitational potential energy, ΔU g, is equal to the force of gravity on the box (g) ultiplied by its change in height, Δy. ΔU g = gδy This forula can be used to solve a variety of probles involving the potential energy of an object. Chapter 5: Work, Power & Energy 117

14 5.1 Q: The diagra below represents a 155-newton box on a rap. Applied force F causes the box to slide fro point A to point B. What is the total aount of gravitational potential energy gained by the box? (A) 8.4 J (B) 79 J (C) 868 J (D) 740 J 5.1 A: (B) ΔU g = gδy = (155N )(1.8) = 79J 5. Q: Which situation describes a syste with decreasing gravitational potential energy? (A) a girl stretching a horizontal spring (B) a bicyclist riding up a steep hill (C) a rocket rising vertically fro Earth (D) a boy juping down fro a tree lib 5. A: (D) The boy s height above ground is decreasing, so his gravitational potential energy is decreasing. 5.3 Q: Which is an SI unit for energy? (A) kg s (C) kg s (B) kg s (D) kg s 5.3 A: (A) is equivalent to a newton-eter, also known as a Joule. 118 Chapter 5: Work, Power & Energy

15 5.4 Q: A car travels at constant speed v up a hill fro point A to point B, as shown in the diagra below. As the car travels fro A to B, its gravitational potential energy (A) increases and its kinetic energy decreases (B) increases and its kinetic energy reains the sae (C) reains the sae and its kinetic energy decreases (D) reains the sae and its kinetic energy reains the sae 5.4 A: (B) The car s height above ground increases so gravitational potential energy increases, and velocity reains constant, so kinetic energy reains the sae. Note that the car s engine ust do work to aintain a constant velocity. 5.5 Q: An object is thrown vertically upward. Which pair of graphs best represents the object s kinetic energy and gravitational potential energy as functions of its displaceent while it rises? 5.5 A: () shows the object s kinetic energy decreasing as it slows down on its way upward, while its potential energy increases as its height increases. 5.6 Q: While riding a chairlift, a 55-kilogra snowboarder is raised a vertical distance of 370 eters. What is the total change in the snowboarder s gravitational potential energy? (A) J (B) J (C) J (D) J Chapter 5: Work, Power & Energy 119

16 5.6 A: (D) J ΔU g = gδy = (55kg)(9.8 s )(370) = 10 5 J 5.7 Q: A pendulu of ass M swings on a light string of length L as shown in the diagra at right. If the ass hanging directly down is set as the zero point of gravitational potential energy, find the gravitational potential energy of the pendulu as a function of θ and L. L 5.7 A: First use soe basic geoetry and trigonoetry to analyze the situation and set up the proble. Redraw the diagra as shown at right, then, solve for the change in vertical displaceent, Δy. adj cosθ= adj = hyp cosθ= Lcosθ hyp Δy = L adj = L Lcosθ = L(1 cosθ) Now, utilize your value for Δy to find the gravitational potential energy above your reference point. Δy=L(1 cosθ) ΔU g = gδy gl(1 cosθ) adj L hyp U g = 0 Δy Springs and Hooke s Law An interesting application of work cobined with the Force and Displaceent graph is exaining the force applied by a spring. The ore you stretch a spring, the greater the force of the spring. Siilarly, the ore you copress a spring, the greater the force. This can be odeled as a linear relationship known as Hooke s Law, where the agnitude of the force applied by the spring is equal to a constant ultiplied by the agnitude of the displaceent of the spring. F S = k x F s is the force of the spring in newtons, x is the displaceent of the spring fro its equilibriu (or rest) position, in eters, and k is the spring constant, which tells you how stiff or powerful a spring is, in newtons per eter. The larger the spring constant, k, the ore force the spring applies per aount of displaceent. In soe cases you ay see this written as F S =-kx, where the negative sign indicates the force is in the opposite direction of the displaceent. 10 Chapter 5: Work, Power & Energy

17 You can deterine the spring constant of a spring by aking a graph of the force fro a spring on the y-axis, and placing the displaceent of the spring fro its equilibriu, or rest position, on the x-axis. The slope of the graph will give you the spring constant. For the case of the spring depicted in the graph at right, you can find the spring constant as follows: k = Slope = rise run = ΔF 0N 0N = Δx = 00 N You ust have done work to copress or stretch the spring, since you applied a force and caused a displaceent. You can find the work done in stretching or copressing a spring by taking the area under the graph. For the spring shown, to displace the spring 0.1, you can find the work done as shown below: 1 1 Work = Area = bh = ( 01. )( 0N) = 1J tri 5.8 Q: In an experient, a student applied various forces to a spring and easured the spring s corresponding Force elongation. The table at right shows his (newtons) data. Plot force versus elongation and draw the best-fit line. Then, using your graph, calculate the spring constant of the spring. Show all your work. Elongation (eters) A: k = Slope = rise run = ΔF 6N 0N = Δx = 4 N Chapter 5: Work, Power & Energy 11

18 5.9 Q: In a laboratory investigation, a student applied various downward forces to a vertical spring. The applied forces and the corresponding elongations of the spring fro its equilibriu position are recorded in the data table. Construct a graph, arking an appropriate scale on the axis labeled Force (N). Plot the data points for force versus elongation. Draw the best-fit line or curve. Then, using your graph, calculate the spring constant of this spring. Show all your work. Force (newtons) Elongation (eters) A: k = Slope = rise run = ΔF Δx =.5N 0.8N = 55 N 5.30 Q: A 10-newton force copresses a spring 0.5 eter fro its equilibriu position. Calculate the spring constant of this spring A: FS 10N F = k x k = = = 40 S x 0.5 N Another for of potential energy involves the stored energy an object possesses due to its position in a stressed elastic syste. An object at the end of a copressed spring, for exaple, has elastic potential energy. When the spring is released, the elastic potential energy of the spring will do work on the object, oving the object and transferring the energy of the spring into kinetic energy of the object. Other exaples of elastic potential energy include tennis rackets, rubber bands, bows (as in bows and arrows), trapolines, bouncy balls, and even pole-vaulting poles. 1 Chapter 5: Work, Power & Energy

19 The ost coon probles involving elastic potential energy in introductory physics involve the energy stored in a spring. As you learned previously, the force needed to copress or stretch a spring fro its equilibriu position increases linearly. The ore you stretch or copress the spring, the ore force it applies trying to restore itself to its equilibriu position. This can be odeled using Hooke s Law: F = k x S Further, you can find the work done in copressing or stretching the spring by taking the area under a force vs. displaceent graph for the spring W = Fr = Area = bh= ( x)( kx) = kx triangle Since the work done in copressing or stretching the spring fro its equilibriu position transfers energy to the spring, you can conclude that the potential energy stored in the spring ust be equal to the work done to copress the spring. The potential energy of a spring (U S ) is therefore given by: U S = 1 kx 5.31 Q: A spring with a spring constant of 4.0 newtons per eter is copressed by a force of 1. newtons. What is the total elastic potential energy stored in this copressed spring? (A) 0.18 J (B) 0.36 J (C) 0.60 J (D) 4.8 J 5.31 A: (A) Us can t be calculated directly since x isn t known, but x can be found fro Hooke s Law: FS 1.N F = k x x = = = 0.3 S k 4 N With x known, the potential energy equation for a spring can be utilized. 1 1 U = kx = (4 )(0.3 ) = 0.18J S N Chapter 5: Work, Power & Energy 13

20 5.3 Q: An unstretched spring has a length of 10 centieters. When the spring is stretched by a force of 16 newtons, its length is increased to 18 centieters. What is the spring constant of this spring? (A) 0.89 N/c (B).0 N/c (C) 1.6 N/c (D) 1.8 N/c 5.3 A: (B) FS 16N F = k x k = = =.0 S x 8c N c 5.33 Q: Which graph best represents the relationship between the elastic potential energy stored in a spring and its elongation fro equilibriu? 5.33 A: () due to the displaceent relationship Q: A pop-up toy has a ass of 0.00 kilogra and a spring constant of 150 newtons per eter. A force is applied to the toy to copress the spring eter. 14 Chapter 5: Work, Power & Energy

21 (A) Calculate the potential energy stored in the copressed spring. (B) The toy is activated and all the copressed spring s potential energy is converted to gravitational potential energy. Calculate the axiu vertical height to which the toy is propelled A: (A) 1 1 U = kx = (150 )(0.05 ) = J S N (B) U g = gδy Δy = U g g = J (0.0kg)(9.8 s ) = Q: A spring with a spring constant of 80 newtons per eter is displaced 0.30 eter fro its equilibriu position. The potential energy stored in the spring is (A) 3.6 J (B) 7. J (C) 1 J (D) 4 J 5.35 A: (A) 1 1 U = kx = (80 )(0.3 ) = 3.6J S N Work-Energy Theore Of course, there are any different kinds of energy which haven t been entioned specifically. Energy can be converted aong its any different fors, such as echanical (which is kinetic, gravitational potential, and elastic potential), electroagnetic, nuclear, and theral (or internal) energy. When a force does work on a syste, the work done changes the syste s energy. If the work done increases otion, there is an increase in the syste s kinetic energy. If the work done increases the object s height, there is an increase in the syste s gravitational potential energy. If the work done copresses a spring, there is an increase in the syste s elastic potential energy. If the work is done against friction, however, where does the energy go? In this case, the energy isn t lost, but instead increases the rate at which olecules in the object vibrate, increasing the object s teperature, or internal energy. The internal energy of a syste includes the kinetic energy of the objects that ake up the syste and the potential energy of the configuration of the objects that ake up the syste. Chapter 5: Work, Power & Energy 15

22 The understanding that the work done on a syste by an external force changes the energy of the syste is known as the Work-Energy Relationship. If an external force does positive work on the syste, the syste s total energy increases. If, instead, the syste does work, the syste s total energy decreases. Put another way, you add energy to a syste by doing work on it and take energy fro a syste when the syste does the work (uch like you add value to your bank account by aking a deposit and take value fro your account by writing a check). When the force applied on an object is parallel to the object s displaceent, the work done increases the kinetic energy of the object. When the force applied is opposite the direction of the object s displaceent, the work done decreases the kinetic energy of the object. This is known as the Work-Energy Theore Q: Given the following sets of velocity and net force vectors for a given object, state whether you expect the kinetic energy of the object to increase, decrease, or reain the sae. v F net (A) (B) (C) (D) 5.36 A: (A) decrease (B) reain the sae (no work done as long as v and F are perpendicular) (C) increase (D) decrease 5.37 Q: A chef pushes a 10-kilogra pastry cart fro rest a distance of 5 eters with a constant horizontal force of 10 N. Assuing a frictionless surface, deterine the cart s change in kinetic energy and its final velocity A: First find the work done by the chef, which will be equal to the cart s change in kinetic energy. W = F r = (10N )(5) = 50J Next, solve for the cart s final velocity. K = 1 v v = K = (50J ) 10kg = 3. s 16 Chapter 5: Work, Power & Energy

23 5.38 Q: A pitcher throws a 143-gra baseball toward the catcher at 45 /s. If the catcher s hand oves back a distance of 6 c in stopping the ball, deterine the average force exerted on the catcher s hand A: There are several paths to solving this proble, including application of the Work-Energy Theore. W = ΔK F r = 1 v F = v r = (0.143kg)(45 s ) = 415N (0.06) 5.39 Q: In the following diagras, a force F acts on a cart in otion on a frictionless surface to change its velocity. The initial velocity of the cart and final velocity of each cart are shown. You do not know how far or in which direction the cart traveled. Rank the agnitude of the work done by the force on each cart fro greatest to least. A kg kg 3 kg 3 kg B v 0 = 5 s v = s v 0 = 3 s v = 3 s C D 5 kg 5 kg 4 kg 4 kg v 0 = 5 s v = 6 s v 0 = 1 s v = s 5.39 A: C, B, A, D (First find the change in K, which ust equal the work done according to the Work-Energy Theore.) 5.40 Q: Given the force vs. displaceent graph below for a net force applied horizontally to an object of ass initially at rest on a frictionless surface, deterine the object s final speed in ters of F ax, r 1, r, r 3, and. You ay assue the force does not change its direction. F ax Net Force r 1 r r 3 Displaceent Chapter 5: Work, Power & Energy 17

24 5.40 A: The work done is the area under the graph, which, according to the Work-Energy Theore, is also equal to the change in the object s kinetic energy, therefore you can set the area of the graph equal to the object s final kinetic energy given the initial kinetic energy is zero. W = Area = 1 bh+ lw+ 1 bh = 1 v 1 r F + (r r )F + 1 (r r )F = 1 1 ax 1 ax 3 ax v v = F ax (r 3 + r r 1 ) v = F ax (r 3 + r r 1 ) Conservation of Energy Energy cannot be created or destroyed... it can only be changed. Chances are you ve heard that phrase before. It s one of the ost iportant concepts in all of physics. It doesn t ean that an object can t lose energy or gain energy. What it eans is that energy can be changed into different fors, and transferred fro syste to syste, but it never agically disappears or reappears. In the world of physics, you can never truly destroy energy. The understanding that the total aount of energy in the universe reains fixed is known as the law of conservation of energy. Objects and systes can possess ultiple types of energy. The energy of a syste includes its kinetic energy, potential energy, and its internal energy. Mechanical energy is the su of an object s kinetic energy as well as its gravitational potential and elastic potential energies. Non-echanical energy fors include cheical potential, nuclear, and theral. Total energy is always conserved in any closed syste, which is the law of conservation of energy. By confining the discussion to just the echanical fors of energy, however, if you neglect the effects of friction you can also state that total echanical energy is constant in any syste. Take the exaple of an F/A-18 Hornet jet fighter with a ass of 0,000 kilogras flying at an altitude of 10,000 eters above the surface of the earth with a velocity of 50 /s. In this scenario, you can calculate the total echanical energy of the jet fighter as follows by assuing ground level is gravitational potential energy level zero: E T = ΔU g + K = gδy + 1 v E T = (0000kg)(9.8 )(10000) + 1 (0000kg)(50 s s ) E T = J 18 Chapter 5: Work, Power & Energy

25 Now, assue the Hornet dives down to an altitude of,000 eters above the surface of the Earth. Total echanical energy reains constant, and the gravitational potential energy of the fighter decreases, therefore the kinetic energy of the fighter ust increase. The fighter s velocity goes up as a result of flying closer to the Earth! For this reason, a key concept in successful dogfighting taught to ilitary pilots is that of energy conservation! You can even calculate the new velocity of the fighter jet since you know its new height and its total echanical energy ust reain constant. Solving for velocity, you find that the Hornet has alost doubled its speed by trading in 8000 eters of altitude for velocity! E T = ΔU g + K = gδy + 1 v 1 v = E T gδy v = (E gδy) T v = v = 469 s J (0000kg)(9.8 ( )(000)) s 0000kg If instead you had been told that soe of the echanical energy of the jet was lost to air resistance (friction), you could also account for that by stating that the total echanical energy of the syste is equal to the gravitational potential energy, the kinetic energy, and the change in internal energy of the syste (Q). This leads to the conservation of echanical energy forula: E = U + K+ Q T Let s take another look at free fall, only this tie, you can analyze a falling object using the law of conservation of energy and copare it to the analysis using the kineatic equations studied previously. The proble: An object falls fro a height of 10 above the ground. Neglecting air resistance, find its velocity the oent before the object strikes the ground. Conservation of Energy Approach: The energy of the object at its highest point ust equal the energy of the object at its lowest point, therefore: E top = E botto U gtop = K botto gδy top = 1 v botto v botto = gδy = (9.8 s )(10) =14 s Chapter 5: Work, Power & Energy 19

26 Kineatics Approach: For an object in free fall, its initial velocity ust be zero, its displaceent is 10 eters, and the acceleration due to gravity on the surface of the Earth is 9.8 /s. Choosing down as the positive direction: Variable Value v 0 v Δy 0 /s FIND 10 a 9.8 /s t? v = v + a ( y y ) v = v + a ( y y ) y y0 y 0 y y0 y v = (9.8 )(10 ) = 14 y s s 0 As you can see, you reach the sae conclusion regardless of approach! 5.41 Q: The diagra below shows a toy cart possessing 16 joules of kinetic energy traveling on a frictionless, horizontal surface toward a horizontal spring. If the cart coes to rest after copressing the spring a distance of 1.0 eter, what is the spring constant of the spring? (A) 3 N/ (B) 16 N/ (C) 8.0 N/ (D) 4.0 N/ A: (A) K = U = kx s K (16 J ) k = = = 3 x (1 ) N 5.4 Q: A child does 0.0 joules of work to copress the spring in a popup toy. If the ass of the toy is kilogras, what is the axiu vertical height that the toy can reach after the spring is released? (A) 0 (B).0 (C) 0.0 (D) Chapter 5: Work, Power & Energy

27 5.4 A: (B) The potential energy in the copressed spring ust be equal to the gravitational potential energy of the toy at its axiu vertical height. U s =U g = gδy Δy = U s g = 0.J (0.01kg)(9.8 ) = s 5.43 Q: A lawyer knocks her folder of ass off her desk of height Δy. What is the speed of the folder upon striking the floor? (A) (gδy) (B) gδy (C) gδy (D) Δy 5.43 A: (A) The folder s initial gravitational potential energy becoes its kinetic energy right before striking the floor. U desk = K floor gδy = 1 v v = gδy 5.44 Q: A 65-kilogra pole vaulter wishes to vault to a height of 5.5 eters. (A) Calculate the iniu aount of kinetic energy the vaulter needs to reach this height if air friction is neglected and all the vaulting energy is derived fro kinetic energy. (B) Calculate the speed the vaulter ust attain to have the necessary kinetic energy A: (A) K =U g = gδy K = (65kg)(9.8 s )(5.5) = 3500J K (B) 1 K = v v= (3500 J ) v = = 65kg 10 s Chapter 5: Work, Power & Energy 131

28 5.45 Q: The work done in accelerating an object along a frictionless horizontal surface is equal to the change in the object s (A) oentu (B) velocity (C) potential energy (D) kinetic energy 5.45 A: (D) Due to the Work-Energy Theore Q: A car, initially traveling at 30 eters per second, slows uniforly as it skids to a stop after the brakes are applied. Sketch a graph showing the relationship between the kinetic energy of the car as it is being brought to a stop and the work done by friction in stopping the car A: 5.47 Q: A -kilogra block sliding down a rap fro a height of 3 eters above the ground reaches the ground with a kinetic energy of 50 joules. The total work done by friction on the block as it slides down the rap is approxiately (A) 6 J (B) 9 J (C) 18 J (D) 44 J 5.47 A: (B) The box has gravitational potential energy at the top of the rap, which is converted to kinetic energy as it slides down the rap. Any gravitational potential energy not converted to kinetic energy ust be the work done by friction on the block, converted to internal energy (heat) of the syste. U gtop = K botto +W friction W friction =U gtop K botto = gδy K botto W friction = (kg)(9.8 )(3) 50J = 9J s 13 Chapter 5: Work, Power & Energy

29 5.48 Q: Four objects travel down an inclined plane fro the sae height without slipping. Which will reach the botto of the incline first? (A) a baseball rolling down the incline (B) an unopened soda can rolling down the incline (C) a physics book sliding down the incline (without friction) (D) an epty soup can rolling down the incline 5.48 A: (C) In all cases, the objects convert their gravitational potential energy into kinetic energy. In the case of the rolling objects, however, soe of that kinetic energy is rotational kinetic energy. Since the physics book cannot rotate, all of its gravitational potential energy becoes translational kinetic energy; therefore, it ust have the highest translational velocity Q: As a box is pushed 30 eters across a horizontal floor by a constant horizontal force of 5 newtons, the kinetic energy of the box increases by 300 joules. How uch total internal energy is produced during this process? (A) 150 J (B) 50 J (C) 450 J (D) 750 J 5.49 A: (C). The work done on the box can be found fro: W = F r = (5N )(30) = 750J Fro the Work-Energy Theore, you know that the total energy of the box ust increase by 750 joules. If the kinetic energy of the box increases by 300 joules, where did the other 450 joules of energy go? It ust have been transfored into internal energy! 5.50 Q: Mass 1 sits on a frictionless surface and is attached by a light string across a frictionless pulley to ass, as shown in the diagra below. 1 What happens to the gravitational potential energy and kinetic energy of 1 and when is released fro rest? 5.50 A: The gravitational potential energy of 1 reains the sae while its kinetic energy increases. The gravitational potential energy of decreases while its kinetic energy increases. Chapter 5: Work, Power & Energy 133

30 5.51 Q: A box of ass is attached to a spring (spring constant k) and sits on a frictionless horizontal surface as shown below. k The spring is copressed a distance x fro its equilibriu position and released. Deterine the speed of the box when the spring returns to its equilibriu position A: The elastic potential energy stored in the copressed spring is converted copletely to kinetic energy when the spring returns to its equilibriu position. 1 1 U = K kx = v v = s k x 5.5 Q: A pendulu of ass M swings on a light string of length L as shown in the diagra at right. If the ass is released fro rest at an angle of theta as shown, find the axiu speed of the pendulu as a function of θ, L, and any required fundaental constants. L 5.5 A: First deterine the gravitational potential energy at the ass s highest point as shown in proble 5.7. Δy=L(1 cosθ) ΔU g = gδy gl(1 cosθ) Next, noting that this gravitational potential energy is converted to kinetic energy at the lowest point of the pendulu, solve for the velocity of the ass at the lowest point. ΔU g = K gl(1 cosθ) = 1 v v = gl(1 cosθ) 5.53 Q: A ball is thrown vertically upwards. Plot the ball s total echanical energy, kinetic energy, and gravitational potential energy versus tie on the sae axes A: E E tot U g K 134 Chapter 5: Work, Power & Energy t

31 5.54 Q: When the ball is half the distance to its peak, which of the following are true? Choose all that apply. Neglect air resistance. (A) The ball s velocity is half its axiu velocity (v=v ax /). (B) The ball s velocity is its axiu velocity divided by the square root of (v=v ax / ). (C) The ball s kinetic energy is equal to its gravitational potential energy. (D) The ball s kinetic energy is equal to half its total echanical energy A: (B), (C), and (D) are all true Q: Andy the Adventurous Adventurer, while running fro evil bad guys in the Aazonian Rainforest, trips, falls, and slides down a frictionless udslide of height 0 eters as depicted below. 15 Cliff 0 Mudslide? Once he reaches the botto of the udslide, he has the isfortune to fly horizontally off a 15-eter cliff. How far fro the base of the cliff does Andy land? 5.55 A: Breaking this proble into sections, let s first analyze Andy s otion on the udslide by utilizing conservation of energy to deterine his horizontal velocity as he flies off the cliff. U g = gδy = 1 v = K v = gδy v = (9.8 s )(0) =19.8 s Once Andy flies horizontally off the cliff, this becoes a projectile proble. First find Andy s tie in the air by analyzing his vertical otion, then use this tie to find how far he travels horizontally before landing soe distance fro the base of the cliff. Δy = v y0 t + 1 a t v y 0 =0 t = y Δx = v y0 t = (19.8 s )(1.75s) = 34.6 Δy a y = (15) 9.8 s =1.75s Chapter 5: Work, Power & Energy 135

32 5.56 Q: Alicia, a 60-kg bungee juper, steps off a 40-eter-high bridge. The bungee cord behaves like a spring with spring constant k=40 N/. Assue there is no slack in the bungee cord. (A) Find the speed of the juper at a height of 15 eters above the ground. (B) Find the speed of the juper at a height of 30 eters above the ground. (C) How close does the juper get to the ground? 5.56 A: First draw a diagra of the bungee juper, labeling the point 15 eters above the ground point A, and the point 30 eters above the ground point B. (A) Next, use conservation of energy to solve for the kinetic energy of the juper when she is at point A. =U g +U A S + K A A K A =U gtop U g U A S A U gtop 1 v = gδy 1 A k(δy) v A = gδy k (Δy) v A = (9.8 )(5) 40 N s 60kg (5) = 73.3 v s A = 8.6 s B A (B) Follow the sae strategy to deterine the speed of the juper at point B. U gtop =U gb +U SB + K B K B =U gtop U GB U SB 1 v = gδy 1 B k(δy) v B = gδy k (Δy) v B = (9.8 )(10) 40 N s 60kg (10) =19 v =11.4 s B s (C) The closest the juper gets to the ground occurs when the juper s speed (and therefore kinetic energy) reaches zero. Again, utilize conservation of energy. U gtop =U gbottom +U SBOTTOM ΔU g =U S gδy = 1 k(δy) g = kδy Δy = g k = (60kg)(9.8 ) s = N If Δy=9.4, the juper ust be fro the ground at her lowest point, or 10.6 above the ground. 136 Chapter 5: Work, Power & Energy

33 Sources of Energy on Earth So where does all this energy initially coe fro? Here on Earth, the energy you deal with everyday ultiately coes fro the conversion of ass into energy, the source of the sun s energy. The sun s radiation provides an energy source for life on Earth, which over the illennia has becoe the source of fossil fuels. The sun s radiation also provides the theral and light energy that heat the atosphere and cause the winds to blow. The sun s energy evaporates water, which eventually recondenses as rain and snow, falling to the Earth s surface to create lakes and rivers, with gravitational potential energy, which is harnessed in hydroelectric power plants. Nuclear power also coes fro the conversion of ass into energy. Just try to find an energy source on Earth that doesn t originate with the conversion of ass into energy! Chapter 5: Work, Power & Energy 137

34 Test Your Understanding 1. Design and perfor an experient which exaines how a force exerted on an object does work on the object as it oves through a distance.. Is it possible to do work on an object without changing the object s energy? Explain why or why not with exaples. 3. Hupty Dupty falls off the wall onto the ground and breaks. If Hupty Dupty initially has gravitational potential energy up on the wall, describe what happened to this energy, keeping in ind the law of conservation of energy. 4. Why can a single object have only kinetic energy, while potential energy requires interactions aong objects or systes? Explain with real-world exaples. 5. Create a conservation of energy proble that utilizes at least three different types of energy. Solve it. 6. In your own words, explain what is eant by open and closed systes. Give exaples of how both energy and linear oentu are conserved in these systes. 7. An object is dropped fro a given height h at the sae tie an identical object is launched vertically upward fro ground level. Deterine the initial velocity of the object launched upward such that the two objects collide at height h/. Having already answered this proble using kineatics at the end of Chapter 3, now answer using a conservation of energy approach. Which approach is easier? 138 Chapter 5: Work, Power & Energy