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Chapter 10

Chapter 10. 0. Energy and Work. 10 Energy and Work. Slide 10-2. Slide 10-3. Slide 10-4. Slide 10-5. Reading Quiz. If a system is isolated , the total energy of the system increases constantly. decreases constantly. is constant. depends on work into the system.

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Chapter 10

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  1. Chapter 10 0 Energy and Work

  2. 10Energy and Work Slide 10-2

  3. Slide 10-3

  4. Slide 10-4

  5. Slide 10-5

  6. Reading Quiz • If a system is isolated, the total energy of the system • increases constantly. • decreases constantly. • is constant. • depends on work into the system. • depends on work out of the system. Slide 10-6

  7. Answer • If a system is isolated, the total energy of the system • increases constantly. • decreases constantly. • is constant. • depends on work into the system. • depends on work out of the system. Slide 10-7

  8. Reading Quiz • Which of the following is an energy transfer? • Kinetic energy • Heat • Potential energy • Chemical energy • Thermal energy Slide 10-8

  9. Answer • Which of the following is an energy transfer? • Kinetic energy • Heat • Potential energy • Chemical energy • Thermal energy Slide 10-9

  10. Reading Quiz • If you raise an object to a greater height, you are increasing • kinetic energy. • heat. • potential energy. • chemical energy. • thermal energy. Slide 10-10

  11. Answer • If you raise an object to a greater height, you are increasing • kinetic energy. • heat. • potential energy. • chemical energy. • thermal energy. Slide 10-11

  12. Forms of Energy Mechanical Energy Thermal Energy Other forms include Slide 10-12

  13. The Basic Energy Model Slide 10-13

  14. Energy Transformations Kinetic energy K = energy of motion Potential energy U = energy of position Thermal energy Eth = energy associated with temperature System energy E = K + U + Eth + Echem + ... Energy can be transformed within the system without loss. Energy is a property of a system. Slide 10-14

  15. Some Energy Transformations Echem Ug KEth Us KUg Echem Ug Slide 10-15

  16. Checking Understanding • A skier is moving down a slope at a constant speed. What energy transformation is taking place? • A. K  Ug • B. Ug Eth • C. Us Ug • D. Ug K • E. K  Eth Slide 10-16

  17. Answer • A skier is moving down a slope at a constant speed. What energy transformation is taking place? • A. K  Ug • B. Ug Eth • C. Us Ug • D. Ug K • E. K  Eth Slide 10-17

  18. Checking Understanding • A child is on a playground swing, motionless at the highest point of his arc. As he swings back down to the lowest point of his motion, what energy transformation is taking place? • A. K  Ug • B. Ug Eth • C. Us Ug • D. Ug K • E. K  Eth Slide 10-18

  19. Answer • A child is on a playground swing, motionless at the highest point of his arc. As he swings back down to the lowest point of his motion, what energy transformation is taking place? • A. K  Ug • B. Ug Eth • C. Us Ug • D. Ug K • E. K  Eth Slide 10-19

  20. Energy Transfers These change the energy of the system. Interactions with the environment. Work is the mechanical transfer of energy to or from a system via pushes and pulls. Slide 10-20

  21. Energy Transfers: Work W K W Eth W Us Slide 10-21

  22. The Work-Energy Equation Slide 10-22

  23. The Law of Conservation of Energy Slide 10-23

  24. The Basic Equation • KfUfEthKiUi • A few things to note: • Work can be positive (work in) or negative (work out) • We are, for now, ignoring heat. • Thermal energy is…special. When energy changes to thermal energy, this change is irreversible. Slide 10-24

  25. Conceptual Example Problem A car sits at rest at the top of a hill. A small push sends it rolling down a hill. After its height has dropped by 5.0 m, it is moving at a good clip. Write down the equation for conservation of energy, noting the choice of system, the initial and final states, and what energy transformation has taken place. Slide 10-25

  26. Checking Understanding • Three balls are thrown off a cliff with the same speed, but in different directions. Which ball has the greatest speed just before it hits the ground? • Ball A • Ball B • Ball C • All balls have the same speed Slide 10-26

  27. Answer • Three balls are thrown off a cliff with the same speed, but in different directions. Which ball has the greatest speed just before it hits the ground? • Ball A • Ball B • Ball C • All balls have the same speed Slide 10-27

  28. Work Slide 10-28

  29. Work Done by Force at an Angle to Displacement Slide 10-29

  30. Energy Equations Slide 10-30

  31. Checking Understanding Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the largest change in kinetic energy? Slide 10-31

  32. Answer Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the largest change in kinetic energy? D. Slide 10-32

  33. Checking Understanding Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the smallest change in kinetic energy? Slide 10-33

  34. Answer Each of the boxes, with masses noted, is pulled for 10 m across a level, frictionless floor by the noted force. Which box experiences the smallest change in kinetic energy? C. Slide 10-34

  35. Example Problem A 200 g block on a frictionless surface is pushed against a spring with spring constant 500 N/m, compressing the spring by 2.0 cm. When the block is released, at what speed does it shoot away from the spring? Slide 10-35

  36. Example Problem • A 2.0 g desert locust can achieve a takeoff speed of 3.6 m/s (comparable to the best human jumpers) by using energy stored in an internal “spring” near the knee joint. • When the locust jumps, what energy transformation takes place? • What is the minimum amount of energy stored in the internal spring? • If the locust were to make a vertical leap, how high could it jump? Ignore air resistance and use conservation of energy concepts to solve this problem. • If 50% of the initial kinetic energy is transformed to thermal energy because of air resistance, how high will the locust jump? Slide 10-36

  37. Slide 10-37

  38. Elastic Collisions Slide 10-38

  39. Power Slide 10-39

  40. Power • Same mass... • Both reach 60 mph... Same final kinetic energy, but different times mean different powers. Slide 10-40

  41. Checking Understanding Four toy cars accelerate from rest to their top speed in a certain amount of time. The masses of the cars, the final speeds, and the time to reach this speed are noted in the table. Which car has the greatest power? Slide 10-41

  42. Answer Four toy cars accelerate from rest to their top speed in a certain amount of time. The masses of the cars, the final speeds, and the time to reach this speed are noted in the table. Which car has the greatest power? Slide 10-42

  43. Checking Understanding Four toy cars accelerate from rest to their top speed in a certain amount of time. The masses of the cars, the final speeds, and the time to reach this speed are noted in the table. Which car has the smallest power? Slide 10-43

  44. Answer Four toy cars accelerate from rest to their top speed in a certain amount of time. The masses of the cars, the final speeds, and the time to reach this speed are noted in the table. Which car has the smallest power? Slide 10-44

  45. Example Problem • In a typical tee shot, a golf ball is hit by the 300 g head of a club moving at a speed of 40 m/s. The collision with the ball happens so fast that the collision can be treated as the collision of a 300 g mass with a stationary ball—the shaft of the club and the golfer can be ignored. The 46 g ball takes off with a speed of 70 m/s. • What is the change in momentum of the ball? • What is the speed of the club head immediately after the collision? • What fraction of the club’s kinetic energy is transferred to the ball? • What fraction of the club’s kinetic energy is “lost” to thermal energy? Slide 10-45

  46. Example Problem A typical human head has a mass of 5.0 kg. If the head is moving at some speed and strikes a fixed surface, it will come to rest. A helmet can help protect against injury; the foam in the helmet allows the head to come to rest over a longer distance, reducing the force on the head. The foam in helmets is generally designed to fail at a certain large force below the threshold of damage to the head. If this force is exceeded, the foam begins to compress. If the foam in a helmet compresses by 1.5 cm under a force of 2500 N (below the threshold for damage to the head), what is the maximum speed the head could have on impact? Use energy concepts to solve this problem. Slide 10-46

  47. Example Problem • Data for one stage of the 2004 Tour de France show that Lance Armstrong’s average speed was 15 m/s, and that keeping Lance and his bike moving at this zippy pace required a power of 450 W. • What was the average forward force keeping Lance and his bike moving forward? • To put this in perspective, compute what mass would have this weight. Slide 10-47

  48. Summary Slide 10-48

  49. Summary Slide 10-49

  50. Additional Questions Each of the 1.0 kg boxes starts at rest and is then pushed for 2.0 m across a level, frictionless floor by a rope with the noted force. Which box has the highest final speed? Slide 10-50

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