1. Chapter 10
Energy and Work

2. 10 Energy and Work
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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.
Answer: C
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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.
Answer: C
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8. Reading Quiz Which of the following is an energy transfer?
Kinetic energy
Heat
Potential energy
Chemical energy
Thermal energy
Answer: B
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9. Answer Which of the following is an energy transfer? Kinetic energy
Heat
Potential energy
Chemical energy
Thermal energy
Answer: B
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10. Reading Quiz
If you raise an object to a greater height, you are increasing
kinetic energy.
heat.
potential energy.
chemical energy.
thermal energy.
Answer: C
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11. Answer If you raise an object to a greater height, you are increasing
kinetic energy.
heat.
potential energy.
chemical energy.
thermal energy.
Answer: C
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12. Forms of Energy Mechanical Energy Thermal Energy Other forms include
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13. The Basic Energy Model
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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.
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15. Some Energy Transformations
Echem  Ug
K  Eth
Echem  Ug
Us  K  Ug
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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
Answer: B
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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
Answer: B
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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
Answer: D
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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
Answer: D
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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.
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21. Energy Transfers: Work
W K
W Eth
W Us
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22. The Work-Energy Equation
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23. The Law of Conservation of Energy
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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.
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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.
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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
Answer: D
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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
Answer: D
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28. Work
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29. Work Done by Force at an Angle to Displacement
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30. Energy Equations
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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?
Answer: D
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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?
Answer: D D.
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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?
Answer: C
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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.
Answer: C
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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?
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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?
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37. Slide 10-37

38. Elastic Collisions
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39. Power
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40. Power Same mass... Both reach 60 mph... Same final kinetic energy, but
different times mean different powers.
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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?
Car
Mass (g)
Speed (m/s)
Time (s) A
100 3 2 B
200 C D
300 E 1 4
Answer: A
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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?
Car
Mass (g)
Speed (m/s)
Time (s) A
100 3 2 B
200 C D
300 E 1 4
Answer: A
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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?
Car
Mass (g)
Speed (m/s)
Time (s) A
100 3 2 B
200 C D
300 E 1 4
Answer: E
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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?
Car
Mass (g)
Speed (m/s)
Time (s) A
100 3 2 B
200 C D
300 E 1 4
Answer: E
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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?
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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 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.
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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.
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48. Summary
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49. Summary
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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?
Answer: E
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51. Answer
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?
Answer: E E.
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52. 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 lowest final speed?
Answer: D
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53. Answer
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 lowest final speed?
Answer: D D.
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54. Additional Questions
Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance?
Answer: E
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55. Answer
Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the largest distance?
Answer: E E.
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56. Additional Questions
Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the smallest distance?
Answer: B
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57. Answer
Trucks with the noted masses moving at the noted speeds crash into barriers that bring them to rest with a constant force. Which truck compresses the barrier by the smallest distance? B.
Answer: B
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58. Additional Questions
A 20-cm-long spring is attached to a wall. When pulled horizontally with a force of 100 N, the spring stretches to a length of 22 cm. What is the value of the spring constant?
5000 N/m
500 N/m
454 N/m
Answer: C
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59. Answer
A 20-cm-long spring is attached to a wall. When pulled horizontally with a force of 100 N, the spring stretches to a length of 22 cm. What is the value of the spring constant?
5000 N/m
500 N/m
454 N/m
Answer: C
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60. Additional Questions
I swing a ball around my head at constant speed in a circle with circumference 3 m. What is the work done on the ball by the 10 N tension force in the string during one revolution of the ball?
30 J
20 J
10 J
0 J
Answer: D
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61. Answer
I swing a ball around my head at constant speed in a circle with circumference 3 m. What is the work done on the ball by the 10 N tension force in the string during one revolution of the ball?
30 J
20 J
10 J
0 J
Answer: D
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