Download Work and Energy LESSON OBJECTIVES Students will be able to

PHYSICS
Unit 4: Work and Energy
LESSON OBJECTIVES
Students will be able to...
 use appropriate metric units for given measurements
 describe the features of potential energy
 describe the features of kinetic energy
 define work
 understand the difference between conservative and non-conservative forces
 describe the Law of Conservation of Energy
 describe how friction causes energy loss
 describe power as energy expended per unit time
SKILLS
Students will be able to...
 use steps for solving physics problems
 solve problems using work, force, and distance
 solve problems using kinetic, potential energy, and the Law of Conservation of Energy
 use bar diagrams as an aid for solving energy problems
 conduct experiments relating to work and energy.
 collect and display experiment data
VOCABULARY:
force
friction
work
power
potential energy
kinetic energy
mechanical energy
non-mechanical energy
Law of Conservation of Energy
bar chart
HOMEWORK:
Terminology and Concepts
#1: Definition and Mathematics of Work, reading
#2: Calculating the Amount of Work Done by Forces, reading and written homework pages 1-4
#3: Potential Energy, reading
#4: Kinetic Energy, reading
#5: Mechanical Energy, reading and written homework page 5 and 6
#6: Power, reading and written homework page 7
The Work-Energy Theorem
#7: Internal vs. External Forces, reading and written homework page 8 and 9
#8: Analysis of Situations Involving External Forces, reading
#9: Analysis of Situations in Which Mechanical Energy is Conserved, reading and written homework
page 10 and 11
#10: Application and Practice Questions, reading and written homework pages 12-17
#11: Bar Chart Illustrations, reading and written homework pages 18 and 19
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EQUATIONS
W=Fd
KEi + PEi + Wext = KEf + PEf
W = mgh
P= W
t
GPE = mgh
EPE = ½ kx2
P = Fd
t
Fspring = kx
P = Fv
KE = ½ mv2
P = ∆E
t
OUTLINE
I. Work
A Defined
B. Calculations
II. Energy
A. Defined
B. Types
1. Non Mechanical
2. Mechanical
a. potential energy
b. kinetic energy
C. Transformation and Conservation
III. Power
A. Defined
B. Calculations
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WORK
1. The illustration shows Beatrice pushing on a heavy box.
Beatrice pushes with a force of 40 N. What information do
you need to determine if Beatrice is doing work on the box?
2. Beatrice pushes the box 2 m to the right. What formula
should you use to calculate the amount of work done on the
box?
3. How much work does Beatrice do pushing the box?
4. Write a definition for each of the following terms:
a. work
b. joule
5. Calculate the amount of work done when a grocery store stocker uses 120 N of force to lift a sack of
flour 1.5 m onto a shelf.
6. How much work is done on a object that is pushed 95 cm with a force of 80 N?
7. If 67 J of work is done to move a person, what force was used to push that person if he was moved
19 m?
8. A crate is moved 25 m across a warehouse by a group of three men. The push the crate and do
500 J of work. What is the total force that was applied to the crate?
9. Mary pulls her book bag off the floor and places it on top of her refrigerator. If the fridge is 250 cm
high, what force did she use if she did 175 J of work?
10. You do 425 J of work to push a box 2.5 m up a ramp. What force did you apply to the box?
GRAVITATIONAL POTENTIAL ENERGY
11. What is the equation for gravitational potential energy?
12. What is the potential energy of a rock that has a mass 10 kg that is sitting on top of a hill 300 m high?
13. A flower pot sitting on a window sill has a mass of 3 kg 30 meters above the ground. How many joules of
potential energy does the flower pot have?
14. A 1200 kg automobile is parked on a steep hill 50 m high. What is the energy?
15. A 2.4 kg toy falls from 2 m to 1 m. What is the change in GPE?
16. If (on earth) an object falls 18 m and loses 36 J of GPE. What is the object’s mass?
17. A 1 Kg object loses 20 J of GPE as it falls. How far does it fall?
18. A small, 3 kg weight is moved from 5 m from the ground to 8 m. What is the change in potential energy?
19. An 80 kg person falls 60 m off of a waterfall. What is her change in GPE?
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20. A Gravitational Potential Energy (GPE) Sensor attached to a 12 Kg ball changes from 12 J to 22 J, by
height change alone. What is the change in height?
21. A man rides up in an elevator 12 m. He gains 6500 J of gravitational potential energy. What is the man’s
mass?
22. When a 5 kg rock is dropped from a height of 6 m on Planet X, it loses 24 J of GPE. What is the
acceleration due to gravity on Planet X?
23. What is the gravitational potential energy of a 450 Kg car at the top of a 25 m parking garage?
24. What is the change in gravitational potential energy of a 45 kg weight that is moved from 2 m to 18 m
on earth? What is it on the moon (g = 1.6 m/s2)?
25. A 0.25 kg book falls off a 2 m shelf on to a 0.5 m chair. What was the change in GPE?
26. A 60 kg girl falls off of a waterfall and loses 10 kJ of GPE. What was her height?
27. When a 0.5 kg rock is dropped from a height of 12 m on Planet Z, it loses 45 J of GPE. What is the
acceleration due to gravity on Planet Z?
KINETIC ENERGY
28. What is the equation for kinetic energy?
29. What is the kinetic energy of a bicycle with a mass of 14 kg traveling at a velocity of 3 m/s north?
30. A ball rolls down a hill with a velocity of 5 m/s. If the ball is 5 kg, what is the kinetic energy of the rolling
ball?
31. How much kinetic energy does an 80 kg man have while running at 1.5 m/s?
32. A bird flies at a speed of 2.3 m/s if it has 14 J of kinetic energy, what is its mass?
33. A child does 12 J of work pushing his 3 kg toy truck. With what velocity does the toy move after the child
is done pushing?
34. A 6 kg object has a speed of 24 m/s. What is its kinetic energy?
35. A rock hits the ground with a speed of 7 m/s and a kinetic energy of 100 J. What is the rock’s mass?
36. A bullet is fired into a 12 kg block of wood. After the bullet stops in the block of wood the block has 29 J
of kinetic energy. At what speed is the block moving?
37. How much kinetic energy does a 4 Kg cat have while running at 9 m/s?
38. What is the mass of an object moving with a speed of 4 m/s and a kinetic energy of 2000 J?
39. A 400 Kg car has 1.8 x 105 J of kinetic energy. How fast is it moving?
40. How fast is a 3 Kg toy car with 20 J of kinetic energy moving?
41. A student runs to physics class with a speed of 6 m/s. If the student has 880 J of kinetic energy, what is
her mass?
42. What is the speed of a 1200 kg car moving with a kinetic energy of 15 kJ?
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ELASTIC POTENTIAL ENERGY
43. A spring with a spring constant of 120 N/m stretches by 0.02 m. What is the potential energy of the
spring?
44. An elastic spring stores 45 J of potential energy when it is stretched by 2 cm. What is the spring
constant?
45. A 50 N force causes a spring to compress 0.09 m. What is the spring constant? What is the potential
energy of the spring?
46. An 80 N force causes a spring to compress 0.15 m. What is the spring constant? What is the potential
energy of the spring?
47. A spring with a spring constant of 200 N/m stretches by 0.03 m. What is the potential energy of the
spring?
48. A spring stores 68 J of potential energy when it is stretched by 6 cm. What is the spring constant?
49. When a 13.2-kg mass is placed on top of a vertical spring, the spring compresses 5.93 cm. Find the force
constant of the spring.
50. If a spring has a spring constant of 400 N/m, how much force is required to compress the spring 25.0 cm
from its resting position?
51. A compressed spring that obeys Hooke's law has a potential energy of 18 J. If the spring constant of the
spring is 400 N/m, find the distance by which the spring is compressed.
52. A spring stretches 10 cm when 200 g is hanging from it. Determine the spring constant.
POWER
53. Ignacio is trying to move a rock uphill. What are the two equations you can use to calculate the power
Ignacio must use to move the rock?
54. If the rock does not move, does Ignacio perform any work? Does he use any power?
55. Ignacio does 50J of work on the rock and it takes him 10 s. How much power, in watts, does he use?
56. Carolina does the same amount of work as Ignacio on the rock but moves the rock in 5 s. How much
power, in watts, does she use?
57. Felix applies the same force but moves the rock twice as far as Ignacio in 5 s. How much power, in
watts, does he use?
58. Calculate the average power, in kilowatts, required to pull a car up a ramp if the amount of work is 250
kJ over a period of 45 s.
59. What amount of power, in horsepower, is used when 1,500 J of work is done in 2 s.
60. Determine the work done using 1.5 x 103 W of power in 2 minutes.
61. A machine uses 1,300 W of power. If the machine runs for 30 seconds, what is the work done by the
machine?
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62. What amount of power is used to push a 50 N box up a 500cm ramp in 60 seconds.
63. Calculate the time, in seconds, needed to do 39 J of work if the power output is 18W.
64. What time is needed to lift a rock using 75 N of force and 100 W of power?
65. A motor does 5,000 joules of work in 20 seconds. What is the power of the motor?
66. A machine does 1,500 joules of work in 30 seconds. What is the power of this machine?
67. A hair dryer uses 72,000 joules of energy in 60 seconds. What is the power of this hair dryer?
68. A toaster oven uses 67,500 joules of energy in 45 seconds to toast a piece of bread. What is the power
of the oven?
69. A horse moves a sleigh 1.00 kilometer by applying a horizontal 2,000-newton force on its harness for 45
minutes. What is the power of the horse? (Hint: Convert time to seconds.)
70. A wagon is pulled at a speed of 0.40 meters/sec by a horse exerting an 1,800-newton horizontal force.
Whatis the power of this horse?
71. Suppose a force of 100 newtons is used to push an object a distance of 5 meters in 15 seconds. Find the
work done and the power for this situation.
72. Emily’s vacuum cleaner has a power rating of 200 watts. If the vacuum cleaner does 360,000 joules of
work, how long did Emily spend vacuuming?
73. Nicholas spends 20 minutes ironing shirts with his 1,800-watt iron. How many joules of energy were used
by the iron? (Hint: convert time to seconds).
74. It take a clothes dryer 45 minutes to dry a load of towels. If the dryer uses 6,750,000 joules of energy to
dry the towels, what is the power rating of the machine?
75. A 1000-watt microwave oven takes 90 seconds to heat a bowl of soup. How many joules of energy does
it use?
76. A force of 100 newtons is used to move an object a distance of 15 meters with a power of 25 watts. Find
the work done and the time it takes to do the work.
77. If a small machine does 2,500 joules of work on an object to move it a distance of 100 meters in 10
seconds, what is the force needed to do the work? What is the power of the machine doing the work?
78. A machine uses a force of 200 newtons to do 20,000 joules of work in 20 seconds. Find the distance the
object moved and the power of the machine. (Hint: A joule is the same as a Newton-meter.)
79. A machine that uses 200 watts of power moves an object a distance of 15 meters in 25 seconds. Find
the force needed and the work done by this machine
"OTHER ENERGY"
80. How are work and energy related?
81. What is the difference between potential and kinetic energy?
82. What is mechanical energy?
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TRANSFORMATION
Energy
includes
is the ability
to do
work
includes
includes
which measures
potential energy
kinetic energy
transformed energy
force
which is energy
associated with
which is
energy of
but never
acting over a
motion
or
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Conservation of Energy
83. A spring gun with a spring constant of 250 N/m is compressed 5 cm. How fast will a 0.025 kg dart go
when it leaves the gun?
84. A student uses a spring (with a spring constant of 180 N/m) to launch a marble vertically into the air. The
mass of the marble is 0.004 kg and the spring is compressed 0.03 m. How high will the marble go?
85. A student uses a spring gun (with a spring constant of 120 N/m) to launch a marble vertically into the air.
The mass of the marble is 0.002 kg and the spring is compressed 0.04 m.
a. How high will the marble go?
b. How fast will it be going when it leaves the gun?
86. A roller coaster has a velocity of 25 m/s at the bottom of the first hill. How high was the hill?
87. How much work is needed to lift a 3 kg mass a distance of 0.75 m?
88. An arrow is fired vertically upwards by a bow and reaches an altitude of 134 m. Find the initial speed of
the arrow on the ground level.
89. A student uses a spring to launch a marble vertically in the air. The mass of the marble is 0.002 kg and
when the spring is stretched 0.05 m it exerts a force of 10 N. What is the maximum height the marble can
reach?
90. A children’s roller coaster is released from the top of a track. If its maximum speed at ground level is 8
m/s, find the height it was released from.
91. A student uses a spring with a spring constant of 130 N/m in his projectile apparatus. When 56 J of
potential energy is required to launch the projectile to a certain height, what is the compression in the
spring?
92. How much work must be done to accelerate an 8x105 kg train: a) from 10 m/s to 15 m/s; b) from 15 m/s
to 20 m/s; c) to a stop an initial speed of 20 m/s?
93. How much work is done in accelerating a 2000 kg car from rest to a speed of 30 m/s?
94. A rock is dropped from a height of 2.7 m. How fast is it going when it hits the ground?
95. A roller coaster is released from the top of a track that is 125 m high. Find the roller coaster speed when
it reaches ground level.
96. A 1500 kg car, moving at a speed of 20 m/s comes to a halt. How much work was done by the brakes?
97. A projectile is fired vertically upward with an initial velocity of 190 m/s. Find the maximum height of the
projectile.
98. A spring gun is used to project a 0.5 kg ball, in order to perform this experiment the spring was initially
compressed by 0.005 m. Find the ball’s speed when it leaves the gun, if the spring constant is 395 N/m.
99. A student uses a spring loaded launcher to launch a marble vertically in the air. The mass of the marble
is 0.003 kg and the spring constant is 220 N/m. What is the maximum height the marble can reach (a)
when compressed 2 cm? (b) when compressed 4 cm?
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