Chapter 9 Energy
Objectives:
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Define and describe work
Define and describe power
State the 2 forms of mechanical energy
State 3 forms of potential energy
Describe how work and kinetic energy are related
State the work-energy thereom
State the law of conservation of energy
Homework:
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Work & Power #2-5, 33-37, 43, 44
Kinetic & Potential Energy #6-8, 26b, 27, 29, 38-40
Work-Energy Theorem #9, 41, 42, 53, 54
Conservation of Energy #11-14, 45, 50-52
The Big Idea!
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_______________ can change from one form to another without a net loss of gain.
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Energy may be the most familiar concept in science, yet is one of the most difficult to define. We
observe the __________ of energy when something is happening – only when energy is being
_________________ from one place to another or _________________________ from on form
to another.
9.1 Work
1. Work is done when a net _______________ acts on an object and the object moves in the
____________________ of the net force.
2. Work is the product of the force on an object and the _________________ through which the
object is moved: the quantity _________________ x ______________________
3. We do work when we ___________ a load against Earth’s gravity. The ______________ the load
or the ___________________ we lift it, the more work we do.
4. If the force is ________________ and the motion takes place in a ________________ line in the
direction of the force, the work done on an object by a net force is the product of the force and
the distance through which the object is moved:
a. Work = ___________________ x _______________________
b. W = __________
5. If we lift two loads, we do _________________ as much work as lifting one load the same
distance, because the ________________ needed is twice as great.
6. If we lift one load _______________ as far, we do twice as much work because the
_________________ is twice as great.
7. Work is done in lifting a barbell. If the barbell could be lifted twice as ____________, the weight
lifter would have to do twice as much ______________.
a. While the weightlifter is holding the barbell over his head, he may get really tired, but
he does NO _____________ on the barbell.
b. Work may be done on the muscles by ________________ and __________________
them, but this work is not done on the barbell.
c. When the weightlifter _______________ the barbell, he is doing work on it.
8. Some work is done against another ___________________.
a. An archer stretches her bowstring, doing work against the _______________ force of
the bow.
b. When the ram of a pile driver is raised, work is required to raise the ram against the
force of __________________.
c. When you do push-ups, you do work against your own _____________________.
9. Some work is done to change the _________________ of an object.
a. Bringing an automobile up to speed or in slowing it down involves _____________.
b. In both categories, work involves a transfer of ________________ between something
and its surroundings.
10. The unit of measurement for work combines a unit of force, _____, with a unit of distance,
____.
a. The unit or work is the newton-meter, also called the _______________
b. One joule (J) of work is done when a force of ____ N is exerted over a distance of ___ m
(lifting an apple over your head).
11. ______________ unites are required to describe greater work.
a. ___________________ (kJ) are thousands of joules. The weight lifter dos work on the
order of kilojoules.
b. ___________________ (MJ) are millions of joules. To stop a loaded truck going at 100
km/h takes megajoules of work.
12. Think! Suppose that you apply a 60-N horizontal force to a 32-kg package, which pushes it 4
meters across the mailroom floor. How much work do you do on the package?
13. Concept Check: When is work done on an object?
9.2 Power
14. Power equals the amount of _________ done divided by the __________ interval during which
the work is done.
15. When carrying a load up some stairs, you do the same amount of ___________ whether you
walk or run up the stairs.
16. _________________ is the rate at which work is done.
17. A high power engine does work ___________________.
a. An engine that delivers twice the power of another engine does not necessarily produce
twice as much ______________ or go twice as ______________.
b. Twice the power means the engine can do twice the work in the same amount of
___________ or the same amount of work can be done in ____________ the time.
c. A powerful engine can get an automobile up to a given speed in _________ time than a
__________ powerful engine can.
18. The unit of power is the joule per second, also known as the ______________.
a. One watt (W) of power is expended when ______ joule of work is done in ______
second.
b. One kilowatt (kW) equals _____________watts.
c. One megawatt (MW) equals ____________________ watts.
19. The 3 main engines of the space shuttle can develop 33,000 MW of ______________ when fuel
is burned at the enormous rate of 3400 kg/s.
20. In the United States, we customarily rate engines in units of ______________________ and
_____________________ in kilowatts, but either may be used.
21. In the metric system of units, automobiles are rated in _____________________. One
horsepower (hp) is the same as ____________ kW, so an engine rated at 134 hp is a ______ kW
engine.
22. Think! If a forklift is replaced with a new forklift that has twice the power, how much greater a
load can it lift in the same amount of time? If it lifts the same load, how much faster can it
operate?
23. Concept Check: How can you calculate power?
9.3Mechanical Energy
24. The 2 forms of mechanical energy are __________________ energy and
____________________ energy.
25. When _________ is done by an archer in drawing back a ______________________, the bent
bow acquires the ability to do ______________ on the arrow.
26. When work is done to raise the heavy ram of a pile driver, the ram acquires the ability to do
work on the object it hits when it _______________.
27. When work is done to wind a ________________ mechanism, the spring acquires the ability to
do work on various gears to run a clock, ring a bell, or sound an alarm.
28. Something has been acquired that enables the object to do ____________.
a. It may be in the form of a _______________________ of atoms in the material of an
object; a physical __________________________ of attracting bodies; or a
rearrangement of electric charges in the molecules of a substance.
29. The property of an object of system that enables them to do work is ______________________.
Like work, energy is measured in _______________.
a. ___________________ energy is the energy due to the position of something or the
___________________ of something.
30. Concept Check: What are the tow forms of mechanical energy?
9.4 Potential Energy
31. Three examples of potential energy are _______________________ potential energy,
_____________________ energy, and __________________________ potential energy.
32. An object may store energy by virtue of its ______________________.
33. Energy that is stored and held in readiness is call _________________________
_____________________ (PE) because in the stored state is has potential for doing
________________.
34. Elastic Potential Energy
a. A stretched or compressed __________________ has a potential for doing work.
b. When a bow is drawn back, energy is __________________ in the bow. The bow can do
work on the arrow.
c. A ____________________ rubber band has potential energy because of its position.
d. These types of potential energy are called _______________ potential energy.
35. Chemical Energy
a. The chemical energy in ___________________ is also potential energy.
b. It is energy of position at the submicroscopic level. This energy is available when the
position of _________________ ________________ within and between
__________________ is altered and a chemical change takes place.
36. Gravitational Potential Energy
a. Work is required to _____________________ objects against Earth’s gravity
b. The potential energy due to elevated positions is ________________________ potential
energy.
c. Water in an elevated reservoir and the raised ram of a pile driver have
________________________ potential energy.
d. The amount of gravitational potential energy possessed by an elevated object is equal to
the ______________ done against ________________ to lift it.
e. The upward force required while moving at constant velocity is equal to the
_______________, mg, of the object, so the work done is lifting it through a height h is
the product __________.
f. Gravitational potential energy = ___________________ x ________________________
g. PE = __________
i. Note that the height is the distance above some chosen ___________________
level, such as the ground or the floor of a building.
ii. The gravitational potential energy, mgh, is _____________________ to that
level and depends only on mg and h.
37. The potential energy of the 100-N boulder with respect to the ground below is __________ J in
each case.
a. The boulder is lifted with _______ N of force.
b. The boulder is pushed up the 4-m incline with ______ N of force.
c. The boulder is lifted with ______ N of force up each 0.5 m stair.
38. Hydroelectric power stations use gravitational potential energy.
a. Water from an upper _______________ flows through a long tunnel to an electric
__________________.
b. Gravitational potential energy of the water is converted to __________________
energy.
c. Power stations _______ electricity at night, when there is much less demand, and pump
water form a lower reservoir back up to the upper reservoir. This process is called
__________________ storage.
d. The pumped storage system helps to smooth out differences between energy
________________ and ___________________.
39. Think! You lift a 100-N boulder 1 m.
a. How much work is done on the boulder?
b. What power is expended if you lift the boulder in a time of 2 s?
c. What is the gravitational potential energy of the boulder in the lifted position?
40. Concept Check! Name 3 examples of potential energy.
9.5 Kinetic Energy
41. The kinetic energy of a _______________ object is equal to the work required to bring it to its
speed from ________________, or the work the object can do while being brought to rest.
42. If an object is moving, then it is capable of doing ________________. It has energy of
__________________, or ___________________ __________________________ (KE).
a. The kinetic energy of an object depends on the ____________ of the object as well as its
______________________.
b. It is equal to half the mass multiplied by the square of the speed.
i. Kinetic energy = ___________________ x __________________
ii. KE = ___________
43. When you throw a ball, you do _________________ on It to give it ________________ as it
leaves your hand. The moving ball can then hit something and ____________ it, doing work on
what it hits.
a. _____________ x ___________________ = kinetic energy
b. _________ = _____________________
44. Note that the speed is ______________________, so if the speed of an object is doubled, its
kinetic energy is _________________________ (22 = 4)
a. It takes 4 times the ______________ to double the speed
b. An object moving twice as fast takes _____________ times as much work to stop.
45. Concept Check: How are work and kinetic energy of a moving object related?
9.6 Work-Energy Theorem
46. The work-energy theorem states that whenever work is done, _____________ changes.
a. To increase the _________________ _________________ of an object, work must be
done on the object.
b. If an object is moving, _______________ is required to bring it to rest.
c. The change in kinetic energy is __________________ to the net work done.
d. The ____________-_______________________ theorem describes the relationship
between work and energy.
47. We abbreviate “change in” with the delta symbol, _______
a. Work = _______________
b. Work equals the change in __________________ __________________________
c. The work in this equation is _____________ ______________________ - that is, the
work based on the net force.
48. If there is no ____________________ in an object’s kinetic energy, then no net work was done
on it.
49. Push a box against the floor.
a. If it doesn’t slide, then you are NOT doing _______________ on the box.
b. On a very slippery floor, if there is no _______________ at all, the work of your push
times the ______________________ of your push appears as kinetic energy of the box.
c. If there is some friction, it is the __________ force of your push minus the frictional
force that is multiplied by ________________________ to give the gain in kinetic
energy.
d. If the box moves at ________________ speed, you are pushing just hard enough to
overcome friction. The net force and net work are ________________, and according to
the work-energy theorem, ___________ = 0. The kinetic energy doesn’t change.
50. The work-energy theorem applies to _________________________ speed as well.
a. The more kinetic energy something has, the more work required to _______________
it.
b. Twice as much kinetic energy means twice as much _________________.
51. Due to _________________________, energy is transferred both into the floor and into the tire
when the bicycle skids to a stop.
52. When a car brakes, the work is the __________________________ force supplied by the brakes
________________________ by the distance over which the friction force acts.
a. A car moving at _______________ the speed of another has ________________ times
as much kinetic energy, and will require ________________ times as much work to
stop/
b. The frictional force is nearly the ______________ for both cars, so the faster one takes
four times as much ______________________ to stop.
53. Kinetic energy depends on speed __________________.
54. Typical stopping distances for care equipped with ___________________ brakes traveling at
various speeds. The work done to stop the car is ____________________
_______________________ x _______________________________________________.
55. Kinetic energy often appears hidden in different forms of energy such as _______________,
_______________________, _____________________, and
_________________________________.
a. Random molecular motion is sensed as ______________________.
b. ____________________ consists of molecules vibrating in rhythmic patterns.
c. ____________________ energy originates in the motion of electrons within atoms.
d. Electrons in motion make ____________________ _________________________.
56. Think! A friend says that if you do 100 J of work on a moving cart, the cart will gain 100 J of KE.
Another friend says this depends on whether or not there is friction. What is your opinion of
these statements?
57. Think! When the brakes of a car are locked, the car skids to a stop. How much farther will the
car skid if it’s moving 3 times as fast?
58. Concept Check: What is the work-energy theorem?
9.7 Conservation of Energy
59. The law of conservation of energy states that energy cannot be ________________ or
________________________. It can be ______________________ form one form to another,
but the total amount of energy never changes.
60. More important that knowing what energy is, is understanding how it behaves – how it
_______________________.
61. We can understand nearly every process that occurs in nature if we analyze it in terms of
transformation of ______________________ from one form to another.
62. _________________________ energy will become the _____________________ energy of the
arrow.
a. As you draw back the arrow in a bow, you do _________________ stretching the bow.
i. The bow then has _______________________ energy
ii. When released, the arrow has ______________________ energy equal to this
potential energy.
iii. It delivers this energy to its ____________________.
iv. The small distance the arrow moves multiplied by the average
__________________ of impact doesn’t quite match the ___________________
energy of the target.
63.
64.
65.
66.
67.
v. However, the arrow and target are a bit _____________________ by the energy
difference.
Energy changes from one form to another without a net ______________ or a net
________________.
The study of the forms of energy and the transformations from one form into another is the
___________ of _______________________________ of ___________________________.
For any _______________________ in its entirety – as simple as a swinging pendulum or as
complex as an exploding galaxy – there is one quantity that does not change:
_____________________.
Energy may change _____________________, but the total energy stays the same.
Part of the ____________ of the wound spring changes into _________. The remaining
_________ goes into heating the machinery and the surroundings due to friction. No energy is
________________.
68. Everywhere along the path of the pendulum bob, the _______ of KE and PE is the same. Because
of the ____________ done against friction, this energy will eventually be
transformed into _______________.
69. When the woman leaps from the burning building, the sum of her _________
and __________ remains ____________________ at each successive
position all the way down to the ground.
70. Each atom that makes up matter is a concentrated bundle of
_________________.
71. When the nuclei of atoms arrange themselves, _______________________ amounts of energy
can be released.
72. The sun shines because some of its ____________________ energy is transformed into
______________________ energy.
73. In nuclear reactors, nuclear energy is transformed into ________________.
74. Enormous compression due to ______________________ in the deep, hot interior of the sun
causes hydrogen nuclei to fuse and become helium nuclei.
a. This high temperature welding of atomic nuclei is called
___________________________________ ____________________________
b. This process releases _______________________ energy, some of which reaches Earth.
c. Part of this energy falls on ___________________, and some of the plants later become
________________.
d. Another part supports life in the food chain that begins with microscopic
______________ animals and ______________, and later gets stored in
_____________.
e. Part of the sun’s energy is used to _______________________ water from the ocean.
f. Some water returns to Earth as rain that is trapped behind a _______________.
g. The water behind a dam has __________________ energy that is used to power a
________________________ plant below the dam.
h. The generating plant transforms the energy of falling water into
_______________________ energy.
i. Electrical energy travels through wires to homes where it is used for
___________________, __________________________, cooking, and operating electric
toothbrushes.
75. Concept Check: What does the law of conservation of energy state?