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Grade 12
Unit 3
PHYSICS 1203
WORK AND ENERGY
CONTENTS
I. TYPE AND SOURCE OF ENERGY
.........
2
MECHANICAL ENERGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FORMS OF ENERGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
5
II. CONSERVATION OF ENERGY, POWER,
AND EFFICIENCY . . . . . . . . . . . . . . . . . . . . . . .
10
CONSERVATION OF ENERGY . . . . . . . . . . . . . . . . . . . . . . . .
POWER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EFFICIENCY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
16
17
III. HEAT ENERGY . . . . . . . . . . . . . . . . . . . . . . . . . .
22
SPECIFIC HEAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LATENT HEAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LAWS OF THERMODYNAMICS . . . . . . . . . . . . . . . . . . . . . . . .
22
25
31
GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
Author:
M. Grace Ferreira, M.A.T., M.N.S.
Editor:
Alan Christopherson, M.S.
Illustrations:
David Sprenger
Alpha Omega Graphics
804 N. 2nd Ave. E., Rock Rapids, IA 51246-1759
© MM by Alpha Omega Publications, Inc. All rights reserved.
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WORK AND ENERGY
success in this LIFEPAC® will affect your success in
other areas of physics.
Energy is the ability to do work. In this
LIFEPAC you will undertake a study of energy—
its sources and forms, its basic laws, and its
transformations.
You have mastered the areas in physics known
as kinematics and dynamics. Now you will add to
that foundation another concept, energy. These
three areas of physics form the backbone of all
future studies of waves, sound, light, electricity and
magnetism, and nuclear and atomic energy. Your
OBJECTIVES
Read these objectives. The objectives tell you what you will be able to do when you have successfully
completed this LIFEPAC.
When
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
you have finished this LIFEPAC, you should be able to:
Define energy.
Identify various forms and sources of energy.
Solve kinetic- and potential-energy problems.
Apply the law of conservation of energy in energy problems.
Solve problems involving power.
Solve problems concerning the efficiency of machines.
Apply thermodynamics to the solution of problems related to heat flow and machines.
Identify and classify phases of matter.
Distinguish between temperature and heat.
Calculate heat energy involving latent heats.
Survey the LIFEPAC. Ask yourself some questions about this study. Write your questions here.
1
I. TYPE AND SOURCES OF ENERGY
Energy is the ability to do work. Energy has a
variety of forms: chemical, heat, electrical,
nuclear, solar, geothermal, hydroelectric,
tidal, and wind. The gasoline your car burns
contains energy in the bonds of the hydrogen and
carbon atoms of which the fuel is comprised.
Substances may contain energy but the substance
should not be confused with the energy it contains.
This section will treat two forms of energy,
kinetic and potential. A later section will deal
with heat energy. Other LlFEPACs will introduce
the study of electrical, light, and nuclear energy.
Chemical energy is covered in the chemistry series
of LlFEPACs; geothermal, tidal, and wind energy
are covered in LlFEPACs on the earth sciences.
SECTION OBJECTIVES
Review these objectives. When you have completed this section, you should be able to:
1. Define energy.
2. Identify various forms and sources of energy.
3. Solve kinetic and potential energy problems.
VOCABULARY
Study these words to enhance your learning success in this section.
acceleration due to gravity
chemical energy
kinetic energy
displacement
light energy
distance
mass
electrical energy
nuclear energy
energy
potential energy
force
solar energy
geothermal energy
tidal energy
heat energy
wind energy
hydroelectric energy
work
Note: All vocabulary words in this LIFEPAC appear in boldface print the first time they are used. If you are unsure of
the meaning when you are reading, study the definitions given
MECHANICAL ENERGY
Mechanical energy has two forms, kinetic and
potential. These two forms are the subject of this
section.
Energy is the ability to do work. Work occurs
whenever a force (F) is exerted through a
distance (d). The product of the net force and the
displacement through which it is exerted is work.
work = F
•
Kinetic energy. An object in motion can do
work by virtue of its motion because it can exert a
force through a distance. The energy it has due to
its motion is called kinetic energy.
d
Net force means that if more than one force is
acting upon an object, the vector sum must be obtained.
The displacement through which the force acts is
parallel to the direction of the force. (Trigonometry is
used to solve problems involving forces not parallel to
the displacement.) If the force were perpendicular to
the displacement, the object would not move in a
straight line but would rotate in a circular path.
Kinetic energy = 1/2 mv2
where m is the mass of the object and v is its
velocity.
Since kinetic energy results from a force acting
over a certain distance,
2
F
•
d = 1/2 mv2
F
•
d = mad
•
d = ma(1/2 at2) = 1/2 ma2t2
Since d = 1/2 at2.
This equation could have been derived from
Newton’s second law:
F
F = ma
Substituting v = at,
F
•
d = 1/2 mv2
✍
Complete these activities.
1.1
A car traveling at 60 mph has how much more energy than a car going at 20 mph?
1.2
How much farther will a car skid if it locks its brakes at 60 mph as compared to a skid from 15 mph?
If the metric (SI) system is used, force is
measured in Newtons (N); displacement is in
meters (m); velocity is in meters/second (m/s); mass is in
kilograms (kg); and work and energy are both
measured in joules (J). A Newton or force is equal to
mass x acceleration; therefore, a Newton is actually
a kg • m/s2. Work is a force x distance the force is
moved, so a joule is actually kg • m2/s2 and energy,
although using a different formula also uses the
measuring unit of joules.
Energy is a bit more complex as there are
different forms of energy; thus, it may be derived in
using a different formula. Kinetic energy is energy
of an object in motion, and potential kinetic energy
is energy due to an object’s position or height above
the earth. There are two types of potential energy:
gravitational and elastic. In this text, we will only
be using the formula concerned with gravitational
energy. Mechanical energy is the sum of all the
kinetic and potential energy of an object.
A joule of energy is defined as a force of 1
Newton exerted over a distance of 1 meter.
Work
Mechanical Energy (ME) = KE + S PE
Notice, however, that in all of these formulas,
the measuring unit of a joule is correct.
1J = 1N
•
m, thus 1 kg
•
m2/s2
In the English system, force is measured in
pounds; mass is in slugs; distance is in feet; and
velocity is in ft/s. So the English unit for work or
energy is a foot-pound.
Work = Force (F) x distance (d)
= pound (slug • ft/s2) • ft,
so the unit is slug • ft2/s2
KE = 1⁄2 mv2, so the unit is slug
•
PE = mgh, so the unit is slug
= slug • ft2/s2
ft/s2
•
ft2/s2
Regardless of whether energy is measured in
metric or English, the unit for measurement is
equal to mass (m) times distance (d) squared
divided by time (t) squared.
Energy = Work = m
In the metric system,
= Force (F) x distance (d)
•
d2/t2
/hr = 36,000 m/hr = 36,000 meters
3,600 sec
36 km/hr = 10m/sec.
36
Kinetic Energy (KE) = ⁄2 mass x velocity squared
= 1⁄2 mv2
= kg • m2/s2
1
km
Unfortunately, since hours are not in a base-ten
form, a conversion factor is needed to convert from
km
/hr to m/sec. In the English system miles must be
changed to feet and hours to seconds to yield a ratio
of 15 mph = 22 ft/sec; so 45 mph = 66 ft/sec.
Potential energy (PE) = mgh, since mass x
gravity (gravity is an acceleration) is a force and
height is a distance, the formula for PE is just like
that of work.
3
✍
Complete these activities.
1.3
A car weighing 3,200 lbs. is traveling at 30 mph. How much kinetic energy does it possess?
(Hint: calculate the mass of the car.)
1.4
In the preceding problem how much less energy would the car have if it were traveling at 15 mph.
1.5
A force of 80 N is exerted on an object on a frictionless surface for a distance of 4 meters.
If the object has a mass of 10 kg, calculate its velocity.
1.6
Why are the chances of death occurring in an accident of a car traveling 60 to 70 mph
fourteen times greater than in a car traveling at 30 to 40 mph?
Potential energy. Kinetic energy is the energy
a body contains by virtue of its motion. The energy
stored in a body by virtue of its position is called
potential energy. A spring has potential energy
when it is compressed or stretched because it can do
work on any object attached to it. A stretched
rubber band or a stretched bowstring also stores
potential energy. A rock resting on the ground has
no stored energy. The potential energy depends also
on the gravitational field.
P
•
does not depend on the path. Since the object was
lifted to that height, work was done on it.
Fd = mgh
Notice that this equation could have been
derived by using the definition that weight is a
force:
weight = F = mg
E = mgh,
and multiply both sides by distance, d:
where m is the mass, g is the acceleration due to
gravity, and h is the height above ground. Where
ground level is 4,000 feet elevation, an object at
4,100 feet has potential energy proportional to 100
feet. The h is vertical height (displacement) and
F
•
d = mg
•
d
If that distance is height, d = h
F
4
•
d = mg
•
h
✍
Complete these activities.
1.7
A rock with mass of 5 kg is carried up a small hill 10 meters high. What is the potential energy
of the rock at the hilltop?
1.8
How much work had to be done in carrying that rock up hill?
1.9
A 20-kg barrel is rolled up a 20-m ramp to the back of a truck whose floor is 5 m above the
ground. What work is done in loading one barrel into the truck?
1.10
How much potential energy does that one barrel (1.9) have when it is in the truck?
FORMS OF ENERGY
In the late eighteenth century a depleted supply
of whales whose blubber was used for lamp oil
accelerated the infant petroleum industry. In the
early 1970s the Arab oil embargo initiated searches
for replacement sources of energy. Technological
advances have been made that will allow oil to
continue as an important source of energy as
research is done to look for alternatives.
Heat energy. When a temperature change
occurs, heat energy is involved. At the microscope
level, atoms and molecules all possess energy of
motion and position. Addition of heat energy
increases the random motion of the atoms and
molecules.
Solar energy. Most energy is ultimately
derived from the sun. Various forms of energy occur
under the title solar energy. Visible light would
properly be called light energy. The infrared
portion of the spectrum could also be called
infrared, or heat, energy. However, energy that is
directly from the sun is termed solar energy. The
future trend is to harness solar energy to do work
economically. Solar ovens and energy collectors to
cook food, to generate electricity, and to heat water,
swimming pools, and houses are already in use.
Chemical energy. The energy that coal
possesses is chemical bonding between hydrogen
and carbon atoms. This energy is chemical
energy. From the molecular point of view, chemical
energy is potential energy because the electron
positions are changed when a chemical reaction
takes place. Work is obtained by breaking the
chemical bonds. When either coal, oil, gas, or wood
is burned, a chemical reaction takes place and
chemical energy is released.
5
Light energy. Light produced by means other
than the sun has light energy. A campfire, light
bulb, or burner element of an electric stove are
sources of light energy. Light can do work by
exciting the cones and rods in your eyes, which your
brain translates as sight, or by triggering the
chemical reactions that develop a photograph.
of bays and rivers. The Rance River in France has
been utilized in this way. The Bay Of Fundy in Nova
Scotia experiences tides higher than fifty feet due to
the funneling of water into a narrow bay. A problem
in harnessing tidal energy is that the strength of
the tide in most places is weak and discontinuous;
that is, the interval between high and low tides is
six hours. This form of energy, unlike hydroelectric
or chemical energy, is due primarily to the influence
of the moon and not of the sun.
Electrical energy. In current, moving
electricity charges light bulbs, turns motors and
cooks food. These moving charges constitute
electrical energy. In an industrial society,
electrical energy is a necessity. To produce electrical
energy, other sources of energy are used: wood, coal,
oil, gas, and falling water. Solar, nuclear, and tidal
energy are being investigated as alternate sources
of energy to produce electricity.
Wind energy. Wind energy is powered by the
sun and can be harnessed in ways similar to water
power. In Holland windmills catch offshore and
onshore winds, which turn wheels to produce
electricity and to grind grain. Small rural
communities and homes use the wind in the same
manner. The problems encountered in trying to use
wind as a large-scale form of energy is its strength
and consistency of direction. In Phoenix, Arizona,
for example, where the average wind speed is two
miles per hour, windmills are unfeasible.
Nuclear energy. The energy derived from
atomic nuclei is termed nuclear energy. Man has
discovered how to generate this energy in two ways:
by splitting nuclei or by fusing nuclei. Currently
only fission has been harnessed for practical energy
production. Fission waste products are difficult to
deal with. Controlled fusion reactions are
technologically not yet practical as a source of
energy.
Geothermal energy. Iceland has 95 percent of
its energy needs satisfied by tapping hot steam
from the ground to power electrical generators.
Geothermal energy is used also to heat homes
and to warm greenhouses, thereby gaining a twomonth advance on their short growing season.
Geothermal energy is derived from water turned to
steam by the heat from magmatic intrusions. In
some locations neither steam nor hot water is found
but extremely hot rock is located by drilling.
Scientists experiment with piping water down to
the rocks to be heated to steam which is then
collected.
Hydroelectric energy. Potential energy
contained by water behind a dam is converted to
hydroelectric energy when the water is released
to a generator. Water power has been used in old
grist mills and early water-powered factories.
Tidal energy. Tidal energy has been harnessed
to produce electricity when water flows in and out
✍
1.11
Prepare a report.
Arguments for and against nuclear energy are frequently in the headlines. Research both sides
of the question and present an unbiased report on the pros and cons of nuclear power plants.
Your sources may include science and environmental textbooks, magazine and newspaper
articles, and literature distributed by activist organizations. Your report must be balanced and
objective. Submit your report for evaluation.
Score
Adult check
______________________
Initial
Date
6
✍
Complete these activities.
1.12
Describe the resultant form of energy.
a. friction
b. nuclear power plant
c. toaster element
d. welding torch
e. light bulb
f. campfire
g. moving car
h. lump of coal
i. stick of TNT
j. eraser on a desk edge
1.13
Not all energy sources derive their energy from the sun. List the ones that their energy from
sources other than the sun.
1.14
All energy sources can be described at the microscopic level as potential or kinetic energy.
Describe the following energy sources as either potential or kinetic.
a. chemical
b. water behind a dam
c. wind
d. stream
e. geothermal
f. nuclear
Review the material in this section in preparation for the Self Test. This Self Test will check
your mastery of this particular section. The items missed on this Self Test will indicate specific
areas where restudy is needed for mastery.
7
SELF TEST 1
Match these items (each answer, 2 points).
1.01
energy of motion
a. work
1.02
potential energy
mass • height
b. energy
1.03
energy that results when a
temperature change occurs
1.04
energy from the sun
1.05
the numerical equivalent of energy
1.06
energy supplied by the moon
1.07
energy that is produced by heat from
the earth
1.08
energy of position
1.09
energy
1.010
energy derived from the nucleus of
an atom
c. kinetic energy
d. potential energy
e. force
f. acceleration due to gravity
g. heat energy
h. solar energy
i. nuclear energy
j. tidal energy
k. geothermal energy
/displacement
Match these items (each answer, 2 points).
Identify the following situations as a source of (a) kinetic energy or (b) potential energy or (c) both.
1.011
a rock at the edge of a cliff
1.012
a plane in flight at 30,000 feet
1.013
chemical energy stored in coal
1.014
water behind a dam
1.015
water flowing downstream
1.016
a car moving on a level road
1.017
a compressed spring
1.018
a child swinging on a swing
1.019
a bouncing ball
1.020
geothermal energy
Complete these calculations (each answer, 5 points).
1.021
Calculate the kinetic energy in joules of a 10 g bullet moving at 300 m/sec.
(Hint: change grams to kilograms.)
1.022
If the speed of an object were to triple, what would be the increase of kinetic energy?
8
1.023
Calculate the increase in potential energy of a 60-kg man who climbs a ladder 10 meters high.
1.024
A horse pulls on an object with a force of 300 newtons and does 12,000 joules of work. How far
was the object moved?
Answer these activities (each answer, 5 points).
1.025
Three mountain climbers set out to climb a mountain from the same altitude and all arrive at
the same location at the top. Mountain climber A took a long gradual slope to the top, B went a
steeper but shorter path, and C tackled the sheer straight side to the top. Assume all three
climbers weigh the same.
A
TH
PA
B
TH
PA
PATH C
Write which climber gained the greatest potential energy and which did the most work, and
explain your answer.
1.026
The expression “The bigger they are, the harder they fall” is used often. Since, in the absence of
air resistance, all objects fall with the same acceleration due to gravity, is this statement a
generally true statement? Explain your answer.
Score
Adult check
56
70
______________________
Initial
9
Date