Grade 12 Unit 3
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. LIFEPAC is a registered trademark of Alpha Omega Publications, Inc.
All trademarks and/or service marks referenced in this material are the property of their respective owners. Alpha Omega Publications, Inc. makes no claim of ownership to any trademarks and/or service marks other than their own and their affiliates’, and makes no claim of affiliation to any companies whose trademarks may be listed in this material, other than their own.
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
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. LIFEPAC is a registered trademark of Alpha Omega Publications, Inc.
All trademarks and/or service marks referenced in this material are the property of their respective owners. Alpha Omega Publications, Inc. makes no claim of ownership to any trademarks and/or service marks other than their own and their affiliates’, and makes no claim of affiliation to any companies whose trademarks may be listed in this material, other than their own.
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.
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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
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Date
6
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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.
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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?
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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
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9
Date
