Conceptual Physics

1. Conceptual Physics Chapter Nine Notes: Energy

2. 9.1 Work • In the past several chapters, we utilized Newton's laws to analyze the motion of objects. Force and mass information were used to determine the acceleration of an object. Acceleration information was subsequently used to determine information about the velocity or displacement of an object after a given period of time. In this manner, Newton's laws serve as a useful model for analyzing motion and making predictions about the final state of an object's motion. In this unit, an entirely different model will be used to analyze the motion of objects. Motion will be approached from the perspective of work and energy. The affect that work has upon the energy of an object (or system of objects) will be investigated; the resulting velocity and/or height of the object can then be predicted from energy information. In order to understand this work-energy approach to the analysis of motion, it is important to first have a solid understanding of a few basic terms. Thus, the sections of this unit will focus on the definitions and meanings of such terms as work, mechanical energy, potential energy, kinetic energy, and power.

3. When a force acts upon an object to cause a displacement of the object, it is said that work was done upon the object. There are three key ingredients to work - force, displacement, and cause. In order for a force to qualify as having done work on an object, there must be a displacementand the force must cause the displacement. There are several good examples of work which can be observed in everyday life - a horse pulling a plow through the field, a father pushing a grocery cart down the aisle of a grocery store, a freshman lifting a backpack full of books upon her shoulder, a weightlifter lifting a barbell above his head, an Olympian launching the shot-put, etc. In each case described here there is a force exerted upon an object to cause that object to be displaced. • Mathematically, work can be expressed by the following equation. • where F is the force, d is the displacement, and the angle (theta) is defined as the angle between the force and the displacement vector. Perhaps the most difficult aspect of the above equation is the angle "theta." The angle is not just any 'ole angle, but rather a very specific angle. The angle measure is defined as the angle between the force and the displacement. To gather an idea of its meaning, consider the following three scenarios.

4. Scenario A: A force acts rightward upon an object as it is displaced rightward. In such an instance, the force vector and the displacement vector are in the same direction. Thus, the angle between F and d is 0 degrees. • Scenario B: A force acts leftward upon an object which is displaced rightward. In such an instance, the force vector and the displacement vector are in the opposite direction. Thus, the angle between F and d is 180 degrees. • Scenario C: A force acts upward on an object as it is displaced rightward. In such an instance, the force vector and the displacement vector are at right angles to each other. Thus, the angle between F and d is 90 degrees.

5. Units of Work • Whenever a new quantity is introduced in physics, the standard metric units associated with that quantity are discussed. In the case of work (and also energy), the standard metric unit is the Joule (abbreviated J). One Joule is equivalent to one Newton of force causing a displacement of one meter. In other words, • The Joule is the unit of work. • 1 Joule = 1 Newton * 1 meter • 1 J = 1 N * m • In fact, any unit of force times any unit of displacement is equivalent to a unit of work. Some nonstandard units for work are shown below. Notice that when analyzed, each set of units is equivalent to a force unit times a displacement unit.  • In summary, work is done when a force acts upon an object to cause a displacement. Three quantities must be known in order to calculate the amount of work. Those three quantities are force, displacement and the angle between the force and the displacement.

6. 9.2 Power • Power • The quantity work has to do with a force causing a displacement. Work has nothing to do with the amount of time that this force acts to cause the displacement. Sometimes, the work is done very quickly and other times the work is done rather slowly. For example, a rock climber takes an abnormally long time to elevate her body up a few meters along the side of a cliff. On the other hand, a trail hiker (who selects the easier path up the mountain) might elevate her body a few meters in a short amount of time. The two people might do the same amount of work, yet the hiker does the work in considerably less time than the rock climber. The quantity which has to do with the rate at which a certain amount of work is done is known as the power. The hiker has a greater power rating than the rock climber. • Power is the rate at which work is done. It is the work/time ratio. Mathematically, it is computed using the following equation.

7. The standard metric unit of power is the Watt. As is implied by the equation for power, a unit of power is equivalent to a unit of work divided by a unit of time. Thus, a Watt is equivalent to a Joule/second. For historical reasons, the horsepower is occasionally used to describe the power delivered by a machine. One horsepower is equivalent to approximately 750 Watts.  Most machines are designed and built to do work on objects. All machines are typically described by a power rating. The power rating indicates the rate at which that machine can do work upon other objects. Thus, the power of a machine is the work/time ratio for that particular machine. A car engine is an example of a machine which is given a power rating. The power rating relates to how rapidly the car can accelerate the car. Suppose that a 40-horsepower engine could accelerate the car from 0 mi/hr to 60 mi/hr in 16 seconds. If this were the case, then a car with four times the horsepower could do the same amount of work in one-fourth the time. That is, a 160-horsepower engine could accelerate the same car from 0 mi/hr to 60 mi/hr in 4 seconds. The point is that for the same amount of work, power and time are inversely proportional. The power equation suggests that a more powerful engine can do the same amount of work in less time.

8. A person is also a machine which has a power rating. Some people are more power-full than others. That is, some people are capable of doing the same amount of work in less time or more work in the same amount of time. A common physics lab involves quickly climbing a flight of stairs and using mass, height and time information to determine a student's personal power. Despite the diagonal motion along the staircase, it is often assumed that the horizontal motion is constant and all the force from the steps are used to elevate the student upward at a constant speed. Thus, the weight of the student is equal to the force which does the work on the student and the height of the staircase is the upward displacement. Suppose that Ben Pumpiniron elevates his 80-kg body up the 2.0 meter stairwell in 1.8 seconds. If this were the case, then we could calculate Ben's power rating. It can be assumed that Ben must apply a 800-Newton downward force upon the stairs to elevate his body. By so doing, the stairs would push upward on Ben's body with just enough force to lift his body up the stairs. It can also be assumed that the angle between the force of the stairs on Ben and Ben's displacement is 0 degrees. With these two approximations, Ben's power rating could be determined as shown below.

9. Ben's power rating is 871 Watts. He is quite a horse. • The expression for power is work/time.And since the expression for work is force*displacement, the expression for power can be rewritten as (force*displacement)/time. Since the expression for velocity is displacement/time, the expression for power can be rewritten once more as force*velocity. This is shown below.

10. This new equation for power reveals that a powerful machine is both strong (big force) and fast (big velocity). A powerful car engine is strong and fast. A powerful piece of farm equipment is strong and fast. A powerful weightlifter is strong and fast. A powerful linemen on a football team is strong and fast. A machine which is strong enough to apply a big force to cause a displacement in a small mount of time (i.e., a big velocity) is a powerful machine. Mechanical Energy Previously, it was said that work is done upon an object whenever a force acts upon it to cause it to be displaced. Work is a force acting upon an object to cause a displacement. In all instances in which work is done, there is an object which supplies the force in order to do the work. If a World Civilization book is lifted to the top shelf of a student locker, then the student supplies the force to do the work on the book. If a plow is displaced across a field, then some form of farm equipment (usually a tractor or a horse) supplies the force to do the work on the plow. If a pitcher winds up and accelerates a baseball towards home plate, then the pitcher supplies

11. the force to do the work on the baseball. If a roller coaster car is displaced from ground level to the top of the first drop of a roller coaster ride, then a chain driven by a motor supplies the force to do the work on the car. If a barbell is displaced from ground level to a height above a weightlifter's head, then the weightlifter is supplying a force to do work on the barbell. In all instances, an object which possesses some form of energy supplies the force to do the work. In the instances described here, the objects doing the work (a student, a tractor, a pitcher, a motor/chain) possess chemical potential energy stored in food or fuel which is transformed into work. In the process of doing work, the object which is doing the work exchanges energy with the object upon which the work is done. When the work is done upon the object, that object gains energy. The energy acquired by the objects upon which work is done is known as mechanical energy. • Mechanical energy is the energy which is possessed by an object due to its motion or due to its position. Mechanical energy can be either kinetic energy (energy of motion) or potential energy (stored energy of position). Objects have mechanical energy if they are in motion and/or if they are at some position relative to a zero potential energy position (for example, a brick held at a vertical position above the ground or zero height position). A moving car possesses mechanical energy due to its motion (kinetic energy).

12. 9.4 Potential Energy • A moving baseball possesses mechanical energy due to both its high speed (kinetic energy) and its vertical position above the ground (gravitational potential energy). A World Civilization book at rest on the top shelf of a locker possesses mechanical energy due to its vertical position above the ground (gravitational potential energy). A barbell lifted high above a weightlifter's head possesses mechanical energy due to its vertical position above the ground (gravitational potential energy). A drawn bow possesses mechanical energy due to its stretched position (elastic potential energy). • An object can store energy as the result of its position. For example, the heavy heavy ball of a demolition machine is storing energy when it is held at an elevated position. This stored energy of position is referred to as potential energy. Similarly, a drawn bow is able to store energy as the result of its position. When assuming its usual position (i.e., when not drawn), there is no energy stored in the bow. Yet when its position is altered from its usual equilibrium position, the bow is able to store energy by virtue of its position. This stored energy of position is referred to as potential energy. Potential energy is the stored energy of position possessed by an object.

13. Elastic Potential Energy • The first form of potential energy which we will discuss is elastic potential energy. Elastic potential energy is the energy stored in elastic materials as the result of their stretching or compressing. Elastic potential energy can be stored in rubber bands, bungee chords, trampolines, springs, an arrow drawn into a bow, etc. The amount of elastic potential energy stored in such a device is related to the amount of stretch of the device - the more stretch, the more stored energy. • Springs are a special instance of a device which can store elastic potential energy due to either compression or stretching. A force is required to compress a spring; the more compression there is, the more force which is required to compress it further. For certain springs, the amount of force is directly proportional to the amount of stretch or compression (x); the constant of proportionality is known as the spring constant (k).

14. Such springs are said to follow Hooke's Law. If a spring is not stretched or compressed, then there is no elastic potential energy stored in it. The spring is said to be at its equilibrium position. The equilibrium position is the position that the spring naturally assumes when there is no force applied to it. In terms of potential energy, the equilibrium position could be called the zero-potential energy position. There is a special equation for springs which relates the amount of elastic potential energy to the amount of stretch (or compression) and the spring constant. The equation is • To summarize, potential energy is the energy which is stored in an object due to its position relative to some zero position. An object possesses elastic potential energy if it is at a position on an elastic medium other than the equilibrium position.

15. Chemical Potential Energy • The chemical energy in fuels is also potential energy. Any substance that can do work through chemical reactions possesses chemical energy. Potential energy is found in fossil fuels, electric batteries, and the food we eat. • Gravitational Potential Energy • The first form of potential energy which we will discuss is gravitational potential energy. Gravitational potential energy is the energy stored in an object as the result of its vertical position or height. The energy is stored as the result of the gravitational attraction of the Earth for the object. The gravitational potential energy of the massive ball of a demolition machine is dependent on two variables - the mass of the ball and the height to which it is raised. There is a direct relation between gravitational potential energy and the mass of an object. More massive objects have greater gravitational potential energy. There is also a direct relation between gravitational potential energy and the height of an object. The higher that an object is elevated, the greater the gravitational potential energy. These relationships are expressed by the following equation:

16. PEgrav = mass * g * height • PEgrav = m * g * h • In the above equation, m represents the mass of the object, h represents the height of the object and g represents the acceleration of gravity (9.8 m/s/s on Earth). • To determine the gravitational potential energy of an object, a zero height positionmust first be arbitrarily assigned. Typically, the ground is considered to be a position of zero height. But this is merely an arbitrarily assigned position which most people agree upon. Since many of our labs are done on tabletops, it is often customary to assign the tabletop to be the zero height position. Again this is merely arbitrary. If the tabletop is the zero position, then the potential energy of an object is based upon its height relative to the tabletop. For example, a pendulum bob swinging to and from above the table top has a potential energy which can be measured based on its height above the tabletop. By measuring the mass of the bob and the height of the bob above the tabletop, the potential energy of the bob can be determined.

17. Since the gravitational potential energy of an object is directly proportional to its height above the zero position, a doubling of the height will result in a doubling of the gravitational potential energy. A tripling of the height will result in a tripling of the gravitational potential energy. • Use this principle to determine the blanks in the following diagram. Knowing that the potential energy at the top of the tall platform is 50 J, what is the potential energy at the other positions shown on the stair steps and the incline?

18. 9.5 Kinetic Energy • Kinetic energyis the energy of motion. An object which has motion - whether it be vertical or horizontal motion - has kinetic energy. There are many forms of kinetic energy - vibrational (the energy due to vibrational motion), rotational (the energy due to rotational motion), and translational (the energy due to motion from one location to another). To keep matters simple, we will focus upon translational kinetic energy. The amount of translational kinetic energy (from here on, the phrase kinetic energy will refer to translational kinetic energy) which an object has depends upon two variables: the mass (m) of the object and the speed (v) of the object. The following equation is used to represent the kinetic energy (KE) of an object. • where m = mass of object • v = speed of object

19. This equation reveals that the kinetic energy of an object is directly proportional to the square of its speed. That means that for a twofold increase in speed, the kinetic energy will increase by a factor of four. For a threefold increase in speed, the kinetic energy will increase by a factor of nine. And for a fourfold increase in speed, the kinetic energy will increase by a factor of sixteen. The kinetic energy is dependent upon the square of the speed. As it is often said, an equation is not merely a recipe for algebraic problem-solving, but also a guide to thinking about the relationship between quantities. • Kinetic energy is a scalar quantity; it does not have a direction. Unlike velocity, acceleration, force, and momentum, the kinetic energy of an object is completely described by magnitude alone. Like work and potential energy, the standard metric unit of measurement for kinetic energy is the Joule. As might be implied by the above equation, 1 Joule is equivalent to 1 kg*(m/s)^2.

20. 9.6 Work-Energy Theorem • Internal vs. External Forces • There are a variety of ways to categorize all the types of forces. Earlier it was mentioned that all the types of forces can be categorized as contact forces or as action-at-a-distance forces. Whether a force was categorized as an action-at-a-distance force was dependent upon whether or not that type of force could exist even when the objects were not physically touching. The force of gravity, electrical forces, and magnetic forces were examples of forces which could exist between two objects even when they are not physically touching. In this lesson, we will learn how to categorize forces based upon whether or not their presence is capable of changing an object's total mechanical energy. We will learn that there are certain types of forces, which when present and when involved in doing work on objects will change the total mechanical energy of the object. And there are other types of forces which can never change the total mechanical energy of an object, but rather can only transform the energy of an object from potential energy to kinetic energy (or vice versa). The two categories of forces are referred to as internal forces and external forces.

21. Forces can be categorized as internal forces or external forces. There are many sophisticated and worthy ways of explaining and distinguishing between internal and external forces. Many of these ways are commonly discussed at great length in physics textbooks. For our purposes, we will simply say that external forces include the applied force, normal force, tension force, friction force, and air resistance force. And for our purposes, the internal forces include the gravity forces, magnetic force, electrical force, and spring force. • The importance of categorizing a force as being either internal or external is related to the ability of that type of force to change an object's total mechanical energy when it does work upon an object.

22. When net work is done upon an object by an external force, the total mechanical energy (KE + PE) of that object is changed. If the work is positive work, then the object will gain energy. If the work is negative work, then the object will lose energy. The gain or loss in energy can be in the form of potential energy, kinetic energy, or both. Under such circumstances, the work which is done will be equal to the change in mechanical energy of the object. Because external forces are capable of changing the total mechanical energy of an object, they are sometimes referred to as non-conservative forces. • When the only type of force doing net work upon an object is an internal force (for example, gravitational and spring forces), the total mechanical energy (KE + PE) of that object remains constant. In such cases, the object's energy changes form. For example, as an object is "forced" from a high elevation to a lower elevation by gravity, some of the potential energy of that object is transformed into kinetic energy. Yet, the sum of the kinetic and potential energies remain constant. This is referred to as energy conservation and will be discussed in detail later in this lesson.

23. 9.7 Conservation of Energy • When the only forces doing work are internal forces, energy changes forms - from kinetic to potential (or vice versa); yet the total amount of mechanical is conserved. Because internal forces are capable of changing the form of energy without changing the total amount of mechanical energy, they are sometimes referred to as conservative forces. • The work-energy theorem describes the relationship between work and energy. The work-energy theorem states that whenever work is done, energy changes. We abbreviate “change in” with the delta symbol, Δ, and say • Work = ΔKE • The study of the various forms of energy and the transformations from one form to another is the law of conservation of energy. The law of conservation of energy states that energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes.

24. 9.8 Machines • There are six types of simple machine: • A machine is a tool used to make work easier. Simple machines are simple tools used to make work easier. Compound machines have two or more simple machines working together to make work easier.

25. Inclined Plane: A plane is a flat surface. For example, a smooth board is a plane. Now, if the plane is lying flat on the ground, it isn't likely to help you do work. However, when that plane is inclined, or slanted, it can help you move objects across distances. And, that's work! A common inclined plane is a ramp. Lifting a heavy box onto a loading dock is much easier if you slide the box up a ramp--a simple machine. • Wedge: Instead of using the smooth side of the inclined plane, you can also use the pointed edges to do other kinds of work. For example, you can use the edge to push things apart. Then, the inclined plane is a wedge. So, a wedge is actually a kind of inclined plane. An axeblade is a wedge. Think of the edge of the blade. It's the edge of a smooth slanted surface. That's a wedge! • Screw: Now, take an inclined plane and wrap it around a cylinder. Its sharp edge becomes another simple tool: the screw. Put a metal screw beside a ramp and it's kind of hard to see the similarities, but the screw is actually just another kind of inclined plane. How does the screw help you do work? Every turn of a metal screw helps you move a piece of metal through a wooden space. And, that's how we build things!

26. Lever: Try pulling a really stubborn weed out of the ground. You know, a deep, persistent weed that seems to have taken over your flowerbed. Using just your bare hands, it might be difficult or even painful. With a tool, like a hand shovel, however, you should win the battle. Any tool that pries something loose is a lever. A lever is an arm that "pivots" (or turns) against a "fulcrum" (or point). Think of the claw end of a hammer that you use to pry nails loose. It's a lever. It's a curved arm that rests against a point on a surface. As you rotate the curved arm, it pries the nail loose from the surface. And that's hard work! • Wheel and Axle: The rotation of the lever against a point pries objects loose. That rotation motion can also do other kinds of work. Another kind of lever, the wheel and axle, moves objects across distances. The wheel, the round end, turns the axle, the cylindrical post, causing movement. On a wagon, for example, the bucket rests on top of the axle. As the wheel rotates the axle, the wagon moves. Now, place your pet dog in the bucket, and you can easily move him around the yard. On a truck, for example, the cargo hold rests on top of several axles. As the wheels rotate the axles, the truck moves.

27. Pulley: Instead of an axle, the wheel could also rotate a rope or cord. This variation of the wheel and axle is the pulley. In a pulley, a cord wraps around a wheel. As the wheel rotates, the cord moves in either direction. Now, attach a hook to the cord, and you can use the wheel's rotation to raise and lower objects. On a flagpole, for example, a rope is attached to a pulley. On the rope, there are usually two hooks. The cord rotates around the pulley and lowers the hooks where you can attach the flag. Then, rotate the cord and the flag raises high on the pole. • A machine transforms energy from one place to another or transforms it from one form into another. • In this section we study two specific simple machines, the lever and the pulley. Below are the three types of lever. We will focus on the first class lever.

28. First Class Lever: If we push down on effort arm, the load is lifted up. We do work on the effort arm, and the load arm does work on the load. • If the heat from friction is small enough to neglect, the work input will be equal to the work output. • Work input = Work output • Since work equals force times distance, we can say • (Force x distance) input = (Force x distance) output

29. Moving the fulcrum, allows us to input a small force through a large distance, and lift a large load through a small distance. However, no machine can multiply work or energy! • The ratio of output force to input force for a machine is called mechanical advantage. The MA (mechanical advantage) can be found by taking the ratio of the output force to the input force. On page 155 of our book, the girl pushes down with a force of 10N through a distance of 1m. The rock, which weighs 80 N is lifted a distance of (1/8)m. The MA (mechanical advantage) is (80N)/(10N), or 8. We can also determine the MA by the ratio of the input distance to output distance. • Pulley: A major purpose of a pulley is to change the direction of the input force. You can pull down one a pulley rope, and the rope will lift the object upward.

30. Pulley’s can be used several ways. • A single pulley changes the direction of the lifting force. For  example, if you are lifting a heavy object with a single pulley anchored to the ceiling, you can pull down on the rope to lift the object instead  of pushing up. The same amount of effort is needed as without a pulley, but it feels easier because you are pulling down. • A fixed pulley is the only pulley that when used individually, uses more effort than the  load to lift the load from the ground. • The fixed pulley when attached to an unmovable object e.g. a ceiling or wall, acts as a first class lever with the fulcrum being located at the axis but with a minor change, the bar becomes a rope.  • The advantage of the fixed pulley is that you do not have to pull or push  the pulley up and down. • The disadvantage is that you have to apply more effort than the load you lift (friction).

31. A movable  pulley  is a pulley that moves with the load. • The movable pulley allows the effort to be less than the weight of the load. The movable pulley also acts as a second class lever. The load is between the fulcrum and the effort. • The main advantage of a movable pulley is that you use less effort to pull the load. • The main disadvantage of a movable pulley is that you have to pull or push the pulley up or down. • If you add a second  pulley, the amount of effort to lift the heavy object seems much less . • For example, to lift a box weighing 150 N, one would need to exert 150 N of force without the help of pulleys. • However, by using just two pulleys, the person would only need to use 75 N of force.

32. A combined pulley makes life easier as the effort needed to lift the load is less than  half the weight of the load. • The main advantage of this pulley is that the amount of effort is less than half of the load. • The main disadvantage is it travels a very long distance.

33. 9.9 Efficiency • A major factor in the usefulness of a machine is its efficiency. • A machine converts the force provided from an input energy into motion that changes the magnitude or direction of that force. This motion against a resistive force is the work done by the machine. According to the Law of Conservation of Energy, the total input energy must equal the total output energy. However, some of the output energy does not contribute to the output work and is lost to such things as friction and heat. • The efficiency of a machine is the ratio of the input energy to the useful output work. • Questions you may have include: • What is the work done by a machine? • What role does the Conservation of Energy play in machines? • What is the efficiency of a machine?

34. The efficiency of a machine is the output work or energy divided by the input work or energy. • Efficiency = Wo/Wi • As an illustration of the losses in all machines, a simple lever loses about 2% of the input energy to internal friction at its fulcrum, such that its efficiency is 98%. If 100 joules of work is input, 98 joules of work is the output. • On the other hand, the efficiency of an automobile is only around 15%. About 75% of the energy is lost through wasted heat from the engine and another 10% is lost due to internal friction, including losses from tire friction. • The usefulness of a machine is determined by its efficiency. A machine converts the force provided from an input energy into output work. The Law of Conservation of Energy requires that the total input energy must equal the total output energy. Some output energy does not contribute to the output work and is lost to friction or heat. The efficiency of a machine is the ratio of the input energy to the useful output work (output divided by input).

35. In any machine, some energy is transformed into atomic or molecular kinetic energy --- making the machine warmer. We say this wasted energy is dissipated as heat. • The efficiency of a machine is the ratio of useful energy output to total energy input, or the percentage of work input that is converted to work output. useful work output • Efficiency = total work input • Efficiency can also be expressed as the ratio of actual mechanical advantage to theoretical mechanical advantage. actual mechanical advantage • Efficiency = theoretical mechanical advantage

36. MECHANICAL ADVANTAGE OF THE INCLINED PLANE • Complex Machines: • A car jack is a simple example of a complex machine that increases the applied force. • The upward force exerted by the jack is greater than the downward force you exert on the handle. • However, the distance you push the handle down is greater than the distance the car is pushed upward. • Because work is the product of force and distance, the work done by the jack is equal to the work you do on the jack. • The jack increases the applied force, but it doesn’t increase the work done.

37. 9.10 Energy for Life • As physicists learned in the nineteenth century, transforming 100% of thermal energy into mechanical energy IS NOT POSSIBLE. Some heat must flow from the engine. Friction adds more to the energy loss. Even the best designed gasoline-powered automobile engines are unlikely to be more than 35% efficient! • On top of these contributors to inefficiency, the fuel does not burn completely. A certain amount of it goes unused. We can look at inefficiency in this way: In any transformation there is a dilution of the amount of useful energy. Useful energy ultimately becomes ther­mal energy, Energy is not destroyed, it is simply degraded. Through heat transfer, thermal energy is the graveyard of useful energy. • Every living cell in every organism is a machine. Like any machine, liv­ing cells need an energy supply. Most living organisms on this planet feed on various hydrocarbon compounds that release energy when they react with oxygen. There is more energy stored in gasoline than in the products of its combustion. ~ There is more energy stored in the molecules in food than there is in the reaction products after the food is metabolized. This energy difference sustains life.

38. 9.11 Sources of Energy • The Sun is the source of practically all our energy on Earth! • Solar Power : The sun is the single most significant source of energy to the planet Earth, and any energy that it provides which isn't used to help plants grow or to heat the Earth is basically lost. Solar power can be used with solarvoltaic power cells to generate electricity. Certain regions of the world receive more direct sunlight than others, so solar energy is not uniformly practical for all areas. • Hydropower: The use of hydropower involves using the kinetic motion in water as it flows downstream, part of the normal water cycle of the Earth, to generate other forms of energy, most notably electricity. Dams use this property as a means of generating electricity. This form of hydropower is called hydroelectricity. Water wheels were an ancient technology which also made use of this concept to generate kinetic energy to run equipment, such as a grain mill.

39. Wind: Modern windmills can transfer the kinetic energy of the air flowing through them into other forms of energy, such as electricity. There are some environmental concerns with using wind energy, because the windmills often injure birds who may be passing through the region. • Nuclear: Certain elements are able to undergo powerful nuclear reactions, releasing energy which can be harnessed and transformed into electricity. Nuclear power is controversial because the material used can be dangerous and resultant waste products are toxic. Accidents that take place at nuclear power plants, such as Chernobyl, are devastating to local populations and environments. Still, many nations have adopted nuclear power as a significant energy alternative. • Biomass: Biomass is not really a separate type of energy, so much as a specific type of fuel. It is generated from organic waste products, such as cornhusks, sewage, and grass clippings. This material contains residual energy, which can be released by burning it in biomass power plants. Since these waste products always exist, it is considered a renewable resource.

40. Geothermal: The Earth generates a lot of heat while going about its normal business, in the form of subterranean steam and magma among others. The energy generated within the Earth's crust can be harnessed and transformed into other forms of energy, such as electricity. • Fuel Cells: Fuel cells are an important enabling technology for the hydrogen economy and have the potential to revolutionize the way we power our nation, offering cleaner, more-efficient alternatives to the combustion of gasoline and other fossil fuels. Fuel cells have the potential to replace the internal-combustion engine in vehicles and provide power in stationary and portable power applications because they are energy-efficient, clean, and fuel-flexible. Hydrogen or any hydrogen-rich fuel can be used by this emerging technology. • THE END!!!!! AT LAST!!!!!!!!!!