LESSON 8: ENERGY, WORK, POWER, AND MACHINES "

S-l FULL OF ENERGY

Content on Energy, Work, Power, and Machines

Going the Distance to Do Work

Have you ever heard someone describe another person as "full of energy"? Or per-haps you've even described yourself as having no energy on a particular day.Actually, all persons, places, and things have energy. The sun can give energy aswell as the food we eat. Energy is only evident to the observer as work is beingdone. In fact, most people define energy as the ability to do work.

Work itself is the transfer of energy. Think about step aerobics. Do you thinkdoing step aerobics is a form of work? Let's take a look and find out. In order forwork to be done, a force must be present and an object must be moved a certaindistance by that force. The food you eat provides the energy for the aerobics. Theparticipant in the aerobics class moves his or her body weight up and down on astep (about 8 inches is standard). Moving your body weight requires a force (new-tons is the unit), and the up-and-down motion is your distance (usually figured inmeters) upon which the force is applied. Yes, you have done work in the case ofstep aerobics.

Let's go to the free weight room and see if work isbeing done. First, we see a woman doing bicep curlswith the free weights. She is lifting 35 pounds in eachhand. ABshe curls the bar up and down, she is doingwork (supplying force to move the weight a certaindistance). Nearby we see a man holding a 200-poundweight bar over his head. You see him grimacing fromthe load. Is he doing work? Although it sure looks likework, he is not doing any. This is because he is notmoving the object through a distance. (Better not tellhim that at this particular time!)

More Power to You

The formula for work is work =force x distance. Force is calculated in units ofnewtons, and distance is calculated in meters. Thus work is a newton-meter, alsocalled a joule. You'll notice that the formula for work says nothing about how longit takes you to perform that task. If I told you that you did the same amount of

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work running up the stadium steps as you did walking slowly up the steps, you'dprobably ask me if I had run those steps lately.

The concept we are talking about here is power. Power is the rate at whichwork is done. Power equals the amount of work done divided by the amount oftime during which the work is done. In formula format, it looks like this: P = f xdlt or P = Wit. The units for power are joules per second (also called a watt). Theunit called watt is a metric unit named after James Watt (the eighteenth-centu-ry developer of the steam engine). When James Watt was trying to sell his steamengine, he had to tell his customers how many horses his engine could replace. Todo this, he had to find out how much work a good horse could do. He found thaton an average, a horse could lift 550 pounds 1 foot in 1 second, and this becameknown as horsepower. One horsepower is the same as 0.75 kilowatts (a kilowattis 1000 watts). So an engine that is rated as a 134-horsepower engine is really a100-kW engine. According to the definition for power, engines that are high inhorsepower can do work rapidly. This does not mean that these engines do morework or necessarily go faster; they do the work in less time. This means theyaccelerate faster.

Energy Stores and Conversions

With all this talk about energy, do you wonder if it can be stored? We store itevery day. As we eat, our cells stockpile energy in our muscles and in our fat cells.Potential energy is the term used to describe energy that is stored and held inreadiness. It has the potential for doing work. Would you be willing to walk undera piano suspended 20 meters above the ground by a single rope? I doubt that Iwould take the chance because the piano is loaded with potential energy. With aslight failure of the rope, that energy could be changed into work and move down-ward. The piano is an example of an object with gravitational potential energy.The amount of gravitational potential energy it possesses is equal to the workdone against gravity in lifting it to that position. The formula for gravitationalpotential energy is gravitational potential energy = (mass) (acceleration due togravity) (height) or PE =m x g x h. Mass is calculated in Kilograms, accelerationdue to gravity is always 110 m/sec2, and the height is calculated in meters. Lookback at the piano example. If the piano had a mass of 2300 kg and was suspend-ed 20 meters above the ground, what would be its potential energy? PE =2300 kgx 10 m/sec2 x 20 m = 460,000 joules. Once again, the product is a newton-meter,which is a joule. .

Once the potential energy is set into motion, it is called kinetic energy.Kinetic energy is defined as the energy of motion. The kinetic energy of an objectdepends on the mass of the object as well as its speed. It is equal to half the massmultiplied by the square of the speed. The formula is written kinetic energy =1/2mass x velocity2 or KE = 1/2 mv2. If and when the rope on the suspended pianobreaks, the potential energy becomes kinetic energy as work is done. To use theKE formula, you must know the velocity with which an object is traveling at thattime. You can find the velocity of an object by using the formula Vf= Vi + (a) (t),or final velocity equals the initial velocity plus the acceleration multiplied by thetime. The final answer for kinetic energy is also given in joules.

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8-1 Full of Energy 85

Lightening the Load with Machines

Some of the objects that perform work are machines. Machines are devices formultiplying forces or changing the direction of forces. There. are six simplemachines that do this. Each of the six types makes work easier for the persondoing the work. If you don't believe it, think of unscrewing a bolt without a screw-driver or lifting up a car without a jack. The six families of simple machines arelevers, pulleys, wheels and. axles, inclined planes, screws, and wedges.

A lever helps you moveobjects by increasing the forceyou exert. A lever is a long, rigidbar with a support that allowsthe bar to pivot. The point of rota-tion is called the fulcrum. Thinkof a seesaw. This is a lever, andthe balance point is the fulcrum.Crowbars and screwdrivers usedto pry the top off objects are alsolevers. Pulleys are machines thatconsist of a rope passing over agrooved wheel. Think about anexample of a fixed pulley. Thiswould be like an old water well. The rope is connected to the bucket and passesover the pulley. You pull down on your end of the rope and the bucket of watercomes up toward you. A fixed pulley does not decrease the force needed to do thejob, but it changes the direction and makes work easier. When several pulleys areused together to make work even easier, this system is called a block and tackle.This combines both fixed and movable pulleys. The wheel and axle is a simplemachine consisting of a wheel and a shaft. As you spin the wheel, the axle turns.Doorknobs and eggbeaters are examples of this.

An inclined plane is a slanting surface that allows you to apply less force inlifting an object to the desired height. Just think of a moving van with pull-downramps at the back of the truck. These ramps are inclined planes that allow themovers to deliver furniture into the van. They increase the distance the moverstravel but decrease the amount of force they must supply to lift each object. Thewedge is a type of inclined plane with either one or two sloping sides. The pointof a needle or the blade of a knife are examples. A screw is an inclined plane in aspiral form. It makes work easier by pressing against the material around it asit moves into the wood. The ridge that spirals around the screw is called itsthread, and the pitch is the number of threads in a given length. When you turna screw, the direction of force is changed and multiplied. The force you need toapply decreases as the threads get closer. You have to make a lot of turns withyour hands, but the work is much easier than if you did not have the screw.

When evaluating a piece of machinery, the concept of mechanical advantage(MA) is often used. MA is the value that tells the number of times a machineincreases the applied force. For example, if the MA was 4 when you used a screw-driver to pry the lid off a paint can, that meant that the screwdriver multiplied

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your force by 4. Each type of simple machine has a formula for figuring the MAof that machine. This will be dealt with in Lesson 8-4, Machines Made Simple.

Fact or Friction

Simple machines combined to form more complex pieces of equipment are calledcompound machines. The value of a compound machine is often based on its effi-ciency.The efficiency of a machine is the amount of useful work obtained from themachine as compared to the work put into the machine. It is important to notethat no machine has 100%efficiency.Why? Friction is the culprit. Remember, fric-tion is the force that opposes motion. The formula for determining the efficiencyof a machine is as follows: efficiency =useful work/work put in x 100%. For exam-ple, if you put 200 joules of work into a machine and the machine did 30 joules ofuseful work, its efficiency is 30/200 x 100% = 15%. This would mean that 85% ofthe work you put into the machine was wasted, probably in overcoming friction.And many complex machines have very low efficiency ratings. Cars have a lowefficiency rating (below 35%), not because of bad design but because some of theenergy converted to heat goes into the cooling system and is wasted through theradiator tq,.the air. Some energy will go out the exhaust, and much is wasted infriction of the moving parts. Substances (such as grease and oil) that reduce fric-tion op moving parts are important to the efficiency rating of the car. When theseare utilized, friction is reduced and efficiency is increased.

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ENERGY UNSCRAMBLE-VOCABULARY ACTIVITY ON

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Directions

After reading Full of Energy, unscramble the words below to complete the statements.

1. (oewrp) This is described as work divided by the time it takes to

perform that work.

2. (nnweot) This is also known as kgmlsec2. It is the unit used to

measure force.

3. (inciket) This is the energy of motion.

4. (ipcht) When this is closer together on a screw, the screw is eas-ier to turn.

5. (rontifci) This is one of the major reasons that machines never

can reach a 100% efficiency rating.

6. (uojel) This is the unit used to designate work done.

7. (eitraloenacc) This is the advantage of a high-horsepower auto-

mobile over a lower-horsepower automobile.

8. (yrgnee) This is the ability to do work.

9. (res soh) James Watt compared his steam engine to these when

he was trying to market the new engine.

10. (iehhgt) To determine the potential energy of an object, you

must know the acceleration due to gravity, mass, and the

11._ (yveticol) To compute a kinetic energy, you need mass and

12. (ihmnceas) These are devices for multiplying force or changing

the direction of a force when moving an object.

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VOCABUlARY ACTIVITYONFull. OFENERGY(continued)

13. (elerv) These simple machines have a fulcrum on which a long,rigid bar rests and pivots freely.

14. (fnike) This is an example of a wedge.

15. (oerhrwsope) This unit was selected by Watt to describe his

steam engine, and it is still used in car descriptions.

16. (ientdrcio) A fixed pulley does not increase the force needed to

do the job, but it does alter the in which the job is being done.

17. (eeswas) This is an example of a type oflever.

18. (orkw) This is determined by multiplying the force times the

distance in which an object is moved.

19. (islmoagkr) Mass is calculated in units of

when determining potential energy.

20. (iflan elvcotiy) This is equal to the initial velocity plus acceler-

ation multiplied by time.

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