Work and Energy : Forms and Changes

Work and Work and Energy: Energy: Forms and Forms and ChangesChanges

What is Work?What is Work?

Remember that a force is a push or a pull. Remember that a force is a push or a pull. Work requires both force and motionWork requires both force and motion– Work is force applied through a distanceis force applied through a distance– If you push against the desk and nothing If you push against the desk and nothing

moves, then you haven't done any work moves, then you haven't done any work

WorkWork

There are two conditions that have to be There are two conditions that have to be satisfied for work to be done on an object satisfied for work to be done on an object – 1. The applied force must make the object move1. The applied force must make the object move– 2. The movement must be in the same direction 2. The movement must be in the same direction

as the applied forceas the applied force Work requires both force AND motionWork requires both force AND motion

Calculating WorkCalculating Work The amount of work done depends on The amount of work done depends on

the amount of force exerted and the the amount of force exerted and the distance over which the force is applied distance over which the force is applied

Work (N-m or Work (N-m or JouleJoule) = F (N) x d (m) ) = F (N) x d (m) where d is distance moved in the where d is distance moved in the direction of the forcedirection of the force

One newton-meter is equal to one joule One newton-meter is equal to one joule so the unit of work is a jouleso the unit of work is a joule

Weight is a Force!

Remember that weight is a force caused by Remember that weight is a force caused by your mass and gravityyour mass and gravity– FFgravitygravity = mg = mg

– To lift something you have to exert a force to To lift something you have to exert a force to overcome this force of gravity, so you are doing overcome this force of gravity, so you are doing work on that objectwork on that object

When is Work Done?When is Work Done?

Give a book a push and it slides along a table Give a book a push and it slides along a table for a distance of 1 m before it stopsfor a distance of 1 m before it stops– You did work on the book only while your hand You did work on the book only while your hand

was in contact with itwas in contact with it Lift books and your arms apply a force Lift books and your arms apply a force

upward to move them, and because the force upward to move them, and because the force and distance are in the same direction, your and distance are in the same direction, your arms have done work on the booksarms have done work on the books

Carry books while walking, and your arms Carry books while walking, and your arms are not doing work. Why not?are not doing work. Why not?

1 m

60 N

W = 60 N x 1 m = 60 J (N-m)

W = 20 N x 3 m = 60 J (N-m)

3 m

20 N

Examples of WorkExamples of Work

PowerPower Running up stairs is harder than walking Running up stairs is harder than walking

up stairs and lifting books quickly is up stairs and lifting books quickly is harder than slowlyharder than slowly– Why? They both do the same amount of Why? They both do the same amount of

workwork– Running does the same work more quicklyRunning does the same work more quickly

Power is the rate at which work is done Power is the rate at which work is done and energy is converted and energy is converted

Power (J/sec or Watt)= Power (J/sec or Watt)= Work (J)Work (J) Time (sec) Time (sec)

The unit of power is Joules/sec, called a The unit of power is Joules/sec, called a Watt.Watt.

Check for UnderstandingCheck for UnderstandingWhat’s work?What’s work?

A scientist delivers a speech to an A scientist delivers a speech to an audience of his peers audience of his peers

A body builder lifts 350 pounds above his A body builder lifts 350 pounds above his headhead

A mother carries her baby from room to A mother carries her baby from room to

roomroom

A father pushes a baby in a carriageA father pushes a baby in a carriage A woman carries a 20 kg grocery bag to A woman carries a 20 kg grocery bag to

her car?her car?

Check for UnderstandingCheck for UnderstandingWhat’s work?What’s work?

A scientist delivers a speech to an audience of A scientist delivers a speech to an audience of his peers his peers NoNo

A body builder lifts 350 pounds above his A body builder lifts 350 pounds above his head head YesYes

A mother carries her baby from room to room A mother carries her baby from room to room NoNo

A father pushes a baby in a carriage A father pushes a baby in a carriage YesYes A woman carries a 20 kg grocery bag to her A woman carries a 20 kg grocery bag to her

car? car? NoNo THE FORCE AND THE MOVEMENT MUST THE FORCE AND THE MOVEMENT MUST

BE IN THE SAME DIRECTION TO BE WORK! BE IN THE SAME DIRECTION TO BE WORK!

Force

Distance moved

Check for UnderstandingCheck for Understanding

How much work does it take to lift a 200 N How much work does it take to lift a 200 N weight 2 m off the floor?weight 2 m off the floor?How much work does it take to hold a 200 How much work does it take to hold a 200 N weight 2 m off the floor?N weight 2 m off the floor?How much work is done if you drop a 2.5 N How much work is done if you drop a 2.5 N book 3 meters? What does the work?book 3 meters? What does the work?


How much work does it take to lift a 200 N How much work does it take to lift a 200 N weight 2 m off the floor? weight 2 m off the floor? 400 J400 J

How much work does it take to hold a 200 How much work does it take to hold a 200 N weight 2 m off the floor? N weight 2 m off the floor? 0 J0 J

How much work is done if you drop a 2.5 N How much work is done if you drop a 2.5 N book 3 meters? book 3 meters? 7.5 J 7.5 J What does the work? What does the work? Gravity!Gravity!

Eureka! Work


1. Two physics students, Ben and Bonnie, are 1. Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 barbell the same distance over his head 10 times in 10 seconds. times in 10 seconds.

Which student does the most work? Which student does the most work?

Which student delivers the most power?Which student delivers the most power?

Explain your answers. Explain your answers.

W = F x d but we need to find the gravitational force W = F x d but we need to find the gravitational force (weight) of the barbells(weight) of the barbells

FFgg = m x g = m x g FFgg = 50kg x 9.8 N/kg = 500 N = 50kg x 9.8 N/kg = 500 N Both use same force to lift the same barbellBoth use same force to lift the same barbell

Now calculate the work done by each: Now calculate the work done by each: W = 500N X 6m (total d) W = 500N X 6m (total d) Both use same work yet, Ben is the most powerful since he Both use same work yet, Ben is the most powerful since he

does the same work in less time does the same work in less time P = W/d P = W/d Ben 500J/10sec = 50 watts Ben 500J/10sec = 50 watts Bonnie 500J/60sec 8.3 wattsBonnie 500J/60sec 8.3 watts


History of WorkHistory of Work

Before engines and motors were invented, Before engines and motors were invented, people had to do things like lifting or pushing people had to do things like lifting or pushing heavy loads by handheavy loads by hand

Using an animal could help, but what they Using an animal could help, but what they really needed were some clever ways to really needed were some clever ways to either make work easier or fastereither make work easier or faster

Simple MachinesSimple Machines Ancient people invented simple machines that would help them overcome Ancient people invented simple machines that would help them overcome

resistive forces resistive forces A A simple machine is a machine that does work with only one movement of is a machine that does work with only one movement of

the machinethe machine– Some machines, such as bicycles, increase speedSome machines, such as bicycles, increase speed– Some machines, such as an axe, change the direction of forceSome machines, such as an axe, change the direction of force– Some machines, such as a car jack, increase force Some machines, such as a car jack, increase force

Simple MachinesSimple Machines

Examples of simple Examples of simple machinesmachines– Inclined PlaneInclined Plane– LeversLevers– Wheel and AxleWheel and Axle– Wedge and ScrewWedge and Screw– GearsGears– PulleyPulley

Inclined PlaneInclined Plane

A flat, slanted A flat, slanted surfacesurface

Eureka! Inclined Plane

LeverLever

Two parts:Two parts:

FulcrumFulcrum

BarBar Eureka! Levers

Wheel and AxleWheel and Axle

Two parts:Two parts: wheel wheel

barbar

Wedges and ScrewsWedges and Screws

Change downward Change downward force into sideways force into sideways forceforce

Eureka! Screw and Wheel

GearsGears

Wheels with Wheels with teethteeth

PulleyPulley

Two kinds: Two kinds:

FixedFixed

MoveableMoveable

Compound MachinesCompound Machines

A compound A compound machine is one machine is one made up of two or made up of two or more simple more simple machines.machines.

EfficiencyEfficiency Efficiency - a measure of how much of the work - a measure of how much of the work

put into a machine is changed into useful put into a machine is changed into useful output workoutput work– Every machine is less than 100% effectiveEvery machine is less than 100% effective– Not 100% of the work done is useful work, because Not 100% of the work done is useful work, because

some gets turned into other forms, like heat some gets turned into other forms, like heat – Machines can be made more efficient by reducing Machines can be made more efficient by reducing

friction with a lubricant, such as oil or grease, which friction with a lubricant, such as oil or grease, which is added to surfaces that rub togetheris added to surfaces that rub together

Mechanical AdvantageMechanical Advantage Two forces are involved when a machine is used to Two forces are involved when a machine is used to

do workdo work– One force is applied to the machine and that is the input One force is applied to the machine and that is the input

forceforce– The force applied by the machine is called the outputThe force applied by the machine is called the output forceforce

Mechanical advantage of a machine is the ratio of Mechanical advantage of a machine is the ratio of the output force to the input force the output force to the input force

Mechanical AdvantageMechanical Advantage Window blinds are Window blinds are

a machine that a machine that changes changes forceforce– A downward pull on A downward pull on

the cord is changed the cord is changed to an upward force to an upward force on the blindson the blinds

– The input and The input and output forces are output forces are equal, so the MA is 1equal, so the MA is 1

– Eureka! Mechanical Advantage

EnergyEnergy Energy is all around you:Energy is all around you:

– LightLight– HeatHeat– WindWind

You use energy when you:You use energy when you:– hit a softballhit a softball– lift your book baglift your book bag– digest fooddigest food

Every change that occurs—Every change that occurs—large or small—involves energylarge or small—involves energy

Changes Require EnergyChanges Require Energy When something is able to change its When something is able to change its

environment or itself, it has energyenvironment or itself, it has energy– Anything that causes change must have energyAnything that causes change must have energy– You use energy to arrange your hair to look the way You use energy to arrange your hair to look the way

you want it toyou want it to– You also use energy when you walk down the halls You also use energy when you walk down the halls

of your school between classes or eat your lunchof your school between classes or eat your lunch

Nature of EnergyNature of Energy What is energy that it can be involved in so What is energy that it can be involved in so

many different activities?many different activities?– Energy-Energy- the ability to do work the ability to do work– If an object or organism does work the If an object or organism does work the

object or organism uses energyobject or organism uses energy– Whenever you do work you transfer energy Whenever you do work you transfer energy

from one thing to anotherfrom one thing to another

Nature of EnergyNature of Energy Because of the direct connection between Because of the direct connection between

energy and work, energy is measured in energy and work, energy is measured in the same unit as work: joules (J)the same unit as work: joules (J)

In addition to using energy to do work, In addition to using energy to do work, objects gain energy because work is being objects gain energy because work is being done on themdone on them

Forms of EnergyForms of Energy The five main forms of energy are:The five main forms of energy are:

– Thermal (heat)Thermal (heat)– ChemicalChemical– ElectromagneticElectromagnetic– NuclearNuclear– MechanicalMechanical

If If you have $100, you could store it in a variety of you have $100, you could store it in a variety of forms—cash in your wallet, a bank account, or coinsforms—cash in your wallet, a bank account, or coins

Regardless of its form, money is money, and the same Regardless of its form, money is money, and the same goes for energy in that these are only different forms of goes for energy in that these are only different forms of the same thingthe same thing

States of EnergyStates of Energy

The most common energy conversion is The most common energy conversion is the conversion between potential and the conversion between potential and kinetic energykinetic energy

All forms of energy can be in either of two All forms of energy can be in either of two states:states:– PotentialPotential– KineticKinetic

Kinetic EnergyKinetic Energy Kinetic energy-Kinetic energy- the energy of motion the energy of motion Depends on both mass and velocityDepends on both mass and velocity

– The faster an object moves, the more kinetic The faster an object moves, the more kinetic energy it hasenergy it has

– The greater the mass of a moving object, the The greater the mass of a moving object, the more kinetic energy it hasmore kinetic energy it has

EEkk = = mass x velocitymass x velocity22

22

What has a greater effect on kinetic energy, What has a greater effect on kinetic energy, mass or velocity? Why?mass or velocity? Why?

Potential EnergyPotential Energy Even motionless objects have Even motionless objects have

energyenergy Potential energy-Potential energy- stored energy stored energy

due to interactions between due to interactions between objectsobjects– If the apple stays in the tree, the If the apple stays in the tree, the

energy will remain storedenergy will remain stored– If the apple falls, that stored energy If the apple falls, that stored energy

is converted to kinetic energyis converted to kinetic energy

Elastic Potential Energy Elastic Potential Energy If you stretch a rubber band and let it go, it If you stretch a rubber band and let it go, it

sails across the roomsails across the room– As it flies through the air, it has kinetic energy due As it flies through the air, it has kinetic energy due

to its motion but where did this kinetic energy come to its motion but where did this kinetic energy come from? from?

– The stretched rubber band had energy stored as elastic potential energy

Elastic potential energy Elastic potential energy is energy stored by is energy stored by something that can stretch or compress, such something that can stretch or compress, such as a rubber band or spring as a rubber band or spring

Chemical Potential EnergyChemical Potential Energy

Gasoline, food, and other substances have Gasoline, food, and other substances have chemical potential energychemical potential energy– Energy stored due to chemical bonds is Energy stored due to chemical bonds is

chemical potential energychemical potential energy– Energy is stored due to the bonds that hold the Energy is stored due to the bonds that hold the

atoms together and is released when the gas is atoms together and is released when the gas is burned burned

Gravitational Potential Gravitational Potential EnergyEnergy

Any system that has Any system that has objects that are attracted objects that are attracted to each other through to each other through gravity has gravitational gravity has gravitational potential energypotential energy– Gravitational potential Gravitational potential

energy energy (GPE) - energy due (GPE) - energy due to gravitational forces to gravitational forces between objects between objects

– Water and EarthWater and Earth– Apple and Earth Apple and Earth


Depends on mass and heightDepends on mass and height EEpp = m (kg) x g (N/kg) x h (m) = m (kg) x g (N/kg) x h (m)

where g is the force caused by gravity (9.8 where g is the force caused by gravity (9.8 N/kg)N/kg)– If you stand on a 3-meter diving board, you If you stand on a 3-meter diving board, you

have 3 times the G.P.E, than you had on a 1-have 3 times the G.P.E, than you had on a 1-meter diving boardmeter diving board

– A person with 3 times a larger mass has 3 A person with 3 times a larger mass has 3 times the potential energytimes the potential energy


A waterfall, a suspension bridge, and a A waterfall, a suspension bridge, and a falling snowflake all have gravitational falling snowflake all have gravitational potential energypotential energy

PotentialEnergy

KineticEnergy

PotentialEnergy

KineticEnergy

Check for UnderstandingCheck for Understanding EEkk = = 1 1 mvmv22

2 2– What is the kinetic energy of a 100 kg man What is the kinetic energy of a 100 kg man

moving 5 m/s?moving 5 m/s?

– What is the kinetic energy of 0.5 kg ball What is the kinetic energy of 0.5 kg ball moving at 30 m/s?moving at 30 m/s?

Check for UnderstandingCheck for Understanding EEkk = = 1 1 mvmv22

2 2– What is the kinetic energy of a 100 kg man What is the kinetic energy of a 100 kg man

moving 5 m/s?moving 5 m/s? 1 1 mvmv2 2 == 1 1 x 100kg x (5m/s) x 100kg x (5m/s)2 2 = 1250 J = 1250 J

2 22 2

– What is the kinetic energy of 0.5 kg ball What is the kinetic energy of 0.5 kg ball moving at 30 m/s?moving at 30 m/s?

1 1 mvmv2 2 == 1 1 x 0.5kg x (30m/s) x 0.5kg x (30m/s)2 2 = 225 J = 225 J 2 2 2 2

Eureka! Kinetic Energy


EEpp = m x g x h = m x g x h – A 100 kg boulder is on the edge of the cliff 10 A 100 kg boulder is on the edge of the cliff 10

m off the ground. How much energy does it m off the ground. How much energy does it have?have?

– A 0.5 kg ball is thrown 15 m into the air How A 0.5 kg ball is thrown 15 m into the air How much potential energy does it have at its much potential energy does it have at its highest point?highest point?

Check for UnderstandingCheck for Understanding E = m x g x h E = m x g x h

– A 100 kg boulder is on the edge of the cliff 10 m A 100 kg boulder is on the edge of the cliff 10 m off the ground. How much energy does it have?off the ground. How much energy does it have?

100kg x 9.8 m/s100kg x 9.8 m/s2 2 x 10m = ~ 10,000 J x 10m = ~ 10,000 J

– A 0.5 kg ball is thrown 15 m into the air How A 0.5 kg ball is thrown 15 m into the air How much potential energy does it have at its much potential energy does it have at its highest point?highest point?

0.5 kg x 9.8 m/s0.5 kg x 9.8 m/s2 2 x 15m = ~ 75 J x 15m = ~ 75 J

Eureka! Potential Energy

The Law of Conservation of The Law of Conservation of EnergyEnergy

The Law of Conservation of Energy-The Law of Conservation of Energy- energy energy can be neither created nor destroyed by can be neither created nor destroyed by ordinary means, it can only be converted ordinary means, it can only be converted from one form to another from one form to another Energy can change from one form to another, Energy can change from one form to another,

but the total amount of energy never changesbut the total amount of energy never changes The total energy of a system remains constantThe total energy of a system remains constant

Energy TransformationsEnergy Transformations

The law of conservation of energy is a The law of conservation of energy is a universal principle that describes what universal principle that describes what happens to energy as it is transferred from happens to energy as it is transferred from one object to another or as it is one object to another or as it is transformedtransformed– You are likely to think of energy as race cars You are likely to think of energy as race cars

roar past or as your body uses energy from roar past or as your body uses energy from food to help it move, or as the Sun warms your food to help it move, or as the Sun warms your skin on a summer dayskin on a summer day

– These situations involve energy changing from These situations involve energy changing from one form to another formone form to another form

Mechanical Energy Mechanical Energy Transformations Transformations

Mechanical energy Mechanical energy is the sum of the is the sum of the kinetic energy and potential energy of the kinetic energy and potential energy of the objects in a systemobjects in a system– The mechanical energy of a system remains The mechanical energy of a system remains

constant or nearly constantconstant or nearly constant– In these cases, energy is only converted In these cases, energy is only converted

between different forms of mechanical energybetween different forms of mechanical energy

Mechanical Energy Mechanical Energy Transformations Transformations

An apple-Earth system has gravitational potential An apple-Earth system has gravitational potential energy due to the gravitational force between energy due to the gravitational force between apple and Earth apple and Earth – The instant the apple comes loose from The instant the apple comes loose from

the tree:the tree: It accelerates due to gravity It accelerates due to gravity It loses height so the gravitational It loses height so the gravitational

potential energy decreasespotential energy decreases Its potential energy is transformed intoIts potential energy is transformed into

kinetic as the speed of the apple increaseskinetic as the speed of the apple increases The potential energy that the apple lost is The potential energy that the apple lost is

gained back as kinetic energy so the total amount of gained back as kinetic energy so the total amount of energy remains the sameenergy remains the same

Energy Transformation in Energy Transformation in Projectile MotionProjectile Motion

Energy transformations also occur during Energy transformations also occur during projectile motion when an object moves in a projectile motion when an object moves in a curved pathcurved path

However, the mechanical energy of the ball-However, the mechanical energy of the ball-Earth system remains constant as it rises Earth system remains constant as it rises and fallsand falls

Energy Transformation in a Energy Transformation in a BasketballBasketball

Ball slows down Ball speeds up

Forceof

gravity

Energy Transformation in a Energy Transformation in a Roller CoasterRoller Coaster

At the point of maximum potential energy, the car has minimum kinetic energy.

Total energy is conserved and constant

Energy Transformations in a Energy Transformations in a SwingSwing

When you ride on a swing part of the fun is the When you ride on a swing part of the fun is the feeling of almost falling as you drop from the feeling of almost falling as you drop from the highest point to the lowest point of the swing’s highest point to the lowest point of the swing’s pathpath– The ride starts with a push that gets you moving, The ride starts with a push that gets you moving,

giving you kinetic energygiving you kinetic energy– As the swing rises, you lose speed but gain heightAs the swing rises, you lose speed but gain height– In energy terms, kinetic energy changes to In energy terms, kinetic energy changes to

gravitational potential energygravitational potential energy

The Effect of FrictionThe Effect of Friction You know that if you don’t continue to pump a You know that if you don’t continue to pump a

swing or get a push, your arcs will become swing or get a push, your arcs will become lower and you eventually will stop swinginglower and you eventually will stop swinging

In other words, the mechanical (kinetic and In other words, the mechanical (kinetic and potential) energy of the swing decreases, as if potential) energy of the swing decreases, as if the energy were being destroyed or lost Is the energy were being destroyed or lost Is this a violation of the law of conservation of this a violation of the law of conservation of energy? energy?

The Effect of FrictionThe Effect of Friction NO!!! Energy in the system is conservedNO!!! Energy in the system is conserved

– With every movement, the swing’s chains rub on With every movement, the swing’s chains rub on their hooks and air pushes on the ridertheir hooks and air pushes on the rider

– Friction and air resistance cause some of the Friction and air resistance cause some of the mechanical energy of the swing to change to thermal mechanical energy of the swing to change to thermal energyenergy

– With every pass of the swing, the temperature of the With every pass of the swing, the temperature of the hooks and the air increases a little, so the hooks and the air increases a little, so the mechanical energy of the swing is mechanical energy of the swing is not destroyed, butnot destroyed, buttransformed into transformed into thermal energy, or heatthermal energy, or heat

Conservation of Energy with a Conservation of Energy with a PendulumPendulum

When is the pendulum moving the fastest?

at the lowest point


When does the pendulum have the most kinetic energy?

at the lowest point


When does the pendulum have the most

gravitational potential energy?

at the highest point

E


KP

E


KP

Gravitational potential energy depends on heightGravitational potential energy depends on height

EP

Kinetic energy depends on speedKinetic energy depends on speed


K

EKP

E = P + KE = P + K

EKP

Total Energy Doesn’t Total Energy Doesn’t ChangeChange

+ =

A Pendulum

PE

No KE

All KE PE

No KE

Transforming Electrical Transforming Electrical EnergyEnergy

Light bulbs transform electrical energy Light bulbs transform electrical energy into light so you can see into light so you can see

The warmth you feel around the bulb is The warmth you feel around the bulb is evidence that some of that electrical evidence that some of that electrical energy is transformed into thermal energy is transformed into thermal energy, or heatenergy, or heat

Transforming Chemical Transforming Chemical EnergyEnergy

Fuel stores chemical potential energy Fuel stores chemical potential energy – An engine transforms chemical An engine transforms chemical

potential energy of gasoline potential energy of gasoline molecules into the kinetic energy molecules into the kinetic energy of a moving car or bus of a moving car or bus

– Several energy conversions occur in this Several energy conversions occur in this process process

– In a car, a spark plug fires, initiating theIn a car, a spark plug fires, initiating theconversion of chemical potential energy conversion of chemical potential energy into thermal energyinto thermal energy

– As the hot gases expand, thermal energy is As the hot gases expand, thermal energy is converted into kinetic energyconverted into kinetic energy

Transforming Chemical Transforming Chemical EnergyEnergy

Some chemical energy Some chemical energy transformations are less obvious transformations are less obvious because they do not result in because they do not result in visible motion, sound, heat or lightvisible motion, sound, heat or light– Every green plant you see converts Every green plant you see converts

the radiant energy from the Sun into the radiant energy from the Sun into the energy stored in chemical bonds the energy stored in chemical bonds in the plant, like in carbohydrates in the plant, like in carbohydrates (sugars)(sugars)

– When we eat we transform the When we eat we transform the potential energy stored in the potential energy stored in the carbohydrate bonds and transform it carbohydrate bonds and transform it into kinetic energyinto kinetic energy

Other Energy ConversionsOther Energy Conversions In a battery, chemical energy is converted In a battery, chemical energy is converted

into electromagnetic energyinto electromagnetic energy The mechanical energy of a waterfall is The mechanical energy of a waterfall is

converted to electrical energy in a generator converted to electrical energy in a generator

Chemical Chemical Thermal Thermal MechanicalMechanical

A E

DC

B


Where is the gravitational potential energy maximum?

Where is the kinetic energy maximum?Where is the gravitational potential energy minimum?Where is the kinetic energy minimum?

A E

DC

B


Where is the gravitational potential energy maximum? A and EWhere is the kinetic energy maximum? CWhere is the gravitational potential energy minimum? CWhere is the kinetic energy minimum? A and E

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Work and Energy : Forms and Changes