Work and Energy Chapter 12 Table of Contents Section 1 Work, Power, and Machines Section 2 Simple Machines Section 3 What is Energy? Section 4 Conservation

Work and EnergyChapter 12

Table of ContentsTable of Contents

Section 1 Section 1 Work, Power, and MachinesWork, Power, and Machines

Section 2 Section 2 Simple MachinesSimple Machines

Section 3 Section 3 What is Energy?What is Energy?

Section 4 Section 4 Conservation of EnergyConservation of Energy

Section 1 Work, Power, and Machines

ObjectivesObjectives DefineDefine work and power.work and power. CalculateCalculate the work done on an object the work done on an object

and the rate at which work is done.and the rate at which work is done. UseUse the concept of mechanical the concept of mechanical

advantage to explain how machines advantage to explain how machines make doing work easier.make doing work easier.

CalculateCalculate the mechanical advantage the mechanical advantage of various machines.of various machines.

Chapter 12


What is Work?What is Work? WorkWork is the transfer of energy to a body by the is the transfer of energy to a body by the

application of a force that causes the body to application of a force that causes the body to move in the direction of the force.move in the direction of the force.

Work is done Work is done onlyonly when when a force causes an object a force causes an object to move in the to move in the directiondirection of the of the forceforce.. This is This is different from the everyday meaning of different from the everyday meaning of workwork..

Work EquationWork Equation workwork = = forceforce distancedistance WW = = FF dd

Chapter 12

A baseball pitcher does work on the A baseball pitcher does work on the ball by transferring energy into it. ball by transferring energy into it.

http://en.wikipedia.org/wiki/Image:Baseball_pitching_motion_2004.jpg

Is work being done in the Is work being done in the following examples?following examples?


What is Work?What is Work? Work is measured in joules.Work is measured in joules.

• Because work is calculated as force times Because work is calculated as force times distance, it is measured in units ofdistance, it is measured in units of newtons newtons times meters, N•m.times meters, N•m.

• These units are also calledThese units are also called joulesjoules (J). (J). In terms In terms of SI base units, a joule is equivalent toof SI base units, a joule is equivalent to 1 1 kg•mkg•m22/s/s22..

Definition of joulesDefinition of joules 1 J = 1 N•m = 1 kg•m1 J = 1 N•m = 1 kg•m22/s/s22

VideoVideo – intro – intro 1min1min Video – work and energy 1minVideo – work and energy 1min

Chapter 12


Math SkillsMath SkillsWork Work Imagine a father playing with his Imagine a father playing with his

daughter by lifting her repeatedly in the daughter by lifting her repeatedly in the air. How much work does he do with each air. How much work does he do with each lift, assuming he lifts her 2.0 m and lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?exerts an average force of 190 N?

1. List the given and unknown values.1. List the given and unknown values.

Chapter 12

Given:Given: force, Fforce, F = 190 N = 190 N

distance, ddistance, d = 2.0 m = 2.0 m

Unknown:Unknown: work, Wwork, W = ? J = ? J


Math SkillsMath Skills2. Write the equation for work.2. Write the equation for work.

Chapter 12

work = force work = force distance distance

W = f W = f dd

3. Insert the known values into 3. Insert the known values into the equation, and solve.the equation, and solve.

WW = 190 N = 190 N 2.0 m = 380 N•m 2.0 m = 380 N•m

WW = 380 J = 380 J

Do the practice problems p Do the practice problems p 379379

1._____________1._____________ 2. _____________2. _____________ 3. _____________3. _____________ 4. _____________4. _____________ 5. _____________5. _____________

Answers practice problems p Answers practice problems p 379379

1. W= (5200 N)(25m)= 1.3 x 101. W= (5200 N)(25m)= 1.3 x 1055 J J 2. W= (1 N)(1 m)= 1 J2. W= (1 N)(1 m)= 1 J 3. W= (125 N)(14m)= 1750 J3. W= (125 N)(14m)= 1750 J 4. W= (30)(165 N)(.8 m)= 3960 J4. W= (30)(165 N)(.8 m)= 3960 J 5. W= (1200kg)(9.8 m/s5. W= (1200kg)(9.8 m/s22)(.5m)= )(.5m)=

5880 J5880 J


PowerPower PowerPower is a quantity that measures the is a quantity that measures the

rate at which work is done or energy is rate at which work is done or energy is transformed.transformed.

Power EquationPower Equation

power work

time

P W

t Power is measured in watts.Power is measured in watts.

• A watt (W) is equal to a joule per second (1 J/s).A watt (W) is equal to a joule per second (1 J/s).• Video – Power 2minVideo – Power 2min

Chapter 12


Math SkillsMath SkillsPowerPower It takes 100 kJ of work to lift an It takes 100 kJ of work to lift an

elevator 18 m. If this is done in 20 s, what elevator 18 m. If this is done in 20 s, what is the average power of the elevator is the average power of the elevator during the process?during the process?

1. List the given and unknown values.1. List the given and unknown values.

Chapter 12

Given:Given: work, Wwork, W = 100 kJ = 1 = 100 kJ = 1 10 1055 J J

time, ttime, t = 20 s = 20 s

The distance of 18 m will not be The distance of 18 m will not be

needed to calculate power.needed to calculate power.Unknown:Unknown: power, Ppower, P = ? W = ? W


Math SkillsMath Skills

3. Insert the known values into the 3. Insert the known values into the equation, and solve.equation, and solve.

2. Write the equation for power.

power work

time

P W

t

P 1105 J

20 s5 103 J/s

P 5 103 W 5 kW

Chapter 12

Do the practice problems Do the practice problems p381p381

1. _______________1. _______________ 2. _______________2. _______________ 3. _______________3. _______________ 4.a. _______________4.a. _______________

b. _______________b. _______________

5 a. _______________5 a. _______________

b. _______________b. _______________

AnswersAnswers 1. P= 1. P= ww = = 3960J3960J = 66W = 66W t 60st 60s 2. P= 2. P= 900MJ900MJ = 900 MW or 900,000,000w = 900 MW or 900,000,000w 1s1s 3 P= 107W3 P= 107W 4. a. W=720 J4. a. W=720 J

P= 36wP= 36w

b.b. W= 300JW= 300J

P= 100P= 100 w w

5. a. P= 146w5. a. P= 146w

b. P= 175 wb. P= 175 w


Machines and Mechanical Machines and Mechanical AdvantageAdvantage

Machines multiply and redirect forces.Machines multiply and redirect forces.• They can change either the They can change either the sizesize or the or the

direction direction of the input force.of the input force.

Chapter 12

Machines cannot increase the amount of Machines cannot increase the amount of work done. Wwork done. Winin= W= Woutout

Instead there is a trade off.Instead there is a trade off.• A machine allows the same amount of A machine allows the same amount of

work to be done by either work to be done by either decreasing the decreasing the distance while increasing the forcedistance while increasing the force or by or by decreasing the force while increasing the decreasing the force while increasing the distance.distance.

Force and WorkForce and Work

Section 1 Work, Power, and MachinesChapter 12


Machines and Mechanical Machines and Mechanical

AdvantageAdvantage Mechanical advantageMechanical advantage tells how much tells how much

a machine multiplies effort force.a machine multiplies effort force.

Mechanical Advantage EquationMechanical Advantage Equation

mechanical advantage

output force

input force

input distance

output distance

Chapter 12

High MA is good!!!High MA is good!!!

MA = resistance force/effort forceMA = resistance force/effort force MA = FMA = Fr r / F/ Fee

NoNo SI units SI units Video – MA Video – MA

1. Resistance Force1. Resistance Force- force of - force of gravitygravity or or frictionfriction working working

against objectagainst object 2. Effort Force2. Effort Force- force applied to - force applied to

overcome the resistanceovercome the resistance force force Situation: Screwdriver being used to open Situation: Screwdriver being used to open

a paint can. a paint can. What are 2 forces in this ex?What are 2 forces in this ex?


Math SkillsMath Skills Mechanical Advantage Mechanical Advantage Calculate the Calculate the

mechanical advantage of a ramp that mechanical advantage of a ramp that is 5.0 m long and 1.5 m high.is 5.0 m long and 1.5 m high.

1. List the given and unknown 1. List the given and unknown values.values.

Chapter 12

Given:Given: input distanceinput distance = 5.0 m = 5.0 moutput distanceoutput distance = 1.5 m = 1.5 m

Unknown:Unknown: mechanical advantagemechanical advantage = = ??


Math SkillsMath Skills 2. Write the equation for mechanical 2. Write the equation for mechanical

advantage.advantage.Because the information we are given Because the information we are given involves only distance, we only need part of involves only distance, we only need part of the full equation:the full equation:


input distance

output distance



5.0 m

1.5 m3.3

Chapter 12

Mechanical Advantage problem Mechanical Advantage problem 22

The person lifting the 200kg load The person lifting the 200kg load experiences a pull equal to only 50kg experiences a pull equal to only 50kg (200kg/4).(200kg/4).

The Mechanical advantage The Mechanical advantage is is calculated by dividing the load by the calculated by dividing the load by the effort (load/effort). effort (load/effort).

MA= MA= Output ForceOutput Force Input ForceInput Force

==200kg 200kg = ??= ?? 50kg50kg

The pulley system offers a The pulley system offers a mechanical advantage of 4.mechanical advantage of 4. (Greater distance but less force (Greater distance but less force

needed)needed)

Section 2 Simple Machines

ObjectivesObjectives

NameName and describe the six types of and describe the six types of simple machines.simple machines.

DiscussDiscuss the mechanical advantage the mechanical advantage of different types of simple of different types of simple machines.machines.

RecognizeRecognize simple machines within simple machines within compound machines.compound machines.

Chapter 12

Simple MachinesSimple Machines The most basic machines are calledThe most basic machines are called

simple machines.simple machines. The six types of simple machines are The six types of simple machines are

divided into two families.divided into two families.

The lever The lever family:family:

The inclined plane The inclined plane family:family:

simple leversimple lever simple inclined simple inclined planeplane

pulleypulley wedgewedge wheel and wheel and axleaxle

screwscrew

Section 2 Simple MachinesChapter 12

3 Functions of Simple Machines3 Functions of Simple Machines

1. Change 1. Change directiondirection of force. (ex- fixed pulley) of force. (ex- fixed pulley)

2.2. Multiplies effort force by increasing distance.Multiplies effort force by increasing distance.

Ex- rampEx- ramp MA= >1MA= >1

3.3. Decreases effort force by decreasing distance.Decreases effort force by decreasing distance.

Ex- tweezers or tongs MA= <1Ex- tweezers or tongs MA= <1

Hey, I thought this was suppose to be

SIMPLE machines! This is too much for

my simple brain!

The Lever FamilyThe Lever Family Levers have aLevers have a rigid arm rigid arm and aand a fulcrum. fulcrum. Video- lever 3 minVideo- lever 3 min


Levers are divided into three classes.Levers are divided into three classes.• All first-class levers All first-class levers have a fulcrum located have a fulcrum located

between the points of application of the input between the points of application of the input and output forces. Ex- Teeter totter, hammer and output forces. Ex- Teeter totter, hammer pulling out nailpulling out nail

• In a second-class lever, In a second-class lever, the fulcrum is at the fulcrum is at one end of the arm and the input force one end of the arm and the input force is applied to the other end. Ex- Wheel is applied to the other end. Ex- Wheel barrow, Doorbarrow, Door

http://upload.wikimedia.org/wikipedia/commons/2/22/LeverSecondClass.svg

ThirdThird--class leversclass levers have the effort force have the effort force placed between the load and the fulcrum. placed between the load and the fulcrum.

33rdrd class levers class levers multiply distance rather than multiply distance rather than force. As a result, they have a mechanical force. As a result, they have a mechanical advantage of less than 1.advantage of less than 1.

• Ex- Fishing pole, tongs, armsEx- Fishing pole, tongs, arms

LeversLeversSection 2 Simple MachinesChapter 12

Video- lever 3 minVideo- lever 3 min

The Lever FamilyThe Lever Family Pulleys are modified levers.Pulleys are modified levers.

• The point in the middle of a pulley is like the The point in the middle of a pulley is like the fulcrum of a lever.fulcrum of a lever.

• A single, fixed pulley has a mechanical A single, fixed pulley has a mechanical advantage of advantage of 11..

• Multiple pulleys are sometimes put together in Multiple pulleys are sometimes put together in a single unit called a a single unit called a block and tackle.block and tackle.


http://upload.wikimedia.org/wikipedia/commons/a/a3/Polea-simple-fija.jpg

http://upload.wikimedia.org/wikipedia/commons/6/62/Polispasto2.jpg

PulleysPulleys


PulleyPulley


Copyright © by Holt, Rinehart and Winston. All rights reserved.

ResourcesChapter menu

Pogo Moose Incident - Fairbanks , Pogo Moose Incident - Fairbanks , Alaska Alaska

"They were laying new power "They were laying new power cables which were strung on the cables which were strung on the ground for miles. The moose are ground for miles. The moose are rutting right now and very rutting right now and very agitated. He was thrashing around agitated. He was thrashing around and got his antlers stuck in the and got his antlers stuck in the cables. When the men (miles cables. When the men (miles away) began pulling the lines up away) began pulling the lines up with their big equipment, the with their big equipment, the moose went up with them. They moose went up with them. They noticed excess tension in the lines noticed excess tension in the lines and went searching for the and went searching for the problem. He was still alive when problem. He was still alive when they lowered him to the ground. they lowered him to the ground. He was a huge 60 inch bull and He was a huge 60 inch bull and slightly peeved!" slightly peeved!"

Wheel and axleWheel and axle

A wheel and axle is a lever or pulley A wheel and axle is a lever or pulley connected to a shaft.connected to a shaft.• EX- The steering wheel of a car, EX- The steering wheel of a car,

screwdrivers, and cranks. screwdrivers, and cranks. Video – wheel and axle Video – wheel and axle 1min1min

The Inclined Plane FamilyThe Inclined Plane Family Inclined planes multiply and redirect Inclined planes multiply and redirect

force.force.• An inclined plane turns a small input force An inclined plane turns a small input force

into a large output force by spreading the into a large output force by spreading the work out over a large distance.work out over a large distance.


The Inclined Plane FamilyThe Inclined Plane Family A wedge is a modified inclined plane.A wedge is a modified inclined plane. Ex- knife, axe, push pin, Ex- knife, axe, push pin, Video- Wedge 2minVideo- Wedge 2min

A screw is an inclined plane wrapped A screw is an inclined plane wrapped around a cylinder.around a cylinder.

Compound MachinesCompound Machines A machine made of more than one A machine made of more than one

simple machine is called asimple machine is called a compound compound machine.machine.

Examples of compound machines are:Examples of compound machines are:• scissors, which use two first class levers scissors, which use two first class levers

joined at a common fulcrum + wedgejoined at a common fulcrum + wedge• a car jack, which uses a lever in a car jack, which uses a lever in

combination with a screwcombination with a screw


Video-Work, energy, Video-Work, energy,

simple machines 15minsimple machines 15min

Rube GoldbergRube Goldberg 18831883 - -19701970 was an American was an American cartoonistcartoonist is best known for his series of popular is best known for his series of popular

cartoons depicting cartoons depicting Rube Goldberg machinesRube Goldberg machines, complex , complex devices that perform simple tasks, using devices that perform simple tasks, using multiple simple machines. multiple simple machines.

http://en.wikipedia.org/wiki/1883

http://en.wikipedia.org/wiki/1970

http://en.wikipedia.org/wiki/Cartoonist

http://en.wikipedia.org/wiki/Rube_Goldberg_machine

http://en.wikipedia.org/wiki/Image:Rubenvent.jpg

Simplified Pencil Sharpener Simplified Pencil Sharpener

Open window Open window (A)(A) and fly kite and fly kite (B)(B). String . String (C)(C) lifts small door lifts small door (D)(D) allowing moths allowing moths (E)(E) to escape and eat red flannel shirt to escape and eat red flannel shirt (F)(F). As . As weight of shirt becomes less, shoe weight of shirt becomes less, shoe (G)(G) steps on switch steps on switch (H)(H) which which heats electric iron heats electric iron (I)(I) and burns hole in pants and burns hole in pants (J)(J). Smoke . Smoke (K)(K) enters hole in tree enters hole in tree (L)(L), smoking out opossum , smoking out opossum (M)(M) which jumps which jumps into basket into basket (N)(N), pulling rope , pulling rope (O)(O) and lifting cage and lifting cage (P)(P), allowing , allowing woodpecker woodpecker (Q)(Q) to chew wood from pencil to chew wood from pencil (R)(R), exposing lead. , exposing lead. Emergency knife Emergency knife (S)(S) is always handy in case opossum or the is always handy in case opossum or the woodpecker gets sick and can't work. woodpecker gets sick and can't work.

http://www.rube-goldberg.com/html/gallery.htm

http://www.rube-goldberg.com/html/gallery.htm

Section 3 What is Energy?

ObjectivesObjectives ExplainExplain the relationship between the relationship between

energy and work.energy and work. DefineDefine potential energy and kinetic potential energy and kinetic

energy.energy. CalculateCalculate kinetic energy and kinetic energy and

gravitational potential energy.gravitational potential energy. DistinguishDistinguish between mechanical and between mechanical and

nonmechan-ical energy.nonmechan-ical energy.

Chapter 12

Energy and WorkEnergy and Work Energy is the ability to do work.Energy is the ability to do work.

• When you do work on an object, you When you do work on an object, you transfer energy to that object.transfer energy to that object.

Energy is measured in joules.Energy is measured in joules.• Because energy is a measure of the ability Because energy is a measure of the ability

to do work, energy and work are expressed to do work, energy and work are expressed in the same units.in the same units.

Section 3 What is Energy?Chapter 12

Potential EnergyPotential Energy The energy that an object has because The energy that an object has because

of the position, shape, or condition of of the position, shape, or condition of the object is calledthe object is called potential energy.potential energy.

Potential energy is stored energy.Potential energy is stored energy.• Elastic potential energyElastic potential energy is the energy is the energy

stored in any type of stretched or stored in any type of stretched or compressed elastic material, such as a compressed elastic material, such as a spring or a rubber band.spring or a rubber band.


Gravitational Potential Gravitational Potential EnergyEnergy

• Gravitational potential energyGravitational potential energy is the is the energy stored in the gravitational field energy stored in the gravitational field which exists between any two or more which exists between any two or more objects.objects.

Gravitational potential energy Gravitational potential energy depends on both mass and height.depends on both mass and height.

Potential EnergyPotential Energy Gravitational Potential Energy EquationGravitational Potential Energy Equation

grav. PE = mass grav. PE = mass free-fall acceleration free-fall acceleration heightheight

PE = mghPE = mgh The height can be relative.The height can be relative.

• The height used in the above equation is The height used in the above equation is usually measured from the ground. usually measured from the ground.

• However, it can be a relative height between However, it can be a relative height between two points, such as between two branches in two points, such as between two branches in a tree.a tree.

• Video- Potential and kinetic energy 2minVideo- Potential and kinetic energy 2min


Math SkillsMath SkillsGravitational Potential EnergyGravitational Potential Energy A 65 kg A 65 kg

rock climber ascends a cliff. What is the rock climber ascends a cliff. What is the climber’s gravitational potential energy at climber’s gravitational potential energy at a point 35 m above the base of the cliff?a point 35 m above the base of the cliff?

1. List the given and unknown values.1. List the given and unknown values.Given:Given: mass, mmass, m = 65 kg = 65 kg

height, hheight, h = 35 m = 35 m

free-fall acceleration, gfree-fall acceleration, g = 9.8 m/s = 9.8 m/s2 2

Unknown:Unknown: gravitational potential energy, PEgravitational potential energy, PE = ? J= ? J


Math Skills, continuedMath Skills, continued2.2. Write the equation for Write the equation for

gravitational potential energy.gravitational potential energy.


PE = mghPE = mgh 3. Insert the known values into the 3. Insert the known values into the

equation, and solve.equation, and solve.

PE = (65 kg)(9.8 m/sPE = (65 kg)(9.8 m/s22)(35 m))(35 m)

PE = 2.2 PE = 2.2 10 1044 kg•m kg•m22/s/s22

PE = 2.2 PE = 2.2 10 1044 J J

Do the practice problems on p Do the practice problems on p 393393

Write the answers on your note packet Write the answers on your note packet beside the practice problem.beside the practice problem.

PE =mghPE =mgh 1. a. 4.9 x 101. a. 4.9 x 1055 J J

b. 5.6 X 10b. 5.6 X 106 6 JJ

c. 2803 Jc. 2803 J 2. 1.4 x 102. 1.4 x 1015 15 J J 3. 15m3. 15m 4. 58kg4. 58kg

Kinetic EnergyKinetic Energy The energy of a moving object due The energy of a moving object due

to the object’s motion is called to the object’s motion is called kinetic energy.kinetic energy.

Kinetic energy depends on mass Kinetic energy depends on mass and speed.and speed.

Kinetic Energy EquationKinetic Energy Equation

kinetic energy 1

2mass speed squared

KE 1

2mv 2

• Kinetic energy depends on speed more than mass.


Kinetic Energy GraphKinetic Energy Graph


Math SkillsMath SkillsKinetic EnergyKinetic Energy What is the kinetic What is the kinetic

energy of a 44 kg cheetah running at energy of a 44 kg cheetah running at 31 m/s?31 m/s?

1. List the given and unknown 1. List the given and unknown values.values.Given:Given: mass, mmass, m = 44 kg = 44 kg

speed, vspeed, v = 31 m/s = 31 m/s

Unknown:Unknown: kinetic energy, KEkinetic energy, KE = ? J = ? J


Math Skills, Math Skills, continuedcontinued2.2. Write the equation for kinetic Write the equation for kinetic

energy.energy.

kinetic energy 1

2mass speed squared

KE 1

2mv 2


KE 1

2(44 kg)(31 m/s)2

KE 2.1104 kggm2 /s2

KE 2.1104 J


Other Forms of EnergyOther Forms of Energy The amount of work an object can do The amount of work an object can do

because of the object’s kinetic because of the object’s kinetic andand potential energies is called potential energies is called mechanical mechanical energy.energy.

Mechanical energy is Mechanical energy is the sum of the the sum of the potential energy and the kinetic energy in potential energy and the kinetic energy in a system.a system.


Other Forms of EnergyOther Forms of Energy Atoms and molecules have kinetic energy.Atoms and molecules have kinetic energy.

• The kinetic energy of particles is related to heat The kinetic energy of particles is related to heat and temperature.and temperature.

Chemical reactions involve potential Chemical reactions involve potential energy.energy.• The amount ofThe amount of chemical energychemical energy associated with associated with

a substance depends in part on the relative a substance depends in part on the relative positions of the atoms it contains.positions of the atoms it contains.

• Video- chemical energy 2minVideo- chemical energy 2min ..


Living things get energy from the Living things get energy from the sun.sun.• Plants usePlants use photosynthesisphotosynthesis to turn the to turn the

energy in sunlight into chemical energyenergy in sunlight into chemical energy

Other Forms of Energy, Other Forms of Energy, The sun gets energy from nuclear The sun gets energy from nuclear

reactions.reactions.• The sun is fueled by nuclear fusion reactions The sun is fueled by nuclear fusion reactions

in its core. Video – nuclear reaction 1minin its core. Video – nuclear reaction 1min Electricity is a form of energy.Electricity is a form of energy.

• Electrical energy Electrical energy is derived from the flow of is derived from the flow of charged particles(electrons), as in a bolt of charged particles(electrons), as in a bolt of lightning or in a wire.lightning or in a wire.


Light can carry energy across empty Light can carry energy across empty space.space.• Light energy travels from the sun to Earth Light energy travels from the sun to Earth

across empty space in the form ofacross empty space in the form of electromagnetic waves.electromagnetic waves.

Section 4 Conservation of Energy

ObjectivesObjectives IdentifyIdentify and describe transformations and describe transformations

of energy.of energy. ExplainExplain the law of conservation of the law of conservation of

energy.energy. DiscussDiscuss where energy goes when it where energy goes when it

seems to disappear.seems to disappear. AnalyzeAnalyze the efficiency of machines.the efficiency of machines.

Chapter 12


BellringerBellringerYou give yourself and your sled You give yourself and your sled gravitational potential energy as you pull gravitational potential energy as you pull your sled to the top of a snowy hill. You your sled to the top of a snowy hill. You get on board your sled and slide to the get on board your sled and slide to the bottom of the hill, speeding up as you go.bottom of the hill, speeding up as you go.

Chapter 12


BellringerBellringer1.1. When does the sled have the most potential When does the sled have the most potential energy? When does it have the least potential energy? When does it have the least potential energy?energy?

2.2. Where does the sled have the most kinetic Where does the sled have the most kinetic energy? the least kinetic energy?energy? the least kinetic energy?

3.3. What happens to the relative amounts of What happens to the relative amounts of potential and kinetic energy as the sled slides down potential and kinetic energy as the sled slides down the hill? What happens to the total energy?the hill? What happens to the total energy?

4.4. After the sled reaches the bottom of the hill, it After the sled reaches the bottom of the hill, it coasts across level ground and eventually stops. coasts across level ground and eventually stops. What happened to the energy the sled had?What happened to the energy the sled had?

Chapter 12

1.1. When does the sled have the most potential When does the sled have the most potential energy? energy? At the top of the hillAt the top of the hill

When does it have the least potential energy? When does it have the least potential energy? bottombottom

2.2. Where does the sled have the most kinetic Where does the sled have the most kinetic energy? energy? Bottom of hillBottom of hill

the least kinetic energy? the least kinetic energy? Top of hillTop of hill

3.3. What happens to the relative amounts of What happens to the relative amounts of potential and kinetic energy as the sled slides potential and kinetic energy as the sled slides down the hill? down the hill?

The potential energy decreases & Kinetic Energy The potential energy decreases & Kinetic Energy increasesincreases

What happens to the total energy? What happens to the total energy? remains the remains the same.same.

4.4. After the sled reaches the bottom of After the sled reaches the bottom of the hill, it coasts across level ground the hill, it coasts across level ground and eventually stops. What and eventually stops. What happened to the energy the sled happened to the energy the sled had? had?

Transformed into heat by friction, Transformed into heat by friction, between sled and snowbetween sled and snow..


Energy TransformationsEnergy Transformations Energy readily changes from one form Energy readily changes from one form

to another.to another. Potential energy can become kinetic Potential energy can become kinetic

energy.energy.• As a car goes down a hill on a roller As a car goes down a hill on a roller

coaster, potential energy changes to coaster, potential energy changes to kinetic energy.kinetic energy.

Chapter 12

Kinetic energy can become potential Kinetic energy can become potential energy.energy.• The kinetic energy a car has at the bottom The kinetic energy a car has at the bottom

of a hill can do work to carry the car up of a hill can do work to carry the car up another hill.another hill.


Energy TransformationsEnergy Transformations

Mechanical energy can change to other Mechanical energy can change to other forms of energy.forms of energy.• Mechanical energy can change to Mechanical energy can change to

nonmechanical energy as a result of nonmechanical energy as a result of friction, air resistance, friction, air resistance, or other means.or other means.

Chapter 12


The Law of Conservation of The Law of Conservation of EnergyEnergy The law of conservation of energy The law of conservation of energy

states that states that energy cannot be created or energy cannot be created or destroyed.destroyed.

Energy doesn’t appear out of nowhere.Energy doesn’t appear out of nowhere.• Whenever the total energy in a system Whenever the total energy in a system

increases, it must be due to energy that increases, it must be due to energy that enters the system from an external source.enters the system from an external source.

Energy doesn’t disappear,Energy doesn’t disappear, but it can be but it can be changed to another form.changed to another form.

Chapter 12

Machines convert energy Machines convert energy However, You only get as much However, You only get as much

energy as you put into the energy as you put into the

system.system.


Efficiency of MachinesEfficiency of Machines Not all of the work done by a machine is Not all of the work done by a machine is

useful work.useful work.• A machine cannot do more work than the A machine cannot do more work than the

work required to operate the machine.work required to operate the machine.• Because of Because of frictionfriction, the work output of a , the work output of a

machine is always less than the work input.machine is always less than the work input.

Chapter 12

EfficiencyEfficiency is the ratio of useful work is the ratio of useful work out to work in. out to work in. • Efficiency is usually expressed as a Efficiency is usually expressed as a

percentage.percentage.• The efficiency of a machine isThe efficiency of a machine is a measure of a measure of

how much useful work it can do.how much useful work it can do.• Video- Efficiency 1.34 minVideo- Efficiency 1.34 min

EfficiencyEfficiency How well a machine converts work input How well a machine converts work input work work

output.output. Efficiency EquationEfficiency Equation Efficiency = WEfficiency = Wout out / W/ Win in x 100%x 100%

Work Input = 48 JWork Input = 48 J Work Output = 42 JWork Output = 42 J

6 J gone? Where did it go?_______6 J gone? Where did it go?_______

Efficiency= ????Efficiency= ????

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ResourcesChapter menu

Efficiency = 42/48 x 100%= 88%Efficiency = 42/48 x 100%= 88%


Efficiency of MachinesEfficiency of Machines Perpetual motion machines are impossible.Perpetual motion machines are impossible.

• Energy is always lost to friction or air resistance.Energy is always lost to friction or air resistance. Machines need energy input.Machines need energy input.

• Because energy always leaks out of a system, Because energy always leaks out of a system, every machine needs at least a small amount of every machine needs at least a small amount of energy input to keep going.energy input to keep going.

Chapter 12

FrictionFriction

No machine is 100% efficient due to No machine is 100% efficient due to friction.friction.

How to reduce friction:How to reduce friction:

Lubrication, smooth etc.Lubrication, smooth etc.Why do we put oil in car engines?Why do we put oil in car engines?

Video Clip


Math SkillsMath SkillsEfficiencyEfficiency A sailor uses a rope and an old, A sailor uses a rope and an old,

squeaky pulley to raise a sail that weighs squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? What is the efficiency of the pulley? Express your answer as a percentage.Express your answer as a percentage.

1. List the given and unknown values.1. List the given and unknown values.Given:Given: work inputwork input = 180 J = 180 J

useful work outputuseful work output = 140 J = 140 J

Unknown:Unknown: efficiencyefficiency = ? % = ? %

Chapter 12


Math SkillsMath Skills2. Write the equation for 2. Write the equation for

efficiency.efficiency. efficiency

useful work output

work input

3. Insert the known values into the 3. Insert the known values into the equation, and solve.equation, and solve.

140 J0.78

180 JTo express this as a percentage, multiply by 100

and add the percent sign, "%."

0.78 100% 78%

efficie

effici

ncy

ency

Chapter 12

Documents

Work and Energy Chapter 12 Table of Contents Section 1 Work, Power, and Machines Section 2 Simple Machines Section 3 What is Energy? Section 4 Conservation