Unit 2: Work, energy, & power

AOIAOI: approaches : approaches to learning.to learning.

U.QU.Q: : How can How can science help me science help me improve my improve my understanding of understanding of scientific scientific terminology & terminology & information?information?

Skills:Skills: TransferTransfer through the through the

formative and summative formative and summative tests.tests.

CollaborationCollaboration : :through through the design and carrying the design and carrying out of the investigationout of the investigation

Communication: Communication: Through essay writingThrough essay writing

ENERGY

Part of our everyday lives:Part of our everyday lives: Energetic peopleEnergetic people Food that is “full of energy”Food that is “full of energy” High cost of electric energyHigh cost of electric energy Risks of nuclear energyRisks of nuclear energy

Reference point

y

ENERGY

Energy:Energy:the amount of work a physical the amount of work a physical

system is capable of performing.system is capable of performing.

Energy can neither be created nor Energy can neither be created nor consumed or destroyedconsumed or destroyed

When anything happens in the When anything happens in the physical world, energy is physical world, energy is somehow involved.somehow involved.

Work

Definition:Definition: A measure of the change a force A measure of the change a force

produces:produces: ““The work done by a force acting on an The work done by a force acting on an

object is equal to the magnitude of the object is equal to the magnitude of the force multiplied by the distance through force multiplied by the distance through which the force acts”.which the force acts”.

FdW

Work Work is done…Work is done…

……by a force when the by a force when the object it acts on moves object it acts on moves

NO work is done by NO work is done by pushing against a pushing against a stationary wall.stationary wall.

Work IS done throwing a Work IS done throwing a ball because the ball ball because the ball MOVES while being pushed MOVES while being pushed during the throw.during the throw.

Work

Equation for work: Equation for work: In words:In words:

The direction of the force (F) is assumed to be The direction of the force (F) is assumed to be the same as the direction of the distance (d)the same as the direction of the distance (d)

A force perpendicular to the direction of motion A force perpendicular to the direction of motion of an object cannot do work on the objectof an object cannot do work on the object

FdW

acts force the

hich through wdistanceforce applied doneWork

The Joule joule (J)joule (J)

The SI unit of energyThe SI unit of energy Amount of work done by a force of one Amount of work done by a force of one

newton when it acts through a distance of newton when it acts through a distance of one meter:one meter:

Example:Example: Push a box 8 m across the floor with a force of

100 N performs 800 J of work:

m)(Nmeter .newton 1 (J) joule 1

JmNmNFdW 800800)8)(100(

Direction of Force When a force and the When a force and the

distance through which it distance through which it acts are parallel, the acts are parallel, the work done is equal to the work done is equal to the product of product of FF and and dd

If the forces are NOT If the forces are NOT parallel, work done is parallel, work done is equal to the product of equal to the product of dd and the projection of and the projection of FF in in the direction of the direction of dd..

Types of Energy

Kinetic – Energy of MotionKinetic – Energy of Motion Potential – Energy of PositionPotential – Energy of Position

Chemical Energy Chemical Energy Food converted to energy in our bodiesFood converted to energy in our bodies

Heat EnergyHeat Energy Heat from burning oil to make steam to drive turbinesHeat from burning oil to make steam to drive turbines

Electric EnergyElectric Energy Electricity turns motors in homes and factoriesElectricity turns motors in homes and factories

Radiant EnergyRadiant Energy Energy from the sunEnergy from the sun

velocity, vvelocity, v(ms(ms-1-1))

mass, m (kg)mass, m (kg)

Translational

Rotational

Kinetic energy= ½ mv2

Kinetic EnergyKinetic energy = the energy a body Kinetic energy = the energy a body

possesses possesses due to its motiondue to its motion

Kinetic Energy

Moving objects can exert forces on Moving objects can exert forces on other moving or stationary objectsother moving or stationary objects Kinetic energy depends on the mass Kinetic energy depends on the mass

and speed of a moving objectand speed of a moving object

221 mvKE Note that Note that vv22 factor means that factor means that

KEKE increases VERY rapidly with increases VERY rapidly with increasing speedincreasing speed

Note that Note that vv22 factor means that factor means that KEKE increases VERY rapidly with increases VERY rapidly with increasing speedincreasing speed

Kinetic Energy

ExampleExample Kinetic energy of a 1000kg car moving at 10 Kinetic energy of a 1000kg car moving at 10

m/s is 50kJm/s is 50kJ

( 50kJ of work must be done to start the car ( 50kJ of work must be done to start the car from a stop, or stop it when it is moving)from a stop, or stop it when it is moving)

Force on a Nail

When a hammer strikes a nail, the When a hammer strikes a nail, the hammer’s kinetic energy is converted into hammer’s kinetic energy is converted into work, which pushes the nail into the woodwork, which pushes the nail into the wood

Force on a Nail

Example:Example: Using a hammer with a 600g head moving at 4 Using a hammer with a 600g head moving at 4

m/s to drive a 5mm nail into a piece of wood, m/s to drive a 5mm nail into a piece of wood, what is the force exerted on the nail on what is the force exerted on the nail on impact?impact?

Nm

smkg

d

mvF

Fdmv

960)005.0(2

)/4)(6.0(

2

nailon done work headhammer of KE

22

221

Potential Energy

Energy possessed by a body due to its Energy possessed by a body due to its position or condition.position or condition.

Elastic potential EnergyElastic potential Energy

Gravitational Potential EnergyGravitational Potential Energy When a stone is dropped, it falls towards the When a stone is dropped, it falls towards the

ground, until it hits the groundground, until it hits the ground (if the ground is soft, the stone will make a small depression in (if the ground is soft, the stone will make a small depression in

the ground)the ground)

Gravitational Potential Gravitational Potential energy = the energy a body energy = the energy a body possesses possesses because of its because of its position relative to the position relative to the groundground

h mg

Gravitational Potential = mgh Energy

Potential Energy

Potential Energy Example

Potential energy of a car pushed off Potential energy of a car pushed off a 45m cliffa 45m cliff

Compare with amount of KE done by Compare with amount of KE done by a car moving at 30m/s (108 km/hr)a car moving at 30m/s (108 km/hr)

kJmsmkgmghPE 441)45)(/8.9)(1000( 2

Examples of Potential Energy

Examples are almost everywhereExamples are almost everywhere Book on the tableBook on the table Skier on the top of a slopeSkier on the top of a slope Water at the top of a waterfallWater at the top of a waterfall Car at the top of a hillCar at the top of a hill A stretched springA stretched spring A nail near a magnetA nail near a magnet

Potential Energy is Relative

Amount of Amount of potential energy is potential energy is a function of the a function of the relative height of relative height of the objectsthe objects

Gravitational PE is Gravitational PE is relativerelative

Power

The RATE of Doing Work…The RATE of Doing Work… Rate is the amount of work done in a Rate is the amount of work done in a

specified period of timespecified period of time The more powerful something is, the faster The more powerful something is, the faster

it can do workit can do work

interval time

donework

t

WPPower

Units of Power

Standard (SI) unit of power is the wattStandard (SI) unit of power is the watt

Example:Example: 500W motor can perform 500J of work in 1 s500W motor can perform 500J of work in 1 s … … or 250J of work in 0.5 sor 250J of work in 0.5 s … … or 5000J of work in 10 sor 5000J of work in 10 s

Watts are very small unitsWatts are very small units Kilowatts are used most commonlyKilowatts are used most commonly

(J/s) ndjoule/seco 1(W) watt 1

kW 1 W 1000 kilowatt 1

Conservation of Energy The Law of Conservation of Energy:The Law of Conservation of Energy:

Energy cannot be created or destroyed, Energy cannot be created or destroyed, although it can be changed from one although it can be changed from one form to another.form to another. This principle has the widest application to This principle has the widest application to

all scienceall science Applies equally to distant stars and Applies equally to distant stars and

biological processes in living cells.biological processes in living cells.

Conservation Principles

Conservation of energy is significant Conservation of energy is significant because, if the laws of nature…because, if the laws of nature…

……are the same at all times (past, present, and are the same at all times (past, present, and future), then energy must be conserved.future), then energy must be conserved.

Energy Transformations Most mechanical processes involve Most mechanical processes involve

conversions between KE, PE, and workconversions between KE, PE, and work A car rolling down a hill into a valleyA car rolling down a hill into a valley

PE at the top of the hill is converted into KE as the car PE at the top of the hill is converted into KE as the car rolls down the hillrolls down the hill

KE is converted to PE as the car rolls up the other sideKE is converted to PE as the car rolls up the other side

Total amount of energy (KE+PE) remains Total amount of energy (KE+PE) remains constantconstant

AA

BB

CC

DD EE

AA BB CCAll potential energy

(stops for an instant)P.E. = 10 JK.E. = 0 J

All potential energy(stops for an instant)

P.E. = 10 JK.E. = 0 J

All kinetic energy(greatest speed)

K.E. = 10 JP.E. = 0 J

DD EEPotential energy &

Kinetic energyP.E. = 5 J; K.E. = 5 J

Potential energy &Kinetic energy

P.E. = 4 J; K.E. = 6 J

Energy Transformations

Example Example

m = 4kg

5m

v

A parcel of mass 4 kg slides down a smooth curved ramp. What is the speed of the parcel when it reaches the bottom.

Top of ramp: all potential energyP.E. = mgh = 4 kg 10 ms-2 5 m = 200 J

Bottom of ramp: all kinetic energy (all P.E. has changed to K.E.)K.E. = ½ mv2 = 200 J½ 4 kg v2 = 200 J

v2 = 100 v = 10 ms-1

Example Example

16.2 m(h1)

11.2 m(h2)

9.0 m (h3)P

Q

R

What is the speed of the What is the speed of the rollercoaster at P, Q and rollercoaster at P, Q and R? R?

At P: P.E. = 0 JAt P: P.E. = 0 J K.E. = maximumK.E. = maximum K.E. = P.E. at the startK.E. = P.E. at the start ½ mv½ mv22 = mgh = mgh11

v = 18 msv = 18 ms-1-1 At Q: P.E. = mghAt Q: P.E. = mgh22

K.E. = loss in P.E.K.E. = loss in P.E. ½ mv½ mv22 = mgh = mgh11 – mgh – mgh22 = mg(h= mg(h11-h-h22)) v = 10 msv = 10 ms-1-1

At R: P.E. = mghAt R: P.E. = mgh33

K.E. = loss in P.E.K.E. = loss in P.E. ½ mv½ mv22 = mgh = mgh11 – mgh – mgh33 = mg(h= mg(h11-h-h33)) v = 12 msv = 12 ms-1-1

Rest Energy

Matter is a form of energyMatter is a form of energy Most important conclusion of special Most important conclusion of special

relativity theory is that matter and relativity theory is that matter and energy are closely relatedenergy are closely related Matter Matter Energy and Energy Energy and Energy Matter Matter

Rest EnergyRest Energy The energy equivalent of an objects massThe energy equivalent of an objects mass

2

200

light) of dmass)(spee(rest energy Rest

cmE

Albert Einstein (1879-1955)

Left school at 16 to work in the Swiss patent Left school at 16 to work in the Swiss patent office (his math teacher called him a “lazy” dog)office (his math teacher called him a “lazy” dog)

Developed 3 papers that would revolutionize Developed 3 papers that would revolutionize physics and modern civilization:physics and modern civilization: Wave and particle theory of lightWave and particle theory of light Brownian motion of particlesBrownian motion of particles Introduction of the theory of relativityIntroduction of the theory of relativity

In 1919, his predictions on gravitational effects In 1919, his predictions on gravitational effects on light were proven…became a world celebrityon light were proven…became a world celebrity

Left Germany in 1933 and spent rest of his life Left Germany in 1933 and spent rest of his life at Princeton Universityat Princeton University Searched for a “unified field theory” that would Searched for a “unified field theory” that would

relate gravitation and electromagnetism.relate gravitation and electromagnetism.

Energy and Civilization The rise of modern civilizationThe rise of modern civilization

Impossible without vast resources of Impossible without vast resources of energyenergy Development of ways to convert energy formsDevelopment of ways to convert energy forms

Most convenient fuels are limitedMost convenient fuels are limited Oil, natural gas, and coalOil, natural gas, and coal

Other sources of energy have various Other sources of energy have various problemsproblems

Population increasing, as is demand for Population increasing, as is demand for energyenergy

Energy Demand and Type

The Energy Problem

Limited Supply, Unlimited DemandLimited Supply, Unlimited Demand The sun – source of most of our energyThe sun – source of most of our energy

Food, wood, plantsFood, wood, plants Water power – The hydrological cycleWater power – The hydrological cycle Wind power – Temperature changesWind power – Temperature changes Fossil FuelsFossil Fuels

Nuclear and hydrothermal powerNuclear and hydrothermal power Not related to the sunNot related to the sun

Solar Cells

Variation due to climate and latitudeVariation due to climate and latitude $70/watt in 1960, $3/watt today$70/watt in 1960, $3/watt today Economics still limit widespread Economics still limit widespread

applicationapplication

Fossil Fuels Limited SupplyLimited Supply

Most large deposits of oil and gas Most large deposits of oil and gas foundfound

Remaining reserves = 100 years??Remaining reserves = 100 years?? No new deposits being formedNo new deposits being formed

Problems with coalProblems with coal Mining needed to extract from earthMining needed to extract from earth Air pollution – dangerous to healthAir pollution – dangerous to health

All Fossil FuelsAll Fossil Fuels Adds COAdds CO2 2 to atmosphere – to atmosphere –

greenhouse effectgreenhouse effect

Hydroelectric Power Kinetic energy of falling Kinetic energy of falling

water converted into water converted into electricity using electricity using turbinesturbines New hydro projects New hydro projects

unlikely due to unlikely due to environmental and land-environmental and land-use constraintsuse constraints

Two-sided argumentsTwo-sided arguments Environmental concernsEnvironmental concerns Development concernsDevelopment concerns

Wind Energy AdvantagesAdvantages

Non-pollutingNon-polluting Don’t contribute to Don’t contribute to

global warmingglobal warming Renewable resourceRenewable resource

DisadvantagesDisadvantages Only work where winds Only work where winds

are powerful and reliableare powerful and reliable Take up a lot of spaceTake up a lot of space Noisy, some Noisy, some

environmental concernsenvironmental concerns

Other Energy Sources

Geothermal EnergyGeothermal Energy Nuclear EnergyNuclear Energy Tidal EnergyTidal Energy

Future Energy Supplies

Fusion EnergyFusion Energy Technology may be many years into the Technology may be many years into the

futurefuture Most alternate energy sources are Most alternate energy sources are

very expensivevery expensive Cost of fossil fuels is still the lowest and Cost of fossil fuels is still the lowest and

easiest to distributeeasiest to distribute