Chapter 3 outline: Main Ideas

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Chapter 3 outline: Main Ideas Work and Power Energy

Kinetic Energy Potential Energy Conservation of Energy

Momentum Linear Momentum Angular Momentum

Energy and Civilization

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Work We have seen that FORCE is the

physical phenomenon behind motion and changes in motion

WORK is a way to determine the measure of CHANGE caused by a force

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Definition of WORK:

Work=Force x Distance This means that something has to move The force must be the force along the direction of motion UNIT: Joule = Nm Other Unit: foot-pound (English System)

Energy: same as work UNIT: Joule

Power: Energy / time UNIT: WATT

Not the usual definition

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Mathematically Work is expressed in a simple

equation:

Where the Force and the distance through which the object is moved are along the same direction

FdW =

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Special Case: Work against gravity Whenever any object is lifted, work must be done

against the gravitational force in order to move the object

To determine the magnitude of this work, we begin with the definition of the force due to gravity on an object of mass m, also called the WEIGHT of the object:

And represent the distance moved (upward or downward) with the letter h in the WORK equation:

mgF =

hmgdFW )(==

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Reminder: Newton 2 According to Newton’s second law:

FORCE is defined as mass x acceleration Because of the relationship between

WORK and ENERGY,

Whenever energy is being expended,

a force is being exerted.

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POWER Power is a measure of the RATE at

which work is accomplished Defined by

The unit of power: WATT (W) 1 W = 1 J/s

t

WP =

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Definition of ENERGY

ENERGY CAN BE THOUGHT OF

AS

“THE ABILITY TO DO WORK”

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Two Main Kinds of Energy

The energy of motion ONLY MOVING OBJECTS have kinetic

energy

The energy of positionMany different types of potential energy

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KINETIC ENERGY (KE) The energy of MOTION SIMPLE definition

2(velocity)mass2

1KE ××=

NO VELOCITY NO KELARGER MASS LARGER KE

LARGER VELOCITY LARGER KE

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Discussion Example

Which object has the greatest amount of kinetic energy?

A. 200 g baseball traveling at 50 m/sB. 4 kg bowling ball traveling at 7

m/sC. 2000 kg truck sitting in the

parking lot

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Do the Math, using our definition of KE

A. 200 g baseball traveling at 50 m/s KE = (1/2) x 0.200 (kg) x [50 (m/s)]2

= (1/2) x (0.200) x (2500) JOULES = 250 Joules

B. 4 kg bowling ball traveling at 7 m/sKE = (1/2) x 4 (kg) x [7 (m/s)]2

= (1/2) x (4) x (49) JOULES = 98 Joules

C. 2000 kg truck sitting in the parking lotKE = (1/2) x 2000 (kg) x [0 (m/s)]2

= (1/2) x (4) x (0) JOULES = 0 Joules

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Potential Energy due to Gravity An object possesses Potential energy

due to its position in the gravitational field. On Earth

we define gravitational potential energy in terms of the distance of the object from the ground.

Potential energy of an object due to gravity is the

gravitational potential energy = m x g x h

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Discussion ExampleYour old car has stalled again.

Your friends push it to the top of a hill and it rolls down the other side.

Half way down the other side the engine starts. At what point did your car have greatest potential energy?

Gravitational PE mass height off the ground

So, the car has the greatest gravitational PE when it is at the TOP OF THE HILL

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Discussion ExampleTwo cars of equal mass are on the same street. The

street is level. One car is driving with a velocity of 20 km/hr due north while the other car is parked.

Which car has more gravitational potential energy?A. Both cars have the same gravitational potential energyB. The parked car has more gravitational potential

energyC. The moving car has more gravitational potential

energyD. The heavier car has the most gravitational potential

They are the same! Gravitational Potential energy

depends only on the distance from the ground, not on the object’s

motion.

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Transformation of Energy

CONSERVATION OF ENERGY

Energy may change form from one type to another, BUT

THE TOTAL AMOUNT OF ENERGY IN THE UNIVERSE

DOES NOT CHANGE

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SIMPLE PENDULUM

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The simple PENDULUM

During the motion of a swinging pendulum, the energy is constantly changing from

KE (kinetic energy) to PE (potential energy)

KE is MAX at lowest point KE is MIN at the top of the path (v - 0) PE is MAX at top of path

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EXAMPLE: Energy TransferA roller coaster (unmotorized) car

starts coasting at the top of one hill, rolls down into a valley and then comes to a stop at the top of a neighboring hill.

The top of the neighboring hill is at a lower

elevation than the starting point. The transfer of energy in this case is potential to kinetic and heat and back to potential and heat.

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Other FORMS of ENERGY Thermal Energy Elastic Potential ENERGY (Springs) Chemical Potential Energy Wave Energy Mass Energy

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Thermal Energy James P. JOULE discovered that

HEAT is a form of ENERGY Two objects with the same temperature have the

same amount of thermal energy. Common unit of HEAT ENERGY

calorieOR

CALORIE HEAT energy is EVER PRESENT!!

Whenever energy is being expended, heat is always produced.

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The Heat Unit: Calorie or Calorie CALORIE (with a big C):

amount of energy needed to raise the temperature of one kilogram of water

by one degree Celsius.

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Other types of potential energy Elastic potential energy

Springsrubber bands

CHEMICAL Potential EnergyEnergy in a batteryEnergy in any combustible

material

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But Wait there’s more… Wave Energy

Energy can take the forms of waves, including sound waves and light waves.

Sound Waves Sound wave energy causes kinetic energy of

molecules in the air or some other medium. Sound waves are longitudinal waves. Sound waves are transmitted by molecular

motion. Sound waves are created by a vibrating object.

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What is HEAT??Heat is the transfer of energy

NOTE: we cannot say that matter “contains”

heatrather, matter contains energy

The amount of energy matter containscan be change by transfer of heat

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POWER = ENERGY PER TIME OR

Power Unit? unit for energy = Joule Unit for time = second So, unit for power = Joule / second

Joule/second = WATT

TIME

ENERGYPOWER =

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ENERGY and POWER New Definition:

Power = Energy / Time This means

Energy = Power x Time

so, another valid unit for ENERGY is Watt x second OR Watt x hour

OR SOMETHING EVEN MORE FAMILIAR Kilowatt x hour

This is the UNIT OF ENERGY

on your ELECTRICITY BILL

(kWh)

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Watt about Horsepower??? Horsepower is a unit invented by

JAMES WATT so that he could impress people about the

strength of his steam engine

one horsepower = 550 ft-lb per second = 746 watts

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More about describing motion

MOMENTUM

A quantity that helps to describe the effect of an object’s motion

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Momentum Momentum is a quantity which

tells us more about the motion than speed or position

Usually, linear momentum is denoted by the letter “p”

p = m x v

MOMENTUM

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Discussion/ Thought A kid rolling down the street on a

skateboard...

or

a large truck at rest?

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Discussion QuestionA student on a bike is traveling down a street with a velocity of 10 m/s toward the south. She passes a delivery van parked on the side of the road. Compare the momentum of the biker with that of the

van.

A) The van has more momentum than the biker due to its much larger mass.

B) The biker has more momentum than the van because the biker has a non-zero velocity.

c) The van and the biker have about the same momentum

D) None of the above

v=0

v->

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Collisions-a way to use momentum Collisions provide a way to better understand

motion. Two main classes of collisions

Elastic collision Inelastic collision

Elastic collision: objects not deformedbouncing balls, bumper cars...Inelastic collision: objects become permanently deformedauto accidents, throwing pie in someone’s face

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Conservation of Momentum When outside forces are not present, the

combined total momentum of objects in motion does not change This is true for all motion

Momentum can be EXCHANGED during the interaction of particles in motion, but the system as a whole can not gain or lose momentum

So, for any interaction:MOMENTUM BEFORE = MOMENTUM AFTER

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HUMAN ENERGY USE Humans convert CHEMICAL ENERGY,

stored in food substances, into KINETIC ENERGY, present whenever our muscles are at work.

If we take in more calories than we expend, our body stores energy as fat to make up the difference.

Estimate the amount of power given off each day by a person consuming 1500 KCal per day on a diet which stores no food as fat (all energy is expended)

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FIGURE IT OUT!!POWER = ENERGY / TIME

Find the energy (Joules) 1500 CALORIES = 1500 kilocalories

# Joules(J) = 4.184 x Calories Energy (J) = 4.184 x 1500 x 103 calories Energy = 6,276,000 Joules

TIME = # SECONDS IN ONE DAY24 hours x 3600 seconds/hour = 86,400 s

POWER = ENERGY / TIME = 6,276,000 Joules / 86,400 s

= 73 WATTS

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Energy and Civilization The source of all fuels used for energy on

our planet is the SUN All food sources and fuel sources were at

one time given energy via the radiation that falls on our planet from the sun

Today, the majority of our energy is provided by fossil fuel sources These sources are ‘finite’

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Fossil Fuels PETROLEUM PRODUCTS COAL NATURAL GAS

All are carbon based All are rich in chemical potential energy.

The are classified as nonrenewable resources. The natural processes that form fossil fuels still

happen on Earth- but at a much slower rate than previously

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Fossil Fuels USE For industrial nations fossil fuels account

for Ninety percent of the total fuel consumption

Where did the energy stored in fossil fuels come from originally? The energy stored in fossil fuels had to come

from somewhere…. originally in The Sun (think about it...)

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SOLAR ENERGY An example of a RENEWABLE energy

resource (sort of) BIG problem: STORAGE

Battery technology still has a long way to go With poor means of storage, Solar Energy is

available ONLY during times when the sun is shining

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Energy: Summary Work is defined as a measure of the change

caused by a force The unit of work is JOULE (N m)

Power is the rate of work done The unit of power is WATT (W)

Energy is the property of something to do work The unit of energy is JOULE Energy occurs in several forms: kinetic, potential,

chemical and rest energy Linear momentum is a measure of the tendency of

a moving object to stay in motion: p = mv

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Energy: Summary(2) Angular momentum is measure of the

tendency of a rotating body to keep rotating All momentum is conserved The special theory of relativity defines the

rules for measurements when there is relative motion between the observer and the object to be measured

General relativity relates gravity to the structure of spacetime

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In-Lecture Quiz Chapter 3 If you touch the desktop, and then touch the

metal legs of the desk, one will “feel” colder. Which object has a higher temperature?

A. Metal Leg

B. Desktop

C. They are the same

D. Not enough information to tell

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In-Lecture Quiz Chapter 3

Name a unit that could be used to measure HEAT.

A. Newton

B. Joule

C. Calorie

D. B and C

E. A, B and C

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In-Lecture Quiz Chapter 3In which of the following cases is

WORK DONE?

A. You try to pull your car out of a ditch, but it does not move.

B. Your room-mate kicks his dirty clothes into the corner of his room.

C. Your mother holds a sleeping baby.

D. You sit at your desk and study physics all night.

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In-Lecture Quiz Chapter 3 Suppose you climb a flight of

stairs to an elevation of 4 meters.

How much work did you do?A. No Work (0 Joules)

B. It depends on how much you weigh

C. It depends on how wide the stairs are

D. It depends on whether you are wearing your favorite shoes

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In-Lecture Quiz Chapter 3A driver increases his velocity from 50 km/hr to 150 km/hr. How much does

he increase the kinetic energy of the car with this increase in speed? A. Kinetic energy is 3 times greater B. Kinetic energy is 6 times greater C. Kinetic energy is 9 times greater D. Kinetic energy is 12 times greater

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In-Lecture Quiz Chapter 3 You realize that you have left your shoes in

your car. You must walk outside with bare feet. It is a HOT sunny afternoon. You have the choice of (A) walking on the grass or (B) walking on the sidewalk.

Think about which one you choose?

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In-Lecture Quiz Chapter 3In a simple pendulum system, we know that

energy is continuously converted from potential to kinetic. At which point in the path of the

pendulum is the kinetic energy greatest?A. Top of path (highest point)B. Bottom of path (lowest point)C. Somewhere in between A & B,

like point CD. It depends on the mass

A

B C

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In-Lecture Quiz Chapter 3 An object which is AT REST can

have:A. A non-zero Kinetic energy

B. A non-zero Gravitational Potential Energy

C. A non-zero Momentum

D. non-zero Mass

E. non-zero Veloctiy

NOTE: There may be more than one choice…

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In-Lecture Quiz Chapter 3

When two objects collide in a perfectly elastic collision, which of the following is true?

A. A. Only momentum is conservedB. Only energy is conservedC. Both Energy and Momentum are conserved

Energy and What are equivalent in Eisenstein’s` famous equation?

A. The speed of light B. Mass C. Momentum

Chapter 3 outline: Main Ideas

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