Unit 3: Energy Energy can change from one form to another without a net loss or gain

Unit 3: Energy

Energy can change from one form to another without a net loss or gain.

Work:

– transfer of energy through motion

– force applied over a distance

(no move, no work)

– unit of measure is joules (J)

W = FdW: work (J)

F: force (N)

d: distance (m)

Work

Name 2 ways to increase work…

more force or more distance

energy in

golfer

energy in

club

energy in

ball

golfer does work on club

club does work on ball

Work is the transfer of energy by applying force over a distance.

Work

How much work is the lifter doing while holding it above her head?

NONE!!because…

The force applied is over zero distance (no

motion)

W = F x d

Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back?

W = (30 N)(1.5 m)

W = 45 J

W = Fd

Work

F = 30 N

d = 1.5 m

Sometimes work is done against another force.

• An archer stretches her bowstring, doing work against the elastic forces of the bow.

• When any object is lifted up,work is done against theforce of gravity.

• When you do push-ups,you do work against yourown weight.

Work

Power:

– rate at which work is done

– unit of measure is watts (W)

P: power (W)

W: work (J)

t: time (s)Name 2 ways to increase power…

more work or

Power

P = W

t

less time (faster)

When carrying a load up some stairs, you do the same amount of work whether you walk or run up the stairs.

Power

Rock climbers do a lot of work at a slow rate.

Their power is small.

A powerful lineman is STRONG (applies a big force) and FAST (moves objects in small times).

A figure skater lifts his 450 N partner 1.0 m in 3.0 s. How much power is required?

W = (450 N)(1.0 m) =

P = 450 J 3.0 s

Power

W = FdP = W

t450 J

P = 150 W

F = 450 N

d = 1.0 m

t = 3.0 s

A high-power engine does work rapidly.

• An engine with twice the power of another engine does not necessarily produce twice as much work or go twice as fast.

• Twice the power means the engine can do twice the work in the same time, or the same work in half the time.

• A powerful engine can get an automobile up to a given speed in less time than a less powerful engine can.

Power

1. Raising a car up in a service station requires work. Raising it twice as high requires…

A. half as much work.

B. the same work.

C. twice the work.

D. four times the work.

Quick Quiz!

2. Raising an auto in a service station requires work. Raising it in half the time requires

A. half the power.

B. the same power.

C. twice the power.

D. four times the power.

Quick Quiz.

When work is done by an archer in drawing back a bowstring... something has been acquired that enables the object to do work.

It may be in different forms:• compression of atoms in an object• a separation of attracting bodies• rearrangement of electric charges in the molecules of a substance.

Forms of Energy

Energy:

the ability to do work

transfer of energy through motionunit of measure is joules (J)

Forms of Energy

Mechanical Energy: energy due to the position or the movement of something.

Forms of Energy

Which boulder

has greater gravitational PE?

Potential Energy (PE)

stored energydepends on position

Elastic PE – stretched

Chemical PE – in bonds

Gravitational PE –

height & weightPE = mgh

The potential energy of the 100 N boulder with respect to the ground below is 200 J in each case.

a. lifted with 100 N of force

Forms of Energy

The potential energy of the 100 N boulder with respect to the ground below is 200 J in each case.

a. lifted with 100 N of force

b. pushed up a 4 m incline with 50 N of force

Forms of Energy

The potential energy of the 100 N boulder with respect to the ground below is 200 J in each case.

a. lifted with 100 N of force

b. pushed up a 4 m incline with 50 N of force

c. lifted with 100 N of force up 4 stairs, 0.5 m each

Forms of Energy

same PE = mghsame W = Fd

think!

You lift a 100 N boulder 1 m.

a. How much work is done on the boulder?

W = Fd = 100 N·m = 100 J

b. What power is expended if you lift the boulder in a time of 2 s?

P = 100 J / 2 s = 50 W

c. What is the gravitational PE of the boulder in the lifted position?

PE = mgh PE = (100 N)(1 m)

Relative to its starting position, the boulder’s PE is 100 J.

Forms of Energy

Forms of Energy

Kinetic Energy (KE)

energy of motion

depends on mass and velocity

KE = ½mv2

What happens to KE if you double the mass?

What happens to KE if you double the velocity?• It takes four times the work to double the speed. • An object moving twice as fast takes four times

as much work to stop.

2x

4x

KE = ½m(2v)2

KE = ½m(4)v2

PE KEKE WORK

Forms of Energy

Energy:

the ability to do work

transfer of energy through motion

unit of measure is joules (J)

PE KE

Forms of Energy

PE KE

Forms of Energy

PE KE Pendulum

Forms of Energy

greatest gravitational PE?

greatest elastic PE?greatest KE?

Forms of Energy

Kinetic energy often appears hidden in different forms of energy, such as heat, sound, light, and electricity.

• Random molecular motion is sensed as heat. • Sound consists of molecules vibrating in patterns. • Light energy originates from the motion of

electrons within atoms. • Electrical energy is electrons in motion making

electric currents.

Forms of Energy

PE KEKE WORKIn what other forms can energy be stored

or transferred?

Forms of Energy

Energy:

the ability to do work

transfer of energy through motion

unit of measure is joules (J)

NUCLEAR

THERMAL

motion of electric chargesPE stored in bonds

motion of objects

(PE+KE)

random motion of particles (heat) MECHANICAL

ELECTRICALCHEMICAL

joules (J)

radiated as waves (light)

ELECTROMAGNETIC

The ability to do work.

stored in the nucleus

ENERGY

Forms of Energy

Forms of Energy

1. The energy due to the position of something OR the energy due to motion is called…

A. potential energy.

B. kinetic energy.

C.mechanical energy.

D.conservation of energy.

Quick Quiz!

2. After you place a book on a high shelf, we say the book has increased

A. elastic potential energy.

B. chemical energy.

C. kinetic energy.

D. gravitational potential energy.

Quick Quiz.

3. An empty truck traveling at 10 km/h has kinetic energy. How much kinetic energy does it have when it is loaded so its mass is twice, and its speed is increased to twice?

A. the same KE

B. twice the KE

C. four times the KE

D.more than four times the KE

Quick Quiz.

To change the kinetic energy of an object, work must be done on the object.

Work is required to start an object moving, or to bring it to rest.

work-energy theorem:

work equals the change in kinetic energy

Conservation of Energy

Work = ∆KE

Fd = ∆½mv2

If there is no change in an object’s kinetic energy, then no net work was done on it.

Push against a box on a floor.

• If it doesn’t slide, then you are not doing work on the box.

• On a frictionless floor, the work of your force times the distance appears as kinetic energy of the box. Fd = ∆½mv2

Conservation of Energy

KE = 0F ½mv2

d

(∑F)d = ∆½mv2

Conservation of Energy

½mv2F ½mv2

d

• With friction, the net force of your push minus the friction force is multiplied by distance to give the gain in kinetic energy.

• If the box moves at a constant velocity, the net force and net work are zero, and, according to the work-energy theorem, ∆KE = 0. (no change in kinetic energy)

f f

∑F = 0

Conservation of Energy

A car moving at twice the velocity of another has four times as much kinetic energy, and will require four times as much work to stop.

Fd = ∆½mv2

A car moving at twice the velocity of another has four times as much kinetic energy, and will require four times as much work to stop.

The friction force is nearly the same for both, so the faster car takes four times as much distance to stop.

Conservation of Energy

Fd = ∆½m(2v)2

F(4)d = ∆½m(4)v2

Fd = ∆½mv2

2 x v gives 4 x d

Conservation of Energy

A car moving at twice the velocity of another has four times as much kinetic energy, and will require four times as much work to stop.

The friction force is nearly the same for both, so the faster car takes four times as much distance to stop.

4 x v gives 16 x d

Fd = ∆½m(4v)2

F(16)d = ∆½m(16)v2

Fd = ∆½mv2

think!A friend says that if you do 100 J of work on a moving cart, the cart will gain 100 J of KE. Another friend says this depends on whether or not there is friction. What is your opinion of these statements?

Careful. Although you do 100 J of work on the cart, this may not mean the cart gains 100 J of KE. How much KE the cart gains depends on the net work done on it.

Conservation of Energy

(∑F)d = ∆½mv2

think!When the brakes of a car are locked, the car skids to a stop. How much farther will the car skid if it’s moving 3 times as fast?

9 times farther.

The car has nine times as much kinetic energy when it travels three times as fast:

Conservation of Energy

Conservation of Energy

Open System:exchange matter and energy with surroundings(cup of hot coffee)

Closed System:exchange only energy with surroundings(cup of hot coffee with a lid)

Isolated System:NO exchange of matter or energy with surroundings(insulated cup of hot coffee)

Law of Conservation of Energytotal energy in an isolated system (universe)does not change

energy cannot be created or destroyed, butmay change forms (conversion)

Conservation of Energy

open system (car)

isolated system (universe)

When the woman leaps from the burning building, the sum of her PE and KE remains constant at each successive position all the way down to the ground.

Conservation of EnergyPE = 10,000 JKE = 0 J

PE = 7500 JKE = 2500 J

PE = 5000 JKE = 5000 J

PE = 2500 JKE = 7500 J

PE = 0 JKE = 10,000 J

PE + KE = 10,000 J

PE KE

Conservation of Energy

energy may change forms (conversion)

List the energy converting steps in lighting a match:

1) muscles use chemical E to move the match, KE

2) friction converts KE of match to thermal E

3) thermal E triggers rxn releasing chemical E

4) chemical E converted to more thermal E and electromagnetic E in heat and light of the flame

Conservation of Energy

energy may change forms (conversion)

1. The work-energy theorem states that the work done on or by a system is equal to …

A. its potential energy.

B. its total mechanical energy.

C. its change in kinetic energy.

D. its change in an open system.

Quick Quiz!

Work = ∆KE

Fd = ∆½mv2

2. When the brakes of a car are locked, the car skids to a stop. How much less will the car skid if it’s moving half as fast?

A. 1/2 as far

B. 1/4 as far

C. 1/8 as far

D. 1/16 as far

Quick Quiz.

Fd = ∆½m(1/2v)2

F(1/4)d = ∆½m(1/4)v2

Fd = ∆½mv2

3. How is an open system similar to a closed system? They both…

A. exchange matter in and out.

B. exchange energy in and out.

C. exchange matter and energy in and out.

D. are identical to an isolated system.

Quick Quiz.

4. When your car engine burns gasoline to move the car, it is converting _____ energy into ______ energy.

A. mechanical, chemical

B. electrical, mechanical

C. potential, thermal

D. chemical, mechanical

Quick Quiz.

Energy Resources

Nonrenewable–limited

amounts

–cannot be replaced

Energy Resources

Fossil Fuels (pollution)

oOiloCoaloNatural Gas

Uranium (no air pollution)

o4.5 billion yrs

Nonrenewable–limited

amounts

–cannot be replaced

Energy Resources

FOSSIL FUELS

microscopic

FOSSIL FUELS

Renewable (green)–replaceable

in short time

Energy Resources

oSolaroWindoGeothermaloHydroelectric (water)

oBiomassoHydrogenfuel cell

Energy Resources

SOLAR

Electromagnetic energy

Energy Resources

Thermal

energy

WIND

Mechanical energy

Energy Resources

GEOTHERMALEnergy from

Earth’s heat.

Thermal energy

Energy Resources

HYDROELECTRIC (WATER)

Energy from the flow of water.

Mechanical energy

(gravitational

PE → KE)

cheapest

Energy Resources

BIOMASSEnergy from burning

organic or living matter.

Chemical energy

Energy Resources

HYDROGEN FUEL CELLEnergy from reacting

H2 & O2

Chemical energy

What is the “waste” for hydrogen fuel

cells?

Energy Resources

Renewable = 7 % Nonrenewable = 93 %

HMMMM....

If burning fossil fuels create pollution, why do you think we use

them so much (85%)?

big supplycheap

Energy Resources

Conservation–using less

energy

or

–using it more efficiently

Energy Resources

Why would you pay more for an efficient air

conditioner if it cost more than a less

efficient one?

saves moneyover time

HMMMM....Energy Resources