General Physical Science

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General Physical Science

Chapter 4

Chapter 4 Work and Energy

Work and Energy

Work

The

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Work

Scalar quantity

–

–Magnitude but no directionMagnitude but no direction

SI base unit = joule (J) SI base unit = joule (J)

–

–Units are NUnits are N--m or kg mm or kg m22_{/ s}_{/ s}22

English

English --ftft--poundpound Work is

Work is againstagainstanother force.another force.

Work

Lifting

–

–apply a force against gravityapply a force against gravity –

–W = F * d (W = F * d (FdFd))

F = mg

Distance measured as height (against gravity) (h)

–

–W = W = mghmgh

Work against friction Work against friction

–

–constant velocity means net force = 0constant velocity means net force = 0

Work

Learning Goals

–

–Define work.Define work. –

–List the units for workList the units for work

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Kinetic and Potential Energy

Work done on an object changes stuff. Work done on an object changes stuff.

–

–Changes speedChanges speed –

–Changes heightChanges height –

–Produces heatProduces heat

All are changes in All are changes in energy.energy.

Energy is the ability to do work Energy is the ability to do work

–

–Work Work bybya system decreases energy.a system decreases energy. –

–Work Work onona system increases energy.a system increases energy.

Work is the process by which energy is transferred

from one object to another.

Energy

Work and energy have the same units! Work and energy have the same units! Many forms of energy

Many forms of energy

– –ElectricalElectrical – –ChemicalChemical – –LightLight –

–many othersmany others

Focus here is on

Focus here is on mechanicalmechanicalenergy.energy.

Mechanical Energy

Two basic forms of mechanical energy Two basic forms of mechanical energy

–

–Kinetic energyKinetic energy

energy of motion

E

Ekk= 1/2 mv= 1/2 mv22

work done on a moving object, must calculate

energy before and after, not just

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School Zone Speed Limits

Example

A 1.0 kg ball is fired from a cannon. What A 1.0 kg ball is fired from a cannon. What is the change in the ball

is the change in the ball’’s kinetic energy s kinetic energy when it accelerates from 4.0 m/s to 8.0 when it accelerates from 4.0 m/s to 8.0 m/s? m/s? Kinetic energy at 4.0 m/s Kinetic energy at 4.0 m/s – –EE_k_k= 1/2 m v= 1/2 m v22 – – = 1/2 x 1.0 x 4.0= 1/2 x 1.0 x 4.022 – – = 8.0 J= 8.0 J

Example

A 1.0 kg ball is fired from a cannon. What A 1.0 kg ball is fired from a cannon. What is the change in the ball

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Mechanical Energy

Potential energy

–

–energy of positionenergy of position –

–Assuming position in gravitational field, then Assuming position in gravitational field, then potential energy is the mass times g times the

potential energy is the mass times g times the

height of the object.

–

–EE_p_p= = mghmgh

Work against gravity: Work against gravity:

–

–//\\EE_p_p= mg= mg//\\hh

Potential Energy

Height is independent of the path used to Height is independent of the path used to obtain the height!

obtain the height! ‘

‘HeightHeight’’(actually (actually //\\h) can be positive or h) can be positive or negative.

negative.

Other types of potential energy Other types of potential energy

– –SpringSpring – –BowBow – –etc.etc.

Kinetic and Potential Energy

Learning Goals

–

–Explain the relationship between Explain the relationship between workworkand and

energy.

energy. –

–Define and distinguish Define and distinguish kinetickineticand and potentialpotential

energies.

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Conservation of Energy

The total energy of an isolated system The total energy of an isolated system remains constant.

remains constant.

Conservation of mechanical energy Conservation of mechanical energy

– –((EE_kk+ E+ Epp))t1t1= (= (EEkk+ E+ Epp))t2t2 – –(1/2 mv(1/2 mv22_{+ mgh)}_{+ mgh)} t1 t1= (1/2 mv= (1/2 mv22+ mgh)+ mgh)t2t2

Example

A 0.10 kg stone is dropped from a height of

10 m. What will the kinetic and potential

energies of the stone be at 10.0 m, 7.0 m, 3.0

m, and 0.0 m. Be sure to neglect air

resistance. resistance. At 10 m At 10 m – –EEpp= = mghmgh= (0.10)(9.8)(10.0) = 9.8 J= (0.10)(9.8)(10.0) = 9.8 J –

–EEkk= 0J; therefore total energy = 9.8J + 0J = 9.8J= 0J; therefore total energy = 9.8J + 0J = 9.8J

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Example

Total energy = 9.8J Total energy = 9.8J At 3.0 m At 3.0 m – –EE_p_p= = mghmgh= (0.10)(9.8)(3.0) = 2.9J= (0.10)(9.8)(3.0) = 2.9J – –E = E = EE_pp+ + EEkk – –9.8J = 2.9J + x9.8J = 2.9J + x – –x = 9.8J x = 9.8J --2.9J = 6.9J2.9J = 6.9J

Example

Total energy = 9.8J Total energy = 9.8J At 0.0 m At 0.0 m – –EE_pp= = mghmgh= (0.10)(9.8)(0.0) = 0.0J= (0.10)(9.8)(0.0) = 0.0J – –E = E = EE_pp+ + EEkk – –9.8J = 0.0 + x9.8J = 0.0 + x – –x = 9.8J x = 9.8J --0.0J = 9.8J0.0J = 9.8J

Speed of Falling Object

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Conservation of Energy

Learning Goals

–

–Explain the relationship between Explain the relationship between workworkand and

energy

energy..

–

–Define and distinguish Define and distinguish kinetickineticand and potentialpotential

energies. energies. Questions: 12 Questions: 12 --1818 Exercises: 17, 19 Exercises: 17, 19

Power

Power is the time rate of doing work Power is the time rate of doing work

–

–More power, more work for given time.More power, more work for given time. –

–More power, fixed amount of work in shorter More power, fixed amount of work in shorter time. time. – –P = W / tP = W / t – –P = P = FF··dd/ t/ t

Units of power the Units of power the wattwatt..

–

–British British ––horsepowerhorsepower

Units summarized in Table 4.3 Units summarized in Table 4.3

Example

A constant force of 150 N is used to push A constant force of 150 N is used to push a student

a student’’s motorcycle 10 m along a flat s motorcycle 10 m along a flat road in 20 s. Calculate the power in watts. road in 20 s. Calculate the power in watts.

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Power

Electric bill Electric bill Kilowatt Kilowatt --hrhr –

–Unit of energyUnit of energy –

–E = P x tE = P x t

Table 4

Table 4--4 gives typical power consumption 4 gives typical power consumption of typical household appliances.

of typical household appliances.

Power

Learning Goals

–

–Define Power and itDefine Power and it’’s units.s units. –

–Distinguish between electrical power and Distinguish between electrical power and electrical energy.

electrical energy.

Questions: 19 Questions: 19 --2323 Exercises: 21 Exercises: 21--25 odd25 odd

Forms of Energy and Consumption

Must understand that all energy is Must understand that all energy is conversion from one form to another! conversion from one form to another!

–

–Heat energyHeat energy –

–Gravitational potential energyGravitational potential energy –

–Electrical energyElectrical energy –

–Chemical energyChemical energy –

–Radiant energyRadiant energy –

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Conservation of Energy

First law of Thermodynamics

–

–Energy cannot be created or destroyedEnergy cannot be created or destroyed –

–The total amount of energy in the universe is The total amount of energy in the universe is constant

constant

Why doesn

Why doesn’’t a pendulum keep swinging?t a pendulum keep swinging?

–

–FrictionFriction –

–Changes the energy to heat energyChanges the energy to heat energy

Forms of Energy and Consumption

Fossil fuels Fossil fuels – –coalcoal – –oiloil – –gasgas Alternative Fuels Alternative Fuels – –HydroelectricHydroelectric – –EthanolEthanol – –WindWind – –SolarSolar Steam generation Steam generation

Direct conversion (Solar cells)

Forms of Energy and

Consumption

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Forms of Energy and Consumption

Learning Goals Learning Goals

–

–Identify some common forms of energyIdentify some common forms of energy –

–Compare the main sources of energy and the Compare the main sources of energy and the main sources of energy consumption.

main sources of energy consumption.

–

–List some List some ‘‘alternativealternative’’energy sources and energy sources and explain their pros and cons.

explain their pros and cons.

Questions: 24 Questions: 24 --27 27

Alternative and Renewable Energy

Sources

Alternative energy sources Alternative energy sources

–

–NOT based on the burning of fossil fuelsNOT based on the burning of fossil fuels

Renewable energy sources Renewable energy sources

–

–Cannot be exhaustedCannot be exhausted

Much overlap between the two Much overlap between the two

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Alternative and Renewable Energy

Sources

Questions: 28 Questions: 28 ––3030

Key Terms; Matching, Multiple Choice, Key Terms; Matching, Multiple Choice, and Fill

and Fill--inin--thethe--Blank Questions; Visual Blank Questions; Visual Connection and Applying your Knowledge Connection and Applying your Knowledge