Energy and WorkChapter 10

Energy

• Energy is the ability to do work• An object that has energy has the ability to

create a force.• Energy is measured in units of joules• 1 joule = 1 Newton meter• There are many different types of energy

The Law of Conservation of Energy

• Energy must be conserved• Energy can change forms • We must account for energy transferred into

or out of a system.• Nature “balances the books” for energy• Our goal is to discover the conditions under

which energy is conserved

Energy in a System

• Every system has a quantity associated with it called total energy.

• The total energy of a system is made up of many kinds of energy

Kinetic Energy (K)• Energy of Motion• All moving objects have kinetic energy• The heavier an object, and the faster it moves,

the more K it has.

Gravitational Potential Energy Ug

• GPE is stored energy associated with an object’s height above the ground.

• As this roller coaster descends, stored potential energy is converted to kinetic energy.

Elastic or Spring Potential Energy (Us)• Us is energy stored when a spring or other

elastic object is stretched.• This can later be transformed into kinetic

energy.• The book may use U to represent potential

energy when it is not important to distinguish between gravitational and spring.

Thermal Energy (Eth)

• Hot objects have more thermal energy than cold ones because the molecules are moving faster.

• Eth = Sum of all the kinetic and potential

energies of all the molecules• Some molecules have enough energy to escape as steam.

Chemical Energy (Echem)

• Energy that is stored in the bonds of molecules

• Energy is released as the bonds are rearranged during chemical reactions.

• Fuel for car or food for our bodies

Nuclear Energy Enuclear

• Energy stored in the nucleus of an atom• If the nuclei is broken apart vast amounts of

energy are released• Transformed into kinetic and thermal energy.• Source of energy for nuclear power plants and weapons

Total Energy of the System (E)

• E = K + Ug + Us + Eth + Echem + …• We will be most interested in these

forms of energy although others may be present.

Energy Transformations

• Energy of one kind may transform into energy of another kind

• We will use an arrow to represent an energy transformation

Echem Ug

K Eth

Echem Eth

Us K Ug

Energy Transformations are changes of energy within the system from one form to another

Energy Transfers

• An exchange of energy between the system and the environment is called an energy transfer

• Twp primary energy transfers– Work –The mechanical transfer of energy to or

from the system by pushing or pulling on it– Heat-The non-mechanical transfer of energy to or

from the system by the environment because of a temperature difference between the two.

Work

• Work is the process of transferring energy between the system and the environment by the application of a mechanical force.

The energy transferW K

• The system is the shot• The environment is the athlete• As the athlete pushes on the shot, he is doing

work on the system . • He transfers energy from himself to the ball.

The transfer W Eth

• The system is the match and matchbox• The environment is the hand• As the hand pulls the match actross, the hand

does work on the system, increasing its thermal energy.

• The match head becomes hot enough to ignite.

The transferW Us

• The system is the slingshot• The environment is the boy• As the boy pulls back the elastic bands, he

does work on the system, increasing its elastic potential energy

• Internal Forces are forces between objects within the system

• Internal forces cannot do work on the system• External forces act on the system, but their

agent is part of the environment• External forces Can do work on the system

Defining the system

• System = the ball only• Internal forces = none• External forces = T, Fg

• System energies = K• Since the earth is not part of the system, the

system has no Ug.

Redefining the system

• System = ball + earth• Internal forces =Fg

• External forces = T• System energies = K, Ug

• ΔK + ΔUg = Fg

Redefining the system

• System = ball + earth + crane• T, Fg Many internal forces of the crane• External forces = none• System energies = K, Ug. Echem

• The system is isolated• Increased potential and kinetic energy comes from a transformation of chemical energy.

Mechanical Energy

• The sum of the kinetic and potential energies is called the mechanical energy

• K + U = K + Ug + Us

• Energy is conserved

Equations• Work• The units of work are joules• 1 joule = 1J = 1 N m

Forces that do no work

• If the object undergoes no displacement while the force acts, no work is done.

Forces that do no work

• A force perpendicular to the displacement does no work

Forces that do no work

• If the part of the object on which the force acts, undergoes no displacement, no work is done.

Energy Equations

• Translational Kinetic Energy of an object moving along a line or some other path.

More Energy Equations

• Gravitational Potential Energy depends only on the height of the object above a reference level

More Energy Equations

• Elastic potential energy• Us is the same whether x is positive

(compressed spring) or negative (extended spring)

• k is the spring constant

Thermal Energy

• When two objects slide against each other with friction present, mechanical energy is always transformed into thermal energy.

• The surface must be included as part of the system

• A system’s thermal energy increases as kinetic energy is transformed into thermal energy

• ΔEth = Ki - Kf