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# Energy and Work - PowerPoint PPT Presentation

Energy and Work. Chapter 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.

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### Energy and Work

Chapter 10

• 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

• 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

• 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

• Energy of Motion

• All moving objects have kinetic energy

• The heavier an object, and the faster it moves, the more K it has.

• 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.

• 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.

• 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

• E = K + Ug + Us + Eth + Echem + …

• We will be most interested in these forms of energy although others may be present.

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

• We will use an arrow to represent an energy transformation

Echem Ug

Echem Eth

Us K Ug

Energy Transfers system from one form to another

• 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 system from one form to another

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

The energy transfer system from one form to anotherW 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 system from one form to anotherW 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 transfer system from one form to anotherW 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

Defining the system system from one form to another

• 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 from one form to another

• System = ball + earth

• Internal forces =Fg

• External forces = T

• System energies = K, Ug

• ΔK + ΔUg = Fg

Redefining the system system from one form to another

• 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 system from one form to another

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

• K + U = K + Ug + Us

• Energy is conserved

Equations system from one form to another

• Work

• The units of work are joules

• 1 joule = 1J = 1 N m

Forces that do no work system from one form to another

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

Forces that do no work system from one form to another

• A force perpendicular to the displacement does no work

Forces that do no work system from one form to another

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

Energy Equations system from one form to another

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

More Energy Equations system from one form to another

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

More Energy Equations system from one form to another

• Elastic potential energy

• Us is the same whether x is positive (compressed spring) or negative (extended spring)

• k is the spring constant

Thermal Energy system from one form to another

• 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