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Energy and Work

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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
• 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
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