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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.

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Energy and work

Energy and Work

Chapter 10


Energy
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
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
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
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 u g
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 u s
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 e th
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 e chem
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 e nuclear
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
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 Transformations

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

  • We will use an arrow to represent an energy transformation


E chem u g
Echem Ug


K e th
K Eth


E chem e th
Echem Eth


U s k u g
Us K Ug



Energy transfers
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 w k
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 w e th
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 w u s
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
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
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 system1
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
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
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
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 work1
Forces that do no work system from one form to another

  • A force perpendicular to the displacement does no work


Forces that do no work2
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
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
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 equations1
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
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


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