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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 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


  • 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
energy transfers
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 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 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 e th
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 transfer w u s
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
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
Redefining the system
  • System = ball + earth
  • Internal forces =Fg
  • External forces = T
  • System energies = K, Ug
  • ΔK + ΔUg = Fg
redefining the system1
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
Mechanical Energy
  • The sum of the kinetic and potential energies is called the mechanical energy
  • K + U = K + Ug + Us
  • Energy is conserved
  • Work
  • The units of work are joules
  • 1 joule = 1J = 1 N m
forces that do no work
Forces that do no work
  • If the object undergoes no displacement while the force acts, no work is done.
forces that do no work1
Forces that do no work
  • A force perpendicular to the displacement does no work
forces that do no work2
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
Energy Equations
  • Translational Kinetic Energy of an object moving along a line or some other path.
more energy equations
More Energy Equations
  • Gravitational Potential Energy depends only on the height of the object above a reference level
more energy equations1
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
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