# Energy and Work - PowerPoint PPT Presentation

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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

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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.

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