when you open a jack in the box, the stored spring energy i

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Energy
Types of Energy

• Kinetic Energy K
• Gravitational potential energy Ug
• Elastic or spring Energy Us
• Thermal Energy Eth
• Chemical Energy Echem
• Nuclear Energy Enuclear

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Energy can change what it looks like it can be
Transformed
Example
When you open a jack in the box, the stored
spring energy is transformed into moving energy
(kinetic)
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Transformation of Energy
Within a closed system, energy can be transformed
(changed) from one type to another with out loss
Ex) a Ball dropped from 10 m The line drawn
around the ball and the surface of the earth
define the system. Defining a system is an
arbitrary event, except that sometimes you want
to make a system simple and sometimes complex.
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Tranformation of Energy
Within a closed system, energy can be transformed
(changed) from one type to another with out loss
Ex) a Ball dropped from 10 m At first the ball
has just Ug (pontential gravitational) As the
Ball falls the energy is converted into K
(kinetic) As the ball nears the ground, it is
solely K
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Energy Transformation Examples

• Name the energy transformations in each example
• Pole vaulter vaulting over a bar
• Kinetic (running) Potential Spring (pole)
• Potential Gravitational (high in air)
  
• Kinetic (Falling)
• K Us Ug K
• Fire a sling shot
• Us K

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Work and Energy
What is Work?
1. Physical or mental effort, labor
2. The activity by which one makes a living.
3. A task or a duty.
4. Something produced as a result of effort a
work of art
5. Plural works The essential or operating
parts of a mechanism
6. The transfer of energy to a body by
application of force.
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The Basic Energy Model
Work done on system (positive) Energy in
Work done by system (negative) Energy out

• System
• K U
• Eth

Heat (energy)
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Work and Energy
Example Fire a sling shot
System Sling shot only
Win Us Wout
System Projectile only
Win K
System Sling shot and Projectile both included
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Energy Transformations

• Energy can be transformed back and forth from
  kinetic form to potential form.
• Once mechanical energy is transformed to thermal
  energy, it typically cannot be transformed back
  into K or U.

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Energy Transfers

• When positive work is done on the system, energy
  is transferred into the system.
• In this case, the total energy of the system is
  increased by the amount of work transferred into
  the system.

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Energy Transfers

• When the system does positive work on its
  environment, energy is transferred out of the
  system.
• This is equivalent to saying that negative work
  is done on the system.
• In this case, the total energy of the system is
  decreased by the amount of work transferred out
  of the system.

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Transfers vs. Transformations

• A transformation is a change of energy from one
  form to another within the system.
• A transfer is the addition or subtraction of
  energy to or from the system through two main
  processes
• Work
• Heat (thermal energy)

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Concept Check

• Determine the energy transformations when a child
  climbs a playground slide and then slides down
  the slide at constant speed.

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The Basic Energy Model
Work done on system (positive) Energy in
Work done by system (negative) Energy out

• System
• K U
• Eth

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Isolated Systems

• If a system is isolated from its environment, no
  energy can be transferred into or out of the
  system.
• No work (positive or negative) is done on the
  system (or by the system)
• No heat energy enters or leaves the system.

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The Law of Conservation of EnergyFor Isolated
Systems

• In an isolated system
• K Ug Us Eth Etotal
• other forms of energy constant

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Conservation of Energy (another equation)

• In an isolated system
• ?K ?Ug ?Us ?Eth 0
• which is to say
• Ki Ugi Usi Ethi
• Kf Ugf Usf Ethf

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Systems That Arent Isolated

• When a system is not isolated
• - Energy is moving in or out
• W Q ?E
• Work heat change in energy

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Systems that arent isolated

• When a system is not isolated
• W Q ?K ?Ug ?Us ?Eth
• Work heat change in energy

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Systems and Conservation of Energy

• Internal Forces Forces within the system
• - They do no work, thus changing to amount of
  energy
• External Forces Forces acting on the system
  from the outside
• - They do work, thus adding or subtracting
  energy from your system

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The Basic Energy Model
Work done on system (positive) Energy in
Work done by system (negative) Energy out

• System
• K U
• Eth

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Work and Energy
What is Work?
Transfer of energy between system and environment
a force acting upon an object to cause a
displacement.
W F d
1 joule 1 J 1 Nm
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Work and Energy

• A force, F, was exerted on an object while the
  object moved a distance, d, as shown in the
  figure.

• If F is a constant force, exerted in the
  direction in which the object is moving, then
  work, W, is the product of the force and the
  objects displacement.

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Work and Energy

• Sarah pushes a crate 3.0 m along the floor at a
  constant speed. She pushes with a constant
  horizontal force of magnitude 70 N. How much
  work does Sarah do on the crate?

• Known F 70 N, d 3.0m, v constant, W
  ?

• W F d

• (70N)(3.0 m) 210 J

70 N
3.0 m
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Work and Energy
Calculating Work at Angles
Only component of the force that is in the
direction of motion transfers energy and is thus
a part of the work done.
Click image to view movie.
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Work and Energy
Calculating Work at Angles
W F cos? x d
The cosine solves for the part of the triangle
that is adjacent to the angle
Question How much work is done if the man pushes
the car for 12 m?
(how much possible energy transfer is there?)
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Work and Energy
Calculating Work at Angles
W F cos? x d
W 125 N cos 25o x 12 m
W 1360 Nm
W 1360 Joules
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Work and Energy
Calculating Work at Angles - Did the Force do
Work?
Complete Energy Transfer into box (system) ()
Force and displacement are in the same direction
displacement
Partial Energy Transfer into box (system) ()
Force is more than 0o but less than 90o off.
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Work and Energy
Calculating Work at Angles - Did the Force do
Work?
No Energy Transfer (0)
Cos 90 0
displacement
Energy transfer out of system (-)
Cos 180o -1
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Kinetic Energy
A 1000 kg car moves at 20 m/s. How much kinetic
energy does the car have?
K ½ mv2
K ½ mv2
K ½ 1000kg(20m/s)2
K 500kg 400 m2/s2
K 200,000 kg m2/s2
K 200,000 J
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Kinetic Energy
A 1000 kg car is at rest. A 200 N force pushes
the car over a distance of 50 meters. What is
the cars new velocity
In a perfect world, all this work is converted
to Kinetic energy.
Ki W Kf
10,000J ½ 1000kgv2
0J W Kf
10,000kgm2/s2 500kgv2
W Kf
Fd Kf
20m2/s2 v2
200N50m Kf
10,000J Kf
4.47m/s v
10,000J K ½ mv2
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Potential Energy
Potential can easily be converted into other
forms of energy and/or do work.
Gravitational -
Based on the height off the ground from some
defined point
Ug mgy
(or mgh)
Mass acc of grav height
Elastic/spring -
Based on distance moved from relaxed position
Us ½ kx2
k spring constant (N/m)
x distance from relaxed position
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Potential Energy
Example A 3kg object is lifted off the ground
1.5 meters. How much potential energy does the
object contain?
Ug mgy
Ug 3kg 9.8 m/s2 1.5m
Ug 44.1 J
How much work was done in raising the object 1.5
meters?
Ui W Uf
0 J W 44.1 J
W 44.1 J
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Conservation of Energy
Within an isolated system, the total amount of
energy will remain unchanged during any course of
events.
Ki (Ug)i Kf (Ug)f
½ mv2 mgy ½ mv2 mgy
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Energy and Collisions
In All isolated systems Ei Ef
In a perfect collision Ki Ki Kf Kf
Most real world collisions Ki Kf Eth
For both sides of the equation to be equal, so Kf
must be less than Ki
Ki (Ug)i Kf (Ug)f
½ mv2 mgy ½ mv2 mgy
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Energy and Collisions
A 3 kg ball moving at 2 m/s collides elastically
with a 1 kg ball at rest. What are the two final
velocities?
There are two variables here. Both the
conservation of momentum and the conservation of
energy equations are insufficient.
If one of the objects is initially at rest, the
following equations can be used
(v1)f
2m1
(v2)f
(v1)i
m1 m2
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Energy and Collisions
A 3 kg ball moving at 2 m/s collides elastically
with a 1 kg ball at rest. What are the two final
velocities?
2m1
(v2)f
(v1)i
(v1)f
m1 m2
2(3kg)
(v2)f
(2m/s)
(v1)f
3kg1kg
(v2)f 3m/s
(v1)f 1 m/s