Work and Energy

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Work and Energy

• PowerPoint notes and practice problems

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When do we do work?

• Two conditions must be met for work to be done
• The application of a force, and
• The movement of something by that force in the
  direction of the force applied
• When we lift a load against Earths gravity
• The heavier the load or
• The higher we lift it
• the more work we do

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WORK

• Work Force x distance
• W Fd
• Joule is the unit for work.
• Something must be moving in order for work to
  occur.

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Calculating work

• How much work is done if a box of books is pushed
  10 meters across the floor with a force of 25
  Newtons?
• 2. How much work is done if a barbell is lifted
  1 meter off the floor using a force of 10
  Newtons?

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Calculating work

• How much work is done if a box of books with a
  mass of 50 kg is pushed 10 meters across the
  floor at 1 m/s?
• F ma
• W Fd

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Calculating work

1. W Fd
   W 25N x 10m 250Nm or 250J
2. W Fd
   W 10N x 1m 10Nm or 10J
3. F ma F 50kg x 1m/s 50N
   W Fd
   W 50N 10m 500Nm or 500J

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Power

• Power is the rate at which work is done. It
  equals the amount of work done divided by the
  time interval during which the work is done.
• The unit of power is joule per second, known as
  the watt. One watt of power is used when one
  joule of work is done in one second

W
work done
power
power
t
time interval
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Calculating work

• If a box of books with a mass of 50 kg is pushed
  10 meters across the floor at 1 m/s, how much
  power was used to push the box?
• F ma
• W Fd
• P W/t

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Calculating work

1. F ma F 50kg x 1 m/s 50N W
   Fd W 50N x 10m 500J P
   W/t 500J /10s 50J/s or 50W

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ENERGY

• Fundamental building blocks of our universe.
• Appears in different forms
• Has the ability to do work
• Any object that has energy has the ability to
  create force.
• Units are measured, like work, in joules (J)

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Mechanical Energy

• Mechanical Energy is the energy due to the
  position of an object or the movement of an
  object
• Mechanical Energy Potential Energy Kinetic
  Energy
• ME PE KE

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POTENTIAL ENERGY

• Comes from the position of an object relative to
  earth.
• Stored energy
• Amount of potential energy must be the same as
  the amount of work done to lift the object
  against gravity.
• Objects do not use potential energy until they
  move.

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POTENTIAL ENERGY

• Usually referred to as gravitational potential
  energy
• gravitational potential energy weight x
  height

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Potential Energy Formula
9.8m/s2
PE

mgh
height
Potential energy
mass
In the last unit we learned that W mg
PE

Wh
height
Potential energy
weight
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Calculating Potential Energy

1. What is the PE of a boulder that weighs 100N
   sitting on a ledge 2m above the ground?
2. What is the PE of a 4N can of paint at the top of
   a 10 meter ladder?
3. If the limb of a tree has a PE of 90J in its
   present position 15 meters above the ground, what
   is its weight? What is the limbs mass?

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Calculating Potential Energy

• PE Wh
• PE 100N x 2m 200J
• PE Wh
• PE 4N x 10m 40J
• PE Wh
• 90J W x 15m 90J/15m 6N
• W mg 6N m x 9.8m/s2 .61 kg

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KINETIC ENERGY

• Energy in motion
• Energy depends on Speed Mass
• Speed increases - KE increases
• Mass increases - KE increases

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Kinetic Energy Formula

• Kinetic energy ½ mass x speed2
• KE ½mv2
• When working this formula, always square speed
  first.

speed
Kinetic energy
mass
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Kinetic Energy Formula

• Kinetic energy is also equal to the net force
  operating on an object and the distance it
  travels or
• Net force x distance kinetic energy
• Fd kinetic energy
• Now if KE Fd and if KE ½mv2, then
• Fd ½mv2

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Calculating Kinetic Energy

1. A ball with a mass of .15 kg is thrown at a speed
   of 40.2 m/s. What is the balls KE?
2. If a car with a mass of 2000 kg is moving at a
   speed of 25 m/s, what is its KE?
3. If a force of 1145N is applied to a railroad
   boxcar and it rolls a distance of 85 meters, what
   is its KE?

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Calculating Kinetic Energy

• KE ½mv2 (.5 x .15kg) x 40.2 m/s2
• KE .075kg x 1616.04 m/s 121.2J
• KE ½mv2 (.5 x 2000kg) x 25 m/s2
• KE 1000kg x 625 m/s 625000J
• KE Fd 1145N x 85m 97325J

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LAW OF CONSERVATION OF ENERGY

• Nature never creates or destroys energy energy
  only gets converted (changed) from one form to
  another.

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Energy Transformation
PE
PE
KE
KE
Energy will continually transfer between forms.
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KE to PE

• PE and KE are always equal to the total amount of
  energy
• PE and KE are in an inverse relationship as one
  increases the other decreases

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Calculate the PE and the KE
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Continual transfer between forms

• Mechanical Energy
• Radiant (Electromagnetic) Energy
• Electrical Energy
• Chemical Energy
• Nuclear Energy
• Thermal Energy

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TYPES OF ENERGY
Mechanical, Electromagnetic, Electrical, Nuclear,
Chemical and Thermal
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What is Mechanical Energy?

• Energy due to a objects motion (kinetic) or
  position (potential).
• The bowling ball has mechanical energy.
• When the ball strikes the pins, mechanical energy
  is transferred to the pins!

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Examples of Mechanical Energy
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What is ElectromagneticEnergy?

• Light energy
• Includes energy from gamma rays, xrays,
  ultraviolet rays, visible light, infrared rays,
  microwave and radio bands

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What is Electrical Energy?

• Energy caused by the movement of electrons
• Easily transported through power lines and
  circuits and converted into other forms of energy

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What is Chemical Energy?

• Energy that is available for release from
  chemical reactions.
• The chemical bonds in a matchstick store energy
  that is transformed into thermal energy when the
  match is struck.

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Examples of Chemical Energy
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What is Thermal Energy?

• Heat energy
• The heat energy of an object determines how
  active its atoms are.
• A hot object is one whose atoms and molecules are
  excited and show rapid movement.
• A cooler object's molecules and atoms will show
  less movement.

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QUIZ TIME!

• What type of energy cooks food in a microwave
  oven?
• 2. What type of energy is the spinning plate
  inside of a microwave oven?

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QUIZ TIME!

• Electrical energy is transported to your house
  through power lines.
• 3. When you plug an electric fan to a power
  outlet, electrical energy is transformed into
  what type of energy?

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QUIZ TIME!

• 4. What energy transformation occurs when an
  electric lamp is turned on?

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5. What types of energy are shown below?
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6. What type of energy transformation is shown
below?
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7. What types of energy are shown below?
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8. What type of energy is shown below?
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9. What type of energy conversion is shown below?
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50 point bonus question

• Draw a flow map showing the flow of energy
  transformations in a car from starting the
  vehicle to driving. You should have 5 different
  types of energy transformations.

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QUIZ TIME!

• 1. What type of energy cooks food in a microwave
  oven?
• ELECTROMAGNETIC ENERGY
• 2. What type of energy is the spinning plate
  inside of a microwave oven?
• MECHANICAL ENERGY

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QUIZ TIME!

• Electrical energy is transported to your house
  through power lines.
• 3. When you plug an electric fan to a power
  outlet, electrical energy is transformed into
  what type of energy?
• MECHANICAL ENERGY

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QUIZ TIME!

• 4. What energy transformation occurs when an
  electric lamp is turned on?
• ELECTRICAL ENERGY
• ?
• ELECTROMAGNETIC ENERGY

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5. What types of energy are shown below?

• Mechanical and Thermal Energy
• (Dont forget friction)

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6. What type of energy transformation is shown
below?

• Electromagnetic to Chemical Energy

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7. What types of energy are shown below?

• Chemical to Electrical, Mechanical to
• Electromagnetic Energy

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8. What type of energy is shown below?

• Chemical Energy (yummy)

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9. What type of energy conversion is shown below?

• Electrical to Thermal Energy

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Energy Transfer
Sound (mechanical)
Electrical
Thermal
Mechanical
Electrical
Chemical
Electrical
Light (Electromagnetic)