Energy

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					                  Energy
• The ability to do work.
• An object has energy if it is able to
  produce change in itself or its
  surroundings.
  Energy exists in different forms
1. Mechanical energy (moving objects and their
   positions)
2. Radiant energy (light and solar energy)
3. Chemical energy (including the food you eat and
   fuels we burn)
4. Thermal or heat energy (molecules moving faster
   means more heat)
5. Electrical energy (electrons moving through a
   wire)
6. Nuclear energy (energy locked in the nucleus of an
   atom)
      Energy can be transferred…
• Fossil fuels like coal and oil can be burned
  to heat water that boils into steam that turns
  a turbine to generate electricity that you
  use to operate a stereo.
  Chemical energy  Thermal energy
  Thermal energy  Kinetic energy
  Kinetic energy  Electrical energy
 Energy cannot be created or destroyed.
• In the example of riding a bicycle down a
  steep hill, you begin with a lot of potential
  energy at the top of the hill and gain
  kinetic energy as you coast down the hill.
• If you are not making the kinetic energy
  (movement down the hill), where does it
  come from? The answer is simple: your
  potential energy at the top is transformed
  into kinetic energy as you speed along.
       Mechanical Energy

• Kinetic & Potential
• Kinetic is the energy of moving objects.
               KE  1 mv2
                    2
            Energy is stored energy.
• Potential
• Gravitational PE is energy due to position.
             PE mgh
     Potential Energy
• Energy that is a result of an
  object’s position or condition.
• All potential energy is Stored
  Energy.
  – Pull back on a bow string and bend
    the bow. The object then possesses
    potential energy.
  Potential Energy

• A rock on a table top has more potential energy
  than when it is on the ground due to its position.
• This is a form of gravitational potential energy.
Gravitational Potential Energy


• Depends on mass and height.
• GPE = m(g)h
  m = mass
  g = acceleration due to gravity
  h = height
  -What are the Units of GPE? Joules
               SI units?
•   m = kg
•   g = m/s2
•   h=m
•   PE = J
                   Question #1
A man lifts a 2 kilogram book from the floor to the
top of a 1.25 meter tall table. What is the change in
the book’s gravitational potential energy?



              a)   0 joules
              b)   +2.50 joules
              c)   -2.50 joules
              d)   +24.525 joules
              e)   -24.525 joules
       Answer #6: (d) +24.525 J
A man lifts a 2 kilogram book from the floor to the
top of a 1.25 meter tall table. What is the change in
the book’s gravitational potential energy?

         GPE  mgh
                         m 
                       9.81 2  (1.25m)
         GPE  (2kg) 
                         s 
         GPE  24.525 joules
         (ADDING energy to the book)
                  Question #2
A mouse now pushes a book (2 kg) off the table
(1.25m). What is the change in the book’s
gravitational potential energy?



             a)   0 joules
             b)   +2.50 joules
             c)   -2.50 joules
             d)   +24.525 joules
             e)   -24.525 joules
      Answer #7: (e) -24.525 J
A man lifts a 2 kilogram book from the floor to the
top of a 1.25 meter tall table. What is the change in
the book’s gravitational potential energy?


      GPE  mgh
                      m 
                    9.81 2  (-1.25m)
      GPE  (2kg) 
                      s 
      GPE  24.525 joules
      (REMOVING energy from the book)
       Kinetic Energy


• Energy that appears in the form of
  motion.
• Depends on the mass and speed of
  the object in motion.
            Kinetic Energy



• KE = (1/2)mv2
• m = mass v = velocity
• Unit for energy is Joule (J)
    Kinetic Energy

• Energy due to motion.
• A brick falling at the same
  speed as a ping pong ball will
  do more damage.
• KE is dependent on mass.
• KE also depends on speed (v)
            Kinetic Energy
Which would affect the kinetic energy of
an object more, doubling its mass or its
              velocity?
• doubling the mass would result in a doubling of
  the KE.
• doubling the velocity would quadruple the KE.
              Question #8
What is the KE of a 1140 kg (2513 lb) car
driving at 8.95 m/s (20 mph)?


             a)   0 joules
             b)   5101.5 joules
             c)   10203 joules
             d)   4.57x104 joules
             e)   I’m lost…
    Answer #8: (d) 45658.4 J


What is the KE of a 1140 kg (2513 lb) car
driving at 8.95 m/s (20 mph)?
           1 2
       KE  mv
           2
                             2
           1            m 
       KE  1140kg 
                      8.95 
           2            s 
       KE  4.57x10 J
                   4
Recall, Law of Conservation of Energy

• Energy can not be created nor destroyed.
• Energy can change from one form to another.
• The total energy in the universe is constant.
     Conservation of Energy
• In a roller coaster all of the
  energy for the entire ride
  comes from the conveyor belt
  that takes the cars up the first
  hill.

				
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posted:7/25/2011
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