Energy

1
Energy

• Book Chapter 15

2
Energy

• Work is a transfer of energy.
• Mechanical energy is when objects have the
  ability to do work.
• There are two types of mechanical energy
• 1. Potential Energy (PE)
• 2. Kinetic Energy (KE)

3
Kinetic Energy is energy in motion

• Any object in motion has kinetic energy.
• The faster the object, or the more massive the
  object is, the more kinetic energy it has.
• KE ½ (mass)(velocity)2 ½ mv2
• Kinetic energy can also be found in acoustical
  energy (sound), thermal energy (heat), etc.

4
Calculating Kinetic Energy
Mass 1500 kg Velocity 25 m/s Kinetic Energy
½ (1500)(25)2 468,750 J
5
Potential Energy is stored energy

• Potential energy can be in many different forms.
• Elastic potential energy stored energy in an
  elongated rubber band or spring.
• Chemical potential energy the amount of energy
  stored in food (calories).
• Gravitational potential energy depends on how
  high an object is above the ground, or the amount
  of work it took to move that object to that
  height.

6
Gravitational Potential Energy

• The higher the object is, the more work that can
  be done on another object. This height depends
  on what reference point is chosen.
• PE (mass)(gravity)(height) mgh

Mass 3 kg Gravity 9.8 m/s2 Height 10 m PE
294 Joules (J)
7
Elastic Potential Energy

• Elastic potential energy is the energy of an
  object that is compressed or stretched.
• Example A guitar string, a rubber band

8
Types of Energy

• There are 6 main forms of energy
• 1. Mechanical Energy
• 2. Heat/Thermal Energy
• 3. Chemical Energy
• 4. Electrical Energy
• 5. Electromagnetic Energy
• 6. Nuclear Energy

9
Law of Conservation of Energy

• Energy can not be created or destroyed. It can
  be transformed from one form into another, but
  the total amount of energy in any system stays
  constant.
• Total amount of energy is the sum of the
  potential and kinetic energies.

10
Conservation of Energy
Half way down PE 5000 J KE 5000 J
At the bottom PE 0 J KE 10,000 J
At the top PE 10,000J KE 0 J
Total Energy of the system is always 10,000 J
11
Conservation of Energy
12
Quiz

• Write the definition of Kinetic Energy.
• Write the definition of Potential Energy
• Calculate the Potential Energy of a 50 kg mass
  that is sitting on a 500 m cliff.
• Does a bird sitting in a tree have kinetic energy?

13
The Pendulum

• An object that is suspended so that it can swing
  back and forth about an axis is called a
  pendulum.
• The swing of a pendulum is an example of simple
  harmonic motion, which means it repeats itself
  over and over.

14
Energy changes in a pendulum
15
The Pendulum

• The time of a back-and-forth swing of a pendulum
  is called the period of a pendulum. It is
  measured in seconds.
• The period of a pendulum only depends on the
  length of the pendulum and the acceleration of
  gravity. (see example)

16
Calculating the period of a pendulum

• Use the equation
• Where
• T period of pendulum
• l length of pendulum
• g gravity

17
Calculating the period of a pendulum

• An astronaut sets up a pendulum on the moon,
  where gravity is 1.6 m/s2. If the pendulum is 1
  meter long, what will the period of the pendulum
  be?

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18
Energy and Mass

• Einsteins equation, Emc2, says that energy and
  mass are equivalent and can be converted into
  each other.
• Energy is released as matter is destroyed, and
  matter can be created from energy.

19
Nonrenewable Energy Resources

• Nonrenewable energy resources exist in limited
  quantities and, once used, cannot be replaced
  over the course of millions of years.
• Nonrenewable energy resources include oil,
  natural gas, coal, and uranium. Oil, natural
  gas, and coal are known as fossil fuels.

20
Renewable Energy Resources

• Renewable energy resources are resources that can
  be replaced in a relatively short period of time.
  Most renewable energy resources originate either
  directly or indirectly from the sun.
• Renewable energy resources
• include hydroelectric, solar,
• geothermal, wind, biomass,
• and, possibly in the future,
• nuclear fusion.

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Renewable Energy Resources

• Energy obtained from flowing water is known as
  hydroelectric energy.
• Solar energy is when sunlight
• is converted into usable energy.
• Geothermal energy is the heat beneath the Earths
  surface.

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Other Renewable Resources

• Sunlight causes living things to grow, converting
  the suns electromagnetic energy into chemical
  energy. The chemical energy stored in living
  things is called biomass energy.
• A hydrogen fuel cell by reacting hydrogen with
  oxygen.

23
Conserving Energy Resources

• Energy resources can be conserved by reducing
  energy needs and by increasing the efficiency of
  energy use. Finding ways to use less energy or
  to use energy more efficiently is known as energy
  conservation.