Energy, Work, and Power

1
Energy, Work, and Power

• What the heck is Energy, and what do Work and
  Power mean?

2
Work, Energy, and Power

• We use these words frequently, in Physics they
  have very specific meanings.
• Energy is something that gets transferred or
  moved around to do something useful with.

3
Energy

• What are different types of energy?
• Kinetic, Potential, chemical, Nucular, solar,
  thermal (heat), sound, wind, electical
  (elec/magnetic), etc.

4
Energy

• Energy you can think of Energy as a way of
  keeping track of this stuff which is
  transferred around.
• Big Idea about Energy Conservation For any
  closed system , energy can be transferred into
  different forms but is never lost.Often called
  the Law of Conservation of Energy

5
Energy

• In this class well first look at forms of
  mechanical energy and how to calculate these.
• Later well look into heat as a form of energy

6
Energy

• Potential Energy (due to gravity) is energy an
  object has because of its height or location. An
  object located above a table has potential energy
  (relative to the table) because it will fall if
  allowed to. (falls due to gravity)
• Potential Energy is relative to what you call the
  reference position.
• Potential Energy depends on height, mass, and the
  pull of gravity PE mgh
• The units of energy are.mgh force x distance
  Newton-meter Joule

7
Energy

• Quick example.. Fluffy the 20kg cat climbs to
  the top of a saguaro cactus (height 12 meters)
  What is the cats potential energy relative to
  the ground? (answer mgh 2400Joules)
• What is Fluffys PE relative to a deer mouse
  hiding in the cactus 5 meters above the ground?
  (answer mgh 1400Joules)

8
Energy

• Our hero Fluffy now jumps from the top of the
  cactus to the ground. What happens to the
  Fluffsters Potential Energy?
• As Fluffy falls she is losing Potential Energy
  but gaining a different energy related to her
  picking up speed..we call that Kinetic Energy or
  KE. What are the paramaters which affect the
  amount of KE?

9
Energy

• Kinetic Energy is the energy an object has
  because it is moving, so it depends on speed or
  velocity.
• Does Fluffy moving at 5 m/s have the same KE as a
  soccer mom driving her 50,000kg SUV at 5 m/s?
• Well..no.because KE depends on speed and mass,
  in fact

10
Energy

• KE ½ mv² (notice that velocity is squared, so
  KE is highly dependent on how fast an object is
  going.)
• If you are driving your car at 60 mph how much
  more energy is that than driving at 30mph?
• Its A LOT (yes, thats the correct answer)
• Doubling the speed causes a 4x increase in
  kinetic energy.tripling your speed makes the
  energy go up by 9x!

11
Energy

• A pendulum is a simple example of an object
  transferring energy from all Potential Energy at
  the top of the swing to all Kinetic Energy at the
  bottom of the swing.

12
Energy and Work

• What about Work?
• We use the word work in many different
  ways..in physics work has a very specific
  meaning.
• Work is an external Force which acts for a
  certain distance to change the energy of an
  object. W F x distance.
• The Force applied must be in the direction the
  object is moving or no work is done. If you
  push an object forward you do positive work on an
  object. If there is friction then it is acting in
  the opposite direction and is doing negative
  work.
• Work has the same units as Energy

13
Power

• Power is another word that is commonly used in
  many ways.in physics it has a very specific
  meaning.
• Power energy/time work/time so its the rate
  of using or making energy per time.
• Power units are joules/sec which is another
  unit.what?

14
Power

• Yes, Watt

15
Summary of equations

• Kin. Energy ½ m v²
• Potential Energy mgh
• Work Force x distance
• Work ..doing work on an object changes its
  energy work and energy have the same
  unitsJoules
• Power energy/time or work/time watts