Transcript Energy, Work, and Power

Energy, Work, and Power
What the heck is Energy, and
what do “Work” and “Power”
mean?
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.
Energy
What are different types of energy?
Kinetic, Potential, chemical, Nuclear solar,
thermal (heat), sound, wind, electrical
(elec/magnetic), etc.
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
Energy
In this class we’ll first look at forms of
mechanical energy and how to calculate
these.
Later we’ll look into heat as a form of
energy
Energy
 Potential Energy (due to gravity) is energy an
object has because of it’s 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
Energy
 Quick example….. Fluffy the 20kg cat climbs to
the top of a saguaro cactus (height = 12 meters)
What is the cat’s potential energy relative to the
ground? (answer: mgh = 2400Joules)
 What is Fluffy’s PE relative to a deer mouse
hiding in the cactus 5 meters above the ground?
(answer: mgh = 1400Joules)
Energy
Our hero Fluffy now jumps from the top of
the cactus to the ground. What happens to
the Fluffster’s 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
parameters which affect the amount of
KE?
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 ……
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?
 It’s A LOT (yes, that’s the correct answer)
 Doubling the speed causes a 4x increase in
kinetic energy….tripling your speed makes the
energy go up by 9x!
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.
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
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 it’s
the rate of using or making energy per
time.
Power units are joules/sec which is
another unit….what?
Power
Yes, Watt
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 units……Joules
Power = energy/time or work/time =
watts