Transcript Work, Energy, Kinetic Energy, Potential Energy

Work, Energy, Kinetic Energy,
Potential Energy, and Power
Work

Work is done on an
object when energy is
transferred to that object

W = Force x distance
Stacking Hay
Energy
Work

The capacity to make something happen is
Energy

The process of making something happen is
Work

Energy and Work have the same units
KWH, Joules, Ft-Lbs., BTUs, Calorie, K-Calories
Conservation of Energy


Energy is not destroyed.
Energy is transferred between
objects.
(Energy in my body is used to slide a
box across the table.)

Energy is converted from one form
to another.
(Energy in the box is converted to heat
as the table heats up from the sliding
box.)
Kinetic Energy
A sliding box is moving, therefore we
say it has motion energy or Kinetic
Energy.
Other examples of Kinetic Energy:
Moving car
Moving water
Moving wheel
Energy can be stored as Potential Energy







Batteries
Gasoline
Natural Gas
Food
Water Towers
A wound up alarm clock
Toyota Prius battery
IowaWaterQuality.org
http://www.uc.edu
http://electronics.howstuffworks.com
Potential Energy
The Energy that is stored in an object
Examples:









A stretched rubber band
A compressed spring
Lego
A boulder sitting at the top of a cliff
A Roller Coaster ready at the top of the track.
A charged battery
A voltage source
The Human body after eating.
Gasoline
The Sun
www.travelblog.org
Steel Dragon 2000 roller coaster in Nagashima, Japan
Gravitational Potential Energy
Gravitational Potential Energy is the energy
an object has because of its position relative
to the earth.
grav. PE = weight x (height above ground)
Glen Canyon Dam in Utah
uses the potential energy of
the Colorado River water to
produce more than
a 1200 MegaWatts or power.
http://pinker.wjh.harvard.edu/
Types of Potential Energy










A stretched rubber band (elastic potential energy)
A compressed spring (elastic potential energy)
A boulder sitting at the top of a cliff (gravitatoinal potential energy)
A Roller Coaster ready at the top of the track. (gravitational
potential energy)
A charged battery (chemical potential energy)
A voltage source (electric potential energy per unit of charge)
The Human body after eating. (chemical potential energy)
Gasoline (chemical potential energy)
The future light and heat from the Sun (nuclear potential energy)
A Dam (gravitational potential energy)
Energy can be stored as Kinetic Energy
(Energy of Motion)








Toy car Flywheels
A moving car
Flying Airplane
Earth, planets, the sun, the galaxies
Potters Wheels
Bicycle Wheels
Flywheel Energy Storage (FES)
Fast Moving Roller Coasters at the bottom
of the track
Potential Energy  Kinetic Energy
Grav. PE  Kinetic Energy
Weight x h  ½ mv2
Animation of Energy Transfer on a Roller Coaster:
http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/energy/ce.html
A good explanation on Roller Coasters:
http://howthingswork.virginia.edu/page1.php?QNum=1319
Flywheels
Engine Flywheels help smooth out the
rotation of the shaft when a fluctuating torque
is exerted on it by its pistons and cylinders.
http://www.craftsmanshipmuseum.com
Kinetic Energy

½ m v2
(translational or linear KE)
m is mass, and v is velocity/speed
mass is how much matter is in the object
weight = mass x gravity

½ I w2
(rotational KE)
I is the moment of inertia, or resistance to rotation, and w is
rotational speed
Inertia is the resistance to a change in motion.
More massive objects have greater inertia.
I is the rotational equivalent of mass
w is the angular speed expressed as revolutions per minute
The moment of inertia of an object
describes how difficult it is to change its
angular motion about its axis.
For example, consider two discs (A and
B) of the same mass but Disc A has a
larger radius than disc B.
Assuming that there is uniform
thickness and mass distribution, it
requires more effort to rotate disc A
(change its angular velocity) because
its mass is distributed farther from its
axis of rotation: mass that is farther out
from the axis must move more quickly
than mass closer in.
In this case, disc A has a larger
moment of inertia than disc B.
http://en.wikipedia.org/wiki/Moment_of_inertia
http://www.bsharp.org/physics/stuff/skater.html  good video clips of skaters
Storing Energy as Kinetic Energy
Get something massive and accelerate it so that it is
moving fast. Due to its mass it will resist slowing
down. Massive objects in motion store a lot of
Kinetic Energy.
This is why high speed car crashes without restraint
devices are often fatal. The energy has to go
somewhere and if that energy is not transferred to
the seatbelt then the energy will be transferred to
other things, like the windshield, the pavement etc.
Consider the toy car:
1.
2.
3.
4.
5.
6.
7.
Take the car apart.
Sketch its gear box
Determine the torque reduction from the wheels to
the flywheel
From the flywheel to the wheels, is this speed
reduction or torque reduction?
Do the wheels have more torque than the flywheel?
How much more?
How would you redesign the car to climb steep
ramps?
What might be some limitations to your redesign?
Power


Power = Work/time
Units for Power are:
Watts, Horsepower, foot-lbs/minute, BTU/hour