Transcript Document 7172147

ENERGY
Energy
Introduction
Energy is always present, but never visible!
 We see the evidence of energy

◦ Pushing a wheelchair
◦ Jumping
◦ Eating

Movement, sound, heat, light all provide
evidence of energy
Trebuchet Video: Consider these
questions
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How is a trebuchet constructed?
Which simple machine do you see?
What is the purpose of the counterweight?
What type of energy does the trebuchet
have when stationary?
What happens when the firing pin is
released?
How would you differentiate between
potential and kinetic energy?
http://www.teachersdomain.org/asset/hew06
_vid_trebuchet/
Work- Energy Correlation
In order for work to be done, a force
must be applied to an object and cause it
to move in the same direction as the
force.
 Work transfers energy from one object
to another
 What is energy?

What is energy?
Energy is the ability of an object
to produce a change in itself or
the environment. It is the ability
to do work.
Types of Energy
Mechanical Energy(ME) - enables an
object to do work
A. Kinetic Energy (KE)
B. Potential Energy(PE)
Kinetic Energy
Kinetic energy is defined as the energy of
a moving object
 Examples:
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◦ Thrown football
◦ Waterfall
◦ Rock falling from a cliff
Potential Energy
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Potential energy is defined as the energy in
matter due to its position or the
arrangement of its parts
Often referred to as stored energy
The more potential an object has, the more
potential energy is has
Has many different forms including
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Gravitational potential energy
Elastic potential energy
Chemical potential energy
Electrical potential energy
Toy observations
1.
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4.
5.
6.
Describe your toy.
Take a few minutes to determine how your
toy works. Make a short statement about how
the toy works.
State how you think your toy uses energy.
How many forms of energy does your toy
display?
Name the types of energy you observe.
Switch toys with another table and repeat the
above observations.
Gravitational Potential Energy
When something is lifted or suspended in
air, work is done on the object against the
pull of gravity
 This work is converted to a form of
potential energy, called gravitational
potential energy
 Once the object falls the potential energy
is converted to kinetic energy

Elastic Potential Energy
Occurs when an object resists being
pulled out of shape
 Examples: a stretched rubber band, a
spring, trampoline, our skin
 The elastic potential energy in a rubber
band can be used to do work- such as a
toy plane . . . A rubber band untwists and
causes a propeller to spin

Chemical Potential Energy
Chemical potential energy is the energy
stored in molecules (including in us)
 Examples: food, gasoline
 When gasoline is burned through
combustion, the arrangement of
molecules changes, and energy is released
 The released energy is used to do work
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Electric Potential Energy
Electrical potential energy is the result of
energy from a battery, a power plant, a
hydroelectric dam, solar cells, or a
windmill
 Electric potential energy can be
converted to sound, light, motion, etc.
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Work and Energy
Work and Energy are related
 When work is waiting to be done, we call
the energy potential
 When work is being done, we call the
energy kinetic
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Rubber Band
In order to make a rubber band fly, you must
do work on it
 While it is stretched, the rubber band has
potential energy
 Once you release it, it has kinetic energy
 If you pull it back a greater distance
(therefore doing more work) the potential is
greater
 The result is that the rubber band will go
faster and further (more potential energy
leads to greater kinetic energy, thus more
work can be done)
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Situations to consider
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Is work done in the following situations?
◦ Pushing against a wall?
 No, there is no displacement, no work, so kinetic
energy does not change
◦ Pushing at constant speed?
 No, work done by the applied force is equal to work
done by the frictional force, so kinetic energy does
not change
◦ Free fall
 Gravity is the only force acting, so there is a net force
and work is done . . . Kinetic energy changes
Kinetic Energy
Because energy is a property
of matter, it can be quantified
 The equation for kinetic energy is:
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1 2
KE  mv
2
Where “m” is the mass and “v” is the velocity of the
object
 The unit for energy is joules
Example 1
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What is the kinetic energy of a 45 kg
object moving at 13 m/s?
m  45kg
v  13m / s
2
1
KE  (45kg)(13m / s)
2
KE  3802.5J
Example 2
The kinetic energy of a boat is calculated
at 52,000 J. If the boat has a mass of
39,000 kg, with what velocity is it moving?
1 2
m  39,000kg
KE  mv
2
KE  52,000 J
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1
52,000 J  (39,000kg)(v) 2
2
52,000 J (2)
 v2
39,000kg
v  1.63m / s
Potential Energy
Remember that potential energy is the
energy of position
 To quantify potential energy use the
following:

PE  mgh

Where “m” is mass, “g” is gravity and “h”
is the height in meters
Example 3
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A 3.8 kg object is lifted to a height of 3
meters. What is the potential energy of
the object?
PE  (3.8kg)(9.8m / s )(3m)
2
m  3.8kg
g  9.8m / s 2
h  3m
PE  mgh
PE  111.7 J
Example 4
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A 30 kg child climbs 15 m up a tree.
When he stops to look around, what is
the child’s potential energy?
m  30kg
h  15m
g  9.8m / s 2
PE  mgh
PE  (30kg)(9.8m / s )(15m)
2
PE  4410J
State whether each of the following has
kinetic energy, potential energy, or both.
1. If an object is at rest, it certainly does NOT possess this
form of energy.
2. Depends upon object mass and object height.
3. The energy an object possesses due to its motion.
4. The amount is expressed using the unit joule
(abbreviated J).
5. The energy stored in an object due to its position (or
height).
6. The amount depends upon the arbitrarily assigned zero
level.
7. Depends upon object mass and object speed.
8. If an object is at rest on the ground (zero height), it
certainly does not possess this form of energy.
Work-Energy Theorem
 Work
is the transfer of energy
from one object to another, or . .
 The change in KINETIC
ENERGY!
W  KE f  KEi  KE
Total Energy

At any point, the total energy of an object
ME = KE + PE
Law of Conservation of Energy
 Energy
cannot be created or
destroyed.
 The total energy in a system before
an interaction equals the energy after
an interaction.
 This is true for a closed, isolated
system (no external forces present)
Law of Conservation of Energy
KEi  PEi  KE f  PE f
Example 1
A 12 kg rock is at the edge of a 95 m cliff.
a. What is the rock’s initial PE and KE?
b. If the rock falls to the ground, what is its
final PE and KE just before it hits?
c. What is the rock’s velocity just before it
hits the ground?
Example 2
A 33 kg cart rests at the top of a hill.
a. If the cart has a PE of 4800J, what is the
height of the hill?
b. When the cart has reached point B
(h = 5.0 m), what is its KE?
Example Problem
A greyhound at a race track can
run at a speed of 16.0 m/s. What
is the KE of the 20.0 kg
greyhound as it crosses the finish
line?
1
2
KE  (20kg)(16.0m / s)
2
KE  2,560 J
Example Problem 2
Determine the kinetic energy of a 625-kg
roller coaster car that is moving with a
speed of 18.3 m/s.
1
2
KE  (625kg)(18.3)  104,653 J
2
Example 3
Missy Diwater, the former platform diver
for the Ringling Brother's Circus, had a
kinetic energy of 12 000 J just prior to
hitting the bucket of water. If Missy's mass
is 40 kg, then what is her speed?
1
12,000 J  (40kg)(v) 2
2
24,000 J  (40kg)v 2
600  v 
2
v  24.5m / s
Example Problem
Legend has it that Isaac Newton
“discovered” gravity when an
apple fell from a tree and hit
him in the head. If a 0.20 kg
apple fell 7.0 m before hitting
Newton, what was its change in
PE during the fall?