Download Energy - Plain Local Schools

Energy
Kinetic and potential
Forms of Energy
Mechanical - energy with which moving
objects perform work Ex: bicycle, sound
 Chemical - energy stored in substances
because of their chemical makeup Ex:
coal, oil, gasoline, foods
 Nuclear Energy - stored in the nucleus of
the atom Ex: splitting or fusing the atom
 Heat Energy - energy produced by
molecular motion Ex: All molecules vibrate

Forms of Energy
Electrical - energy as a flow of electrons
through a conductor, such as a wire Ex:
computers, television, appliances
 Light (Electromagnetic) - form of radiant
energy that moves in waves Ex: solar
panels
 Conservation of Matter and Energy energy and matter cannot be created or
destroyed, only transformed

Work

Work - a form of mechanical energy
always measured in Joules (J)
Units
Work Formula
Work = Force x distance
W = F x
d
Joules Newtons meters
Power

Power - the amount of work completed in an
specific amount of time, units are Watts
Units
Power Formula
Power = Work / time
P = W / t
Watts = Joules / second
***See sample problems
Daily Quiz #2 Sm 2
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1. What type of energy refers to the movement
of an object?
2. Which type of energy is often referred to as
stored energy?
3. Which type of energy is calculated by
multiplying the force of an object by the distance
it moved?
4. What are the units for energy?
5. What are the units for power?
6. Power is calculated by dividing Work by what?
Bonus : Which burns more calories running or
walking a mile?
Machines
A machine is a device to make work
easier.
 Machines do this by changing the size or
the direction of the applied force.
 The force that is applied to a machine is
called the effort force, fE.
 The work done on a machine is called
work input, Wi

Work must always be done on a machine
if the machine is to do any work.
 The distance through which the machine
moves is the effort distance, dE.
 Wi = FE X dE
 The force applied by the machine is called
the resistance force, FR.
 The resistance force is often the weight of
the object being moved. It opposes the
effort force.

The distance the object moves is the
resistance distance, dR.
 Machines can only multiply force.
Because machines cannot multiply work,
work output can never be greater than
work input.
 Mechanical Advantage is the number of
times a machine multiplies the effort force.
 Efficiency is the comparison of the work
output to work input. It is usually
expressed as a percent.

Efficiency = WO X 100
WI
Ex) What is the efficiency of a machine that
Has a work output of 200 J and a work
input of 100 J?
Efficiency = 200 J X 100 = 200 %
100 J
High efficiency means that much of the work
is changed to useful work output.
The less friction in a machine the higher the
efficiency.
Energy can be classified as
potential or kinetic

Potential energy: energy of position
The boulder has
more gravitational
potential energy when
measured from point A
compared to B.
PE = mgh
PE = mgh
m = mass in kilograms
 g = acceleration due to gravity
(9.8 m/s2)
 h = height in meters
 Potential Energy is measured in
kg • m/s2 • m =
newton meter =
Joules

Example 1:
 If
8kg
10m
the boulder
has a mass of
8kg, and
distance B is
10m, what is the
potential energy
of the boulder
relative to the
plateau?
Answer:
 PE
8kg
10m
= mgh
 = 8kg • 9.8m/s2 • 10m
 =784 J
Example 2:
 If
8kg
distance A is
20m, what is the
potential energy
of the boulder
20m relative to the
bottom?
Answer:
 PE
8kg
= mgh
 = 8kg • 9.8m/s2 • 20m
 = 1568 J
20m
Example 3
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When work is done to stretch something, it
is called elastic potential energy.
Find the elastic
potential energy stored
in a drawn bow if it
takes an average force
of 100N to pull the
arrow back a distance
of 0.5 meters.
Answer:

PE = mgh = Fd
Fxd = 100N x 0.5m
= 50J
Kinetic energy
Energy of motion
 KE = ½ mv2

KE = 1/2mv2
m = mass in kilograms
 v = velocity in meters/sec
 Kinetic energy is measured in

kg • m/s • m/s =

kg m/s2 • m =

newton meter =

Joules
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Example 4
The bird has a mass of 2 kg. It is
flying at a speed of 5 m/s. Find its
kinetic energy.
 KE = ½ mv2
 = ½ • 2kg • (5m/s)2
 = ½ • 2kg • 25 m2/s2
 = 25 joules
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Kinetic and potential energy
conversions
Describe the energy
conversions in this picture:
 At the top: All PE, no KE
 ¼ of the way down: 3/4 PE, 1/4 KE
 ½ way down: 1/2 PE, 1/2 KE
 ¾ down 1/4 PE, 3/4 KE
 At the bottom: No PE, All KE
 Is the sum of KE + PE a
Yes! It’s always
constant? 10,000 J in this case.
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Where are PE and KE maximums
in this picture?
What happens
when the cord
is cut?
Potential energy
is converted to
kinetic energy!
Kinetic and potential energy
convert to one another
PE max
PE max
no KE
no KE
KE max
KE max
no PE
no PE
How is mass related to energy?
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Albert gave us
the answer!
2
E=mc
E=mc2
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E = energy in joules
m = mass in kilograms
c = speed of light
(3 x 108 m/s)
A small mass gives a great amount
of energy
Nuclear energy- clean power for
the world?
Example:
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How much energy can be given off by 5
grams of mass? ( 10 paper clips = 5 g)
5g = 0.005 kg
E=mc2
= 0.005kg x (3 x 108 m/s)2
= 0.005 kg x 9 x 1016 m2/s2
= 0.045 x 1016
= 450,000,000,000,000 J
WOW!!!
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450,000,000,000,000 J is the
amount of energy in 3 million
gallons of gasoline!