Transcript Energy

Energy
Kinetic and potential
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

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

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

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.

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?

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!!!

450,000,000,000,000 J is the
amount of energy in 3 million
gallons of gasoline!