Download Representing Energy , Energy Transfers and Energy

Big Ideas
About the
Conservation
of Energy
By Laura Zinszer 2011
Columbia Public Schools & Physics First
Now that we know the
types of Energy . . .
Let’s analyze the total energy
in a system. . .
All Energy is
measured in
a unit called
a Joule (J) or
kilo-joule (kj).
Total Energy is the all of the
potential (usually Eg) and
all of the kinetic energy.
TE = Ep + Ek
To determine the total
energy, we will need to
calculate the Ep and Ek,
and then add all
the energy
together.
Gravitational Potential
Energy (Eg)
Eg = m * g * h
m = mass (Kg or g)
g = 9.8 (m/s2)
h = height (m or cm)
Joules (Kg and m)
Kilo-joules (g and cm)
Gravitational Potential
Energy (Eg).
Why would a ball on the ground,
have no gravitational potential
energy?
(Eg)
Why is this true?
The higher that an object
is elevated, the greater
the Eg …
Gravitational Potential
Energy (Eg)
For example, a 3.5 kg
ball held at a vertical
position of 20 meters
above the ground has
gravitational potential
energy.
(What is the Eg)
Gravitational Potential
Energy (Eg)
Eg = m * g * h
Eg = 3.5kg * 9.8
Eg = 686 J
m/s2
.
* 20m
Kinetic Energy (Ek)
Ek = ½ m
2
*V
m = mass (Kg or g)
V = velocity (m/s or cm/s)
Kinetic Energy (Ek)
The amount of Ek
depends on the
mass and
velocity of the
object.
Therefore . . .
The greater the velocity or speed of
an object, then the greater the Ek!
Kinetic Energy (Ek)
If the car has a
mass of 2000 kg,
and travels at a
velocity of 15
m/s, then the
Kinetic Energy
equals…
Kinetic Energy (Ek)
2
Ek= ½ m * V
2
Ek= ½ 2000kg * (15 m/s)
2
2
Ek= 1000 kg * 225 m /s
Ek= 225,000 J
How do Eg
and Ek
relate in an
energy
system?
As the car moves down the track, Why
does the Eg decreases and Ek increase?
As the height is lower, the Eg becomes
less. The car speeds up, so Ek increases!!!
What is the car’s position on the track
when the Eg = Ek?
One-half of the distance down the track.
Why does the Total Energy (TE)
remains constant?
Total Energy (TE)
TE = Eg + Ek
The TE will always remain constant!!
As the pendulum swings, the
Eg is transformed into Ek …
but the Total Energy (TE)
remains constant.
What is the position of the pendulum
when all of the energy is Eg?
The pendulum is at the top on
either side where it is stopped.
What is the position of the pendulum
when all of the energy is Ek?
The pendulum is at the bottom of
the swing…and is at its fastest!!
What is the position of the pendulum
when Eg = Ek?
The pendulum is ½ way through
the drop.
As the coaster travels on the track,
the Eg is exchanged for the Ek.
but the Total Energy (TE) remains
constant.
The BIG IDEA for energy is
Total Energy in the system is equal to
the Eg + Ek ….
The Total Energy in the system
will always remain constant.
Conservation of Energy!
Energy can not be
created or
destroyed;
it can only be
transformed from
one type of energy
form to another.
Let’s Look at some Energy Graphs
As the
swinging
pendulum
falls…
Eg Ek TE
Eg Ek TE
Eg Ek TE
Eg Ek TE
Eg Ek TE
The Eg is
transformed
into Ek
Eg Ek TE
Eg Ek TE
Eg Ek TE
but the
Total Energy
in the system
at each
position
remains
constant.
Eg Ek TE
Eg Ek TE
Eg Ek TE
Eg Ek TE
As the kids on the sled go down the
hill… the energy graph would look
like this…
Eg Ek TE
Eg Ek TE
Eg Ek TE
Eg Ek TE
Eg Ek TE
But the Total Energy in
this system remains
constant as the sled
goes down the hill!
Practice
Problem #1
Formula
Work
Answer
Units
A 5 kg ball is held at a vertical height of 32 meters.
How much Gravitational Potential Energy does the ball
have? (Show all math formulas, calculations, and
units)
Problem #3
A car has a mass of 1300kg and is traveling at a
velocity of 31m/s. How much Kinetic Energy does the
car have?
Energy Efficiency
A toy car
was going
down a track.
It hit a rubberband
but only bounced
part of the way up.
Energy Efficiency
final Eg / beginning Eg
l
100 = Efficiency (%)
Energy Efficiency
The car had a final
Eg of 28 J when it
bounced up after
hitting the
rubberband.
The car had a
beginning Eg
of 32 J.
Eg (initial)
of 32 J
Eg (final) of 28 J
Energy Efficiency
28 J = .875 x 100
32 J
Energy Efficiency = 87.5%
Eg of 32 J
Eg of 28 J
Energy Efficiency
If only 87.5% went back to the final
Eg, what happened to 12.5% of the
energy that disappeared?
Eg of 32 J
Eg of 28 J
It became thermal energy
due to the friction when the
car hit the rubberband.
How does the amount of work
done compare to Eg of an
object? (Bucket of Lead)
Eg = m * g * h
W=f*d
Work & Gravitational
Potential Energy (Eg)
Eg = 1.74 kg * 9.8 * 1.5 m
W = 16.8 N * 1.5 m
Bucket of Lead
Work & Gravitational
Potential Energy (Eg)
Eg = 1.74 kg * 9.8 * 1.5 m
Eg = 25.6 J
W = 16.8 N * 1.5 m
W = 25.2 J
Remember that work does not have to
be vertical. We could pull the bucket
of lead across the floor and there
would be work done without any Eg.
Work = 2.3 N * 1.5 m = 0.45 J
Why is less work done? Same
distance moved?