Download Kinetic and Potential Energy

Energy Conversion
and Conservation
After the Lesson:
• You will be able to identify and describe
conversions from one type of energy to
another.
• You will be able state the law of
conservation of energy.
Definition of Energy
Conversion
The change from one form of energy to
another.
Radiant
Thermal
Electrical
Chemical
Mechanical
Nuclear
Sound
Magnetic
Series of Conversions-1
Often a series of energy conversions is needed to do a task.
For example, Strike a match:
•The mechanical energy used to scratch the match is converted to
thermal energy.
•The thermal energy causes the match to release stored chemical
energy
•The chemical energy is then converted to thermal energy and to
the energy you see as light.
Series of Conversions-2
In a car engine another series of conversions occurs.
•Electrical energy produces a hot spark.
•The thermal energy of the spark releases chemical energy in the
fuel.
•When the fuel burns, this chemical energy in turn becomes
thermal energy.
•Thermal energy is converted to mechanical energy used to move
the car, and to electrical energy that produces more sparks.
Kinetic and Potential Energy
One of the most common conversions is the conversion of
potential energy to kinetic energy.
•When you stretch a rubber band, you give it elastic. potential energy.
•If you let it go, the rubber band flies across the room.
•When the rubber band is moving, it has kinetic energy.
•The potential energy of the stretched rubber band is converted to
the kinetic energy of the moving rubber band
Energy Conversion in Juggling
Any object that rises or falls experiences a
change in its kinetic and gravitational
potential energy. Look at the orange in the
figure.
•When it moves, the orange possesses kinetic energy.
•As it rises, it slows down. Its kinetic energy
decreases. But because its height increases, its
potential energy increases.
• At the highest point in its path, it stops moving. At
this point, it no longer possesses kinetic energy, but
it possesses potential energy.
• As the orange falls, the entire energy conversion is
reversed—kinetic energy increases while potential
energy decreases. potential energy to kinetic energy.
Energy Conversion in a
Waterfall
There is a conversion between potential and
kinetic energy on a large scale at Niagara
Falls.
•The water at the top of the falls has
gravitational potential energy because it is
higher than the bottom of the falls.
•But as the water falls, its height
decreases and so it loses potential energy.
•At the same time, its kinetic energy
increases because its velocity increases.
•Thus potential energy is converted into
kinetic energy.
Energy Conversion in a Pole
Vault
•As a pole vaulter runs, he has kinetic energy
because he is moving.
•When he plants his pole to jump, the pole bends.
His kinetic energy is converted to elastic
potential energy in the pole.
•As the pole straightens out, the vaulter is lifted
high into the air. The elastic potential energy of
the pole is converted to the gravitational
potential energy of the pole vaulter.
•Once over the bar, the vaulter’s gravitational
potential energy is converted into kinetic energy
as he falls to the safety cushion below.
Energy Conversion in a
Pendulum
A continuous conversion between kinetic energy and potential energy takes
place in a pendulum.
•At the highest point in its swing, the pendulum in the figure has only gravitational
potential energy.
•As the pendulum starts to swing
downward, it speeds up and its
gravitational potential energy changes
to kinetic energy.
•At the bottom of its swing, all its
energy is kinetic energy.
•Then, as it swings to the other side
and slows down, it regains gravitational
potential energy, and at the same time
loses kinetic energy.
•At the top of its swing on the other
side it again has only gravitational
potential energy. And so the pattern
of energy conversion continues.
Energy Conservation
If you set a pendulum in motion, do you think it will remain in motion
forever?
No, it will not. Does that mean that energy is destroyed over time?
•The answer is no.
•The law of conservation of energy states that when one form of
energy is converted to another, no energy is destroyed in the
process.
According to the law of conservation of energy, energy cannot be
created or destroyed.
•So the total amount of energy is the same before and after any
process. All energy can be accounted for!!
Energy and Friction
So what happens to the kinetic energy of
the pendulum?
•As the pendulum moves, it encounters
friction at the pivot of the string and
from the air through which it moves.
•When an object experiences friction,
the motion (and thus the kinetic
energy) of the atoms or molecules
increases.
•This means its thermal energy
increases. So the mechanical energy of
the moving pendulum is converted to
thermal energy.
•The pendulum slows down, but its
energy is not destroyed.
Energy and Friction
The fact that friction converts mechanical energy to
thermal energy should not surprise you. After all, you take
advantage of such thermal energy when you rub your cold
hands together to warm them up. The fact that mechanical
energy is converted to thermal energy because of friction
explains why no machine is 100 percent efficient. Now you
know that mechanical energy is converted to thermal energy
in a machine.
Energy and Matter
You might have heard of Albert Einstein’s
theory of relativity. Einstein’s theory included
a small change to the law of conservation of
energy.
•He explained that energy can sometimes be
created—by destroying matter!
•This discovery means that in some situations
energy alone is not conserved.
•But scientists say that matter and energy
together are always conserved.
•Just as different forms of energy can be
converted to one another, matter can
sometimes be converted to energy.
Some Sample P.E. Problems!!

Calculate the Potential Energy:
1.
A 250 N book bag raised 5.6m off the ground.
• Formula to use: PE= w*h
• PE= 250N*5.6m
• PE= 1400 Joules
2. A 28.6 N object being held 128cm above the floor.
• Formula to use: PE= w*h
• Remember 128cm equals 1.28m
• PE= 28.6N*1.28m
• PE= 36.608 Joules
3. A 629 Kg car at the top of a small hill, 4,122mm tall.
• Formula to use: PE= mgh
• Remember 4,122mm equals 4.122m
• PE= 629Kg * 9.8m/s2 * 4.122m
• PE= 25,408.8324 Joules