Download 9.7 Conservation of Energy - Fort Thomas Independent Schools

Chapter 9.7
Conservation of Energy
9.6 Work-Energy Theorem
For moving objects such as cars:
The more kinetic energy it has, the more work is required to
stop it.
Twice as much kinetic energy means twice as much work.
Brakes do work on wheels (you do work by pushing the brake
pedal). When a car brakes, the work is the friction force
(supplied by the brakes) multiplied by the distance over which
the friction force acts.
KE is transformed by work (friction) into thermal energy, sound
energy and larger-scale vibrations.
9.7 Conservation of Energy
The law of conservation of energy states that energy
cannot be created or destroyed. It can be
transformed from one form into another, but the
total amount of energy never changes.
For any system in its entirety—as simple
as a swinging pendulum or as complex as
an exploding galaxy—there is one quantity
that does not change: energy.
Energy may change form, but the total
energy stays the same.
9.7 Conservation of Energy
When energy is transformed, it is conserved, meaning that it
will change form without losing its original amount of energy.
9.7 Conservation of Energy
When the woman leaps from the
burning building, the sum of her PE
and KE remains constant at each
successive position all the way down
to the ground.
9.7 Conservation of Energy
Elastic potential energy
will become the kinetic
energy of the arrow
when the bow does
work on the arrow.
As you draw back the arrow in a bow, you
do work stretching the bow.
The bow then has potential energy.
When released, the arrow has kinetic
energy equal to this potential energy.
It delivers this energy to its target.
9.7 Conservation of Energy
Part of the PE of the wound spring changes into KE. The
remaining PE goes into heating the machinery and the
surroundings due to friction. No energy is lost.
9.7 Conservation of Energy
Same energy
transformation applies
The 2 J of heat can be called nonuseful work (work that is not part of the
object’s total mechanical energy).
10 J of PE does 8 J
useful work on the
arrow and 2 J of
non-useful work on
the molecules that
compose the bow
and string and
arrow. The arrow
has 8 J of KE.
9.7 Conservation of Energy
Everywhere along the path of the pendulum bob, the sum of
PE and KE is the same. Because of the work done against
friction, this energy will eventually be transformed into heat.
Non-useful work can also be called non-useful energy!
Useful vs. Non-Useful Energy
• Useful energy is total mechanical energy
TME = PE + KE
• Non-useful energy are those forms that cause the TME
to decrease.
– Thermal energy (heat from friction)
– Sound energy
– Vibrations not related to the original motion of the object
• Example: friction decreases KE, causing the object to
decrease in speed and causing the object and it
surrounding to increase in temperature.
• Can also be referred to as useful and non-useful work.
9.7 Conservation of Energy
• Why does a tennis ball eventually stop
bouncing?
• Eventually, all of the total mechanical energy is
transformed into non-useful energy (heat,
sound, movement of fibers)
50 J
PE
New height less than before means less PE
stored 35 J PE
20 J PE
50 J
KE
Bounce!
35 J KE
Bounce!
20 J KE
(bounce and so
on!)
Slides showing transformation of
KE and PE
• Source:
http://www.physicsclassroom.com/mmedia
/index.cfm
Watch how KE and gravitational PE
transform
Where is the KE at the maximum?
Where is the PE at the maximum?
How is PE stored?
Watch the change in height vs. the
change in speed!
How does the change in height affect KE and PE?
What happens to KE and TME when the
brakes are applied? What work is being
done?
Watch the transfer of KE and PE.
What happens to the PE when the skier moves down the hill?
What happens to the KE and TME when the skier travels over the
unpacked snow?
What work is done?
Same work, more force, less
displacement (from left to right)
9.7 Conservation of Energy
Each atom that makes up matter is a concentrated bundle of
energy.
When the nuclei of atoms rearrange themselves, enormous
amounts of energy can be released.
The sun shines because some of its nuclear energy is
transformed into radiant energy.
In nuclear reactors, nuclear energy is transformed into heat.
9.7 Conservation of Energy
Enormous compression due to gravity in the deep, hot interior
of the sun causes hydrogen nuclei to fuse and become helium
nuclei.
• This high-temperature welding of atomic nuclei is called
thermonuclear fusion.
• This process releases radiant energy, some of which
reaches Earth.
• Part of this energy falls on plants, and some of the plants
later become coal.
9.7 Conservation of Energy
• Another part supports life in the food chain that begins with
microscopic marine animals and plants, and later gets
stored in oil.
• Part of the sun’s energy is used to evaporate water from the
ocean.
• Some water returns to Earth as rain that is trapped behind a
dam.
9.7 Conservation of Energy
The water behind a dam has potential energy that is used to
power a generating plant below the dam.
• The generating plant transforms the energy of falling
water into electrical energy.
• Electrical energy travels through wires to homes where it
is used for lighting, heating, cooking, and operating
electric toothbrushes.
9.7 Conservation of Energy
What does the law of conservation of
energy state?