Transcript Unit 3 Section 3 Notes

Unit 3 Section 3 Notes
Conservation of Energy
Energy Transformations
• Energy is most noticeable as it
transforms from one type to another.
• What are some examples of electrical
energy transforming to heat energy?
• A light bulb
• A hair dryer
Energy Transformations
• An example of transforming chemical
energy is a car engine. Chemical
potential energy in gasoline is
transformed into kinetic energy of the
car as it moves.
Energy Transformations
• Imagine you are sitting in the front car of
a roller coaster. A conveyor belt pulls
the car slowly up the first hill. When you
reach the crest of the hill, you are barely
moving. Then, you go over the edge
and start to race down the hill. You
speed faster and faster until you reach
the bottom of the hill.
Energy Transformations
• In the course of a roller coaster ride,
energy changes forms many times. The
conveyor belt is the most important part
of the ride; the energy from that initial
work is stored as gravitational potential
energy at the top of the first hill. After
that, most of the energy goes through a
series of transformations turning into KE
and then back to PE.
Energy Transformations
• In many situations, there is a conversion
between kinetic and potential energy.
• The total amount of potential and kinetic
energy in a system is called the
mechanical energy.
• Mechanical energy = PE + KE
• Mechanical energy is due to the position
and motion of the object.
Energy Transformations
• A small quantity of the stored energy is
transferred to the wheels as heat and to
the air as vibrations that make a roaring
sound.
• Energy readily changes from one form
to another.
• Whatever forms the energy takes during
a transformation, the total amount of
energy always remains the same.
Other Examples of PE to KE
Energy Transformations
• What happens to the mechanical
energy of an apple as it falls from a
tree?
Energy Transformations
• As the apple falls to the ground, its
height decreases. Therefore, its GPE
decreases.
• The potential energy is NOT lost… it is
converted into kinetic energy as the
velocity of the apple increases.
• What happens to the mechanical
energy?
Energy Transformations
• The mechanical energy does not
change because the loss in potential
energy is simply transferred into kinetic
energy.
• The energy in the system remains
constant!!
Energy Transformations
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Energy Transformations
• Think about the changes in energy
when you are on a swing…
• At what point do you have the most
potential energy?
• At what point do you have the most
kinetic energy?
• What happens to the mechanical
energy?
Law of Conservation of Energy
• The Law of Conservation of Energy
states that energy cannot be created
or destroyed.
• The big picture… the total energy in the
universe remains constant.
Law of Conservation of Energy
• But how? If I stop pumping while I’m
swinging, I stop!! So, where’s the
energy?
Law of Conservation of Energy
• You need to remember friction…..
• As you slow down on the swing, the
hooks and the chain rub against each
other and air pushes against the rider.
Law of Conservation of Energy
• Friction causes some of the mechanical
energy of the swing to change to
thermal energy and the temperature of
the hooks and chain heat up a little.
• The energy is still there, just in a
different form!!
Energy Transformations
• Objects lose mechanical energy that
can be in the form of sound or heat.
• When energy appears to disappear, it
has just changed to a nonmechanical
form.
Conservation of Energy
• Energy is transformed… not destroyed!!
• So even though the total amount of
energy remains constant, less energy is
available for doing useful work.
Energy Transformations
• Energy transformations in a bouncing
ball:
Energy in Our Bodies
• Even the energy converted in your body
follows the law of conservation of
energy.
• Chemical potential energy is transferred
to kinetic energy that allows your body
to move!!
Energy and Calories
• A Calorie (C) is a unit to measure
energy in foods.
• 1 Calorie is equal to about 4,184 Joules.
• A person uses about:
• 55 Calories while sleeping for 1 hour
• 210 Calories while walking for 1 hour
• 850 Calories while running for 1 hour