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Physical Science
Ch. 4: Energy
• Energy is the ability of an object to do
work. The SI unit for measuring energy
is the Joule (j).
• 1 J is the amount of energy required to
move a 1 N weight, 1 meter.
• Kinetic energy is energy in motion.
• The amount of kinetic energy which an
object has depends on the objects mass
and velocity.
i.e. the bigger it is or the faster it is
moving, the more kinetic energy it has.
• What other property that we’ve talked
about is kinetic energy closely related to?
• Potential energy is stored energy. It is the
result of an objects position or condition.
Examples:
-a ball sitting on the top shelf
-anything that is flammable
-a skydiver ready to jump
• Gravitational potential energy is due to an objects
position above the ground.
Energy was required to elevate the object to that
height, and since that energy can’t be destroyed
(Law of Conservation of Energy), then it is stored
in the object until released.
• Why does is burn when
you slide down the rope
in gym class?
• P.E. due to the elasticity of a material is called
elastic potential energy.
• Examples of E.P.E.:
– A drawn bow
– A rubber band
– A compressed spring
• Chemical potential energy is energy
released through a chemical reaction.
• Mechanical energy is the combined total of
the kinetic and potential energy in an
object or a system.
K.E. + P.E. = M.E.
• The Law of Conservation of Energy states
that energy can not be created or destroyed,
but may change from one form to another.
Therefore, the total amount of energy in the
universe is constant.
• Example:
A bouncing ball will
continually alternate
between kinetic and
potential energy as it
goes up and down.
(Changing from one
form of energy to
another.)
• Explain what happens to the kinetic,
potential, and mechanical energies of a
pendulum as it swings back and forth.
• Sometimes energy might seem to be lost,
like the bouncing ball gradually coming to
a stop, or the pendulum gradually
stopping. This energy is not actually lost,
it is just transferred into a different form,
such as thermal energy (heat).
• An automobile engine
takes gasoline
(chemical potential
energy) and burns it
(thermal energy) and
uses the gases from the
burning to turn drive
pistons which turn the
axles (kinetic energy).
• A calorimeter is a device used to measure
changes in thermal energy (find the calories
in food), just like a thermometer is used to
measure changes in kinetic energy.
• Changes in thermal energy are measured in
joules, but the English unit is the calorie.
1 J = .2399 calories
1 calorie = 4.19 J
• So, calorimeters can tell how many calories of
energy are released by certain foods. This will be
the same as the amount of energy the foods will
release in your body.
• This also tells you how much energy you need to
produce to burn up these foods.
• So if a Big Mac has 850 calories, then that’s
how much energy it will produce in your body.
That’s also how much energy you’ll need to
produce to burn it off.
• The average person takes in 2,000-2,500
calories per day.
• Here’s the problem, certain foods
contain large percentages of
proteins and carbohydrates, which
your body burns with relative ease.
However, your body has a harder
time breaking down fatty foods. So
if you eat a diet high in fat, then
you’ll either have to work harder to
get rid of the calories, or you’ll gain
weight.
Calculating Changes in Thermal Energy
Q = M x rt x Cp
Q = change in thermal energy
M = mass
rt = change in temperature
Cp = specific heat of the material
• A perpetual motion device will continue
running indefinitely on an initial source of
energy, without ever having to draw more
energy from an external source.
These are also called free energy
machines.
• Perpetual motion machines will eventually
solve all of our energy problems, and
provide an everlasting source of cheap
energy. Right (?)………….
• Wrong! It sounds great in
theory, but creating a perpetual
motion device is against the
law.
• Which law would that be?
• Nevertheless, this has not stopped some
“scientists” from continually trying to
create a free energy machine.
• How do you think the following devices are
designed to work?
Work
• Work is the amount of
force which is applied
over a given distance.
W=FxD
• Work is a form of energy
and therefore is
measured in joules (j).
• And if you remember from Newton’s 2nd
Law of Motion,
Force = Mass x Acceleration
So you could also say that Work = M x A x D
• In a nuclear fission reaction (like an
atomic blast), atoms are split apart and
produce a tremendous amount of energy.
Do you think that this is a violation of the
Law of Conservation of Energy? Why or
why not.
• In order to push her car 300 m, Kim
had to do 27,600 J of work. How
much force was she producing?
• Ralph’s diet consists primarily of proteins
and carbohydrates, while Homer eats a lot
of fatty foods. What effect, if any, will this
have on the boys if their daily caloric intake
is the same (2,000 cal./day)? Explain your
answer.
• What happens to the amount of gravitational
potential energy of a projectile as it flies
through the air?
• The kinetic energy?
• In an automobile engine the amount of kinetic
energy produced from chemical potential
energy (gasoline) is relatively low, due to
undesirable energy transfers.
What are some ways that you could decrease
the amount of undesirable energy transfers
occurring in your engine?
• As a pendulum swings back and forth, the
mechanical energy is 120 J. At 1 point during the
swing, if the pendulum is experiencing 10 J of kinetic
energy, how much potential energy will there be?
• What type of potential energy are we talking about?
• About where in the swing would this scenario take
place?
• A parachutist jumps out of a plane and
accelerates for 5 sec., after which time he
reaches terminal velocity. After falling for 10
seconds at that speed, he deploys the
parachute and drifts to the ground. Tell what
happens to each of the following during these
3 phases:
• Kinetic energy
• Potential energy
• Mechanical energy
• A car with a mass of 1,200 kg accelerates
at 10 m/s/s over a distance of 500m. How
much work was done by the car’s engine?
A car’s engine takes in 15,000 J of chemical
potential energy and produces 4,000 J of kinetic
energy.
•How much thermal energy was produced by the engine?
•What would an oil change do for the system?
• A red Pacer accelerates at 3 m/s/s over a
distance of 500 m. If the car has a mass of
900 kg, how much work was done by the
car’s engine?
• If a 7 kg bowling ball is dropped from
a window which is 45 m above the
ground, how much work is being done
by gravity?
• Heili is running the
marathon at the 2008
world championships.
He carb loads for 2
days prior to the event
to insure a good
energy supply during
the race. As his body
burns these calories,
what 2 types of energy
are produced as a
result?
• The formula used to
calculate the gravitational
potential energy of an object
is GPE = M x 9.8 x H, where
M is mass, G is acceleration
due to gravity, and H is
height above the ground.
• A baseball player pops up the first pitch. If the
ball (.25 kg) reaches a height of 15 m, how
much GPE did it have at it’s highest point?
• How much kinetic energy did it have when it
was caught?
• How much mechanical energy did it have?