Download Potential and Kinetic Energy

Potential and Kinetic Energy
Have you ever swung at a playground? Can you describe your motion
while swinging? Do you ever feel like the swing stops at a certain point
or the swing goes faster? What do you think causes these changes in
speed? Do you think your energy is also changing as you swing?
Energy can change within a system.
Energy can take many forms. What do these forms have in common?
They all have the ability to cause changes to a system, or a set of
connected things that work together. As you swing on the playground,
you and the swing form a system.
The increase or decrease of energy in
physical variable:
a system has a direct effect upon the
something measurable
system’s physical variables. These
that can change
variables include temperature, speed,
position, pressure, and motion. If
someone turns on a stove burner under a pot of water, energy from the burner will cause the pot to
heat up. The heat will cause the water to come to a boil. This shows the relationship between
energy and the system of the water- filled pot.
Energy can be classified as potential or kinetic.
There are two main forms of energy in a system: potential and kinetic. Potential energy (PE) is
stored energy. Kinetic energy (KE) is energy in the form of motion. The total amount of potential
energy and kinetic energy in a system is known as mechanical energy.
Think back to the swing example. This diagram shows how the swing moves back and forth as a
person rides it. When the person has swung all the way back (position A), the swing pauses a
moment. At this moment, the swing has only potential
energy. The swing then falls forward, gradually gaining
speed. As it falls, its potential energy changes to kinetic
energy. At position B, the swing has only kinetic energy.
As the swing continues forward, it gradually slows down.
Its kinetic energy changes back to potential energy until it
reaches the farthest point in its arc (position C). Here, the
swing pauses again for a moment. At this moment, the
swing has only potential energy. It then falls backward
through its arc. Its potential energy changes to kinetic
energy, and the cycle is repeated.
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Potential and Kinetic Energy
At every point in this cycle, the system of swing and person has the same amount of mechanical
energy. In other words, when the system has less potential energy, it has more kinetic energy. And
when the system has more kinetic energy, it has less potential energy. At each moment, the
system’s kinetic energy and potential energy add up to the same value.
You may think that as a swing gains speed, its potential energy is
destroyed and kinetic energy is created. In fact, energy can only change
forms. It cannot be created or destroyed. This is called the law of
conservation of energy. Why can’t you swing forever? Where does the
energy go?
As you swing, your body collides with particles of air. These particles are
tiny, but there are lots of them. Each time you collide with air particles, a
little of your kinetic energy is transferred to the particles. This force, called
air resistance, gradually slows you down. Another force that slows you
down is called friction. As the swing moves, its parts rub against each
other. As this happens, some of the swing’s energy changes to heat and
leaves the system. If there were no friction or air resistance, you could
swing forever!
As air particles
collide with
the parachute,
the force of air
resistance slows
down the skydiver.
Potential energy and kinetic energy are similar but not the same.
Scientists measure both potential energy and kinetic energy in joules (J).
A joule describes the amount of energy needed to do a certain amount of
work or cause a certain amount of change. So, more joules of energy can
perform more work or cause more change. Scientists can use the same
unit to measure both types of energy because kinetic energy and potential energy are related.
Remember, a system’s mechanical energy equals its potential energy plus its kinetic energy.
There are also differences between potential energy and kinetic energy. Potential energy is stored
energy. In other words, it has the potential to become kinetic energy. The chemicals that make up
food, batteries, and fuel all contain potential energy. When you eat food, your body converts the
food’s potential energy into kinetic energy you can use to move and function. When fuel is burned
in a car engine, the fuel’s potential energy is converted to kinetic energy that powers the car.
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Potential and Kinetic Energy
Potential energy depends on gravity. The higher an object is, the more potential energy the object
will have. The force of gravity is stronger on an apple high in a tree than an apple low in a tree. So,
the higher apple will have greater potential energy than the lower apple.
Kinetic energy is the energy of motion. It depends on an object’s mass or velocity. A large car will
have greater kinetic energy than a small car. What if two cars have the same mass but are moving
at different velocities? The car moving faster will have greater kinetic energy.
Look at this picture of an apple tree. Which apples have the most
potential energy? Which apples have the least potential energy?
Everyday Life: Energy and Roller Coasters
Mechanical energy is part of everyday life. Have you ever ridden
a roller coaster at an amusement park? Engineers who design
roller coasters must understand the relationship between potential
and kinetic energy.
For example, engineers take advantage of potential energy when
the coaster car is at the top of the first hill of the roller coaster.
This hill is usually the highest point on the coaster. So, a car here
will have the greatest potential
energy. As the coaster car rolls
down the hill, its potential energy
is converted to kinetic energy. At
the bottom of this hill, the car’s
velocity is very high. The car has
lots of kinetic energy. This kinetic
energy propels the car up the
next hill. As the car climbs this
hill, its kinetic energy decreases.
Where does it go? It is converted
to potential energy.
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Potential and Kinetic Energy
What do you know?
A student sets up the pendulum system shown below. He holds the pendulum at the top of its arc.
Draw the path of the pendulum after the student releases it. Label the following points in the
pendulum’s path. If the total mechanical energy of the system is 100 J:
• 
Where does the pendulum have 100 J of kinetic energy? How many joules of potential energy
does the pendulum have at this point?
• 
Where does the pendulum have 100 J of potential energy? (Label both points.)
• 
• 
How many joules of kinetic energy does the pendulum have at each of these points?
Where does the pendulum have 50 J of potential energy? (Label both points.)
• 
How many joules of kinetic energy does the pendulum have at each of these points?
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Potential and Kinetic Energy
Egg Drop Experiment
This can be a messy experiment but a fun
way to see how gravitational potential
energy is converted to kinetic energy. As a
family, you can design a compartment to
safely protect an egg so that it does not
break when you drop it from a height of
several meters. You may want to perform
this experiment outdoors. Be careful when
dropping the eggs—make sure you are
standing on a sturdy foundation so that you
will not slip or fall.
Have your child explain when the potential
energy and kinetic energy were the highest
and lowest during each test.
You may construct your compartments from
a wide variety of materials including egg
cartons, milk cartons, cereal boxes,
newspapers, and bubble wrap. You will also
need eggs and a meter stick or tape
measure.
• 
Which materials were the most successful
in protecting an egg? Why do you think
so?
• 
Did you use friction or air resistance to
your advantage in your design?
• 
How do you think engineers use models
and tests similar to this activity when they
design safety features for cars? What
would be the advantages of using models
instead of actual cars?
• 
Can you apply what you learned in this
activity to other areas of your life?
You may need to test many different designs
(and break many different eggs!) before you
design a compartment that successfully
protects an egg. Record which designs and
materials have the most success. Suggest
possible reasons why these types of material
were most successful in protecting an egg.
After completing the experiment, you can
discuss how these concepts may apply to
other areas in life—for example, designing
cars to safely withstand a crash or sneakers
to cushion a runner’s feet.
Here are some questions to discuss with
students:
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