LUX MIDDLE SCHOOL
8th grade Science
Objective 8.3.4: Investigate energy and power:
b. Describe potential and kinetic energy
Scientist: Dorina Marta Mihut
Lead Teacher: Angela Zabawa
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KINETIC AND POTENTIAL ENERGY
Potential Energy
An object can store energy as the result of its
position.
For example: the heavy ram of a pile driver is storing energy when it
is held at an elevated position.
This stored energy of position is referred to as potential energy.
Potential energy is the stored energy of position possessed by an object.
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Gravitational potential energy is the energy stored in an object as the result
of its vertical position (i.e., height). The energy is stored as the result of the
gravitational attraction of the Earth for the object.
The gravitational potential energy of the heavy ram of a pile driver is
dependent on two variables - the mass of the ram and the height to which it
is raised. There is a direct relation between gravitational potential energy
and the mass of an object; more massive objects have greater gravitational
potential energy. There is also a direct relation between gravitational
potential energy and the height of an object; the higher that an object is
elevated, the greater the gravitational potential energy. These relationships
are expressed by the following equation:
PEgrav = mass * g * height
Similarly, a drawn bow is able to store energy as the result of its position.
When assuming its usual position (i.e., when not drawn), there is no energy
stored in the bow.
Yet, when its position is altered from its usual equilibrium position, the bow
is able to store energy by virtue of its position. This stored energy of position
is elastic potential energy.
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Kinetic Energy
Kinetic energy is the energy of motion.
An object which has motion - whether it is vertical or horizontal
motion - has kinetic energy.
There are many forms of kinetic energy:
• translational (the energy due to motion from one location to
another).
• vibration (the energy due to vibration motion),
• rotational (the energy due to rotational motion)
We will focus upon translational kinetic energy.
The amount of translational kinetic energy of an object depends
upon two variables:
1. the mass (m) of the object and
2. the speed (v) of the object.
The following equation is used to represent the kinetic energy
(KE) of an object:
where m = mass of object
v = speed of object
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Analysis of a Situation in Which Mechanical
Energy is Conserved
A roller coaster operates on the principle of energy transformation.
Work is initially done on a roller coaster car to lift to its initial summit.
Once lifted to the top of the summit, the roller coaster car has a large
quantity of potential energy and virtually no kinetic energy (the car is almost
at rest). If it can be assumed that no external forces are doing work upon the
car as it travels from the initial summit to the end of the track (where finally
an external braking system is employed), then the total mechanical energy of
the roller coaster car is conserved. As the car descends hills and loops, its
potential energy is transformed into kinetic energy (as the car speeds up); as
the car ascends hills and loops, its kinetic energy is transformed into
potential energy (as the car slows down). Yet in the absence of external
forces doing work, the total mechanical energy of the car is conserved.
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EXPERIMENT
In this lab we will explore the effect of the height of a ramp and the
mass of an object on the potential and kinetic energy.
Hypothesis
As the height of a ramp increases, potential and kinetic energy will………………………
As the mass increases, potential and kinetic energy will…………………………………
Materials
•
•
•
•
3 balls ( different mass) ramp (a piece of plywood)
meter stick
balance
stop watch
Procedure
1. Weigh each ball on the balance to determine its mass (in grams). Record the mass in
the data table.
2. Draw a starting line from the top of the plywood.
3. Place one block of wood (1 book) under the end of the plywood to make a ramp.
Measure the height of the ramp (m) and record it.
4. Place one of the balls on the starting line.
5. Release the ball and start the stop watch.
6. When the ball has used all its energy, i.e., when it comes to a complete stop, record the
time.
7. Measure and record the distance (in meters) that the ball traveled.
8. Repeat steps 4-7 with the other two balls.
9. Place one additional block (2 books) under the end of the plywood. Measure the new
height of the ramp and record it on the data table.
10. Repeat steps 4-7 with each of the three balls.
11. Using the third block of wood (3 books), raise the plywood ramp still higher. Measure
the new height and record it on the data table.
12. Repeat steps 4-7 with each ball.
Observations
1. Height of the ramp (m) = ………..
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Mass (g)
Distance (m)
Time (sec)
Velocity
(m/s)
Time (sec)
Velocity
(m/s)
Time (sec)
Velocity
(m/s)
Ball 1
Ball 2
Ball 3
22. Height of the ramp (m) = …………
Mass (g)
Distance (m)
Ball 1
Ball 2
Ball 3
3. Height of the ramp (m) = …………
Mass (g)
Distance (m)
Ball 1
Ball 2
Ball 3
Questions
1. When in this investigation
__________________________
did
each
ball
have
potential
energy?
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_______________________________________________________________________
2.
When
did
each
ball
have
kinetic
energy?
_______________________________________________
________________________________________________________________________
3. What is the relationship between energy, mass and height (potential and kinetic)?
________________
________________________________________________________________________
5.
What
evidence
do
you
have
that
supports
your
hypothesis?______________________________
________________________________________________________________________
6. The velocity of an object (V) is calculated by dividing the distance (d) traveled by time
(t). Using the formula V = d/t, calculate the velocity of each ball traveled down the ramp.
7. As the mass of the ball increased, did the balls speed up or slow down?
____________________
Why/Why not?_________________________________________
________________________________________________________________________
8. As the height of the ramp increased, did the balls speed up or slow down?
____________________
Why/Why not?_________________________________________
________________________________________________________________________
9. Calculate the potential energy of the ball at the starting line.
Calculate the kinetic energy of each ball traveling down the ramp elevated with one block
of wood.
Ball 1 ______________________
Ball 2 ______________________
Ball 3 ______________________
10. When a car is going downhill, the driver must apply more pressure on the brakes to
stop
than
if
the
car
was
on
ground
level.
Why?
____________________________________________
11. Why is it harder to stop a four-person bobsled than a three-person bobsled?
______________
________________________________________________________________________
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