Make a Model of Mechanical Energy!
Energy
changes. It is also defined as the ability to do
is the ability of an object to cause
work
causes an object to move). It was while thinking
about why objects move that scientists first
began to develop the idea of energy. In 1583 at
the age of 19, the Italian scientist Galileo
Galilei (1564–1642) observed that a swinging
church lamp swung upward very nearly as
high as the point from which it had previously
swung downward. He used his pulse to time
the swinging lamp and discovered that the
time of the upswing was equal to that of
the downswing. Galileo couldn’t explain why
the lamp swung back and forth, but his
observations laid the groundwork for future
scientists who further experimented and
explained the types of energy involved in the
movement of a swinging lamp.
In 1807, Thomas Young, an English physicist
(1773–1829), was the first to use the word
energy. He defined energy as the ability to do
work. Work is the amount of force on an object
times the distance the object moves in the
direction of the force. The first form of energy
as defined by Young was
(what is accomplished when a forcemechanical energy
(the energy of motion). It is the energy of an
object that is moving or is capable of motion.
Today, physicists generally think of mechanical
energy as being the sum of the
energy, KE
because of its motion) and
PE
object). Kinetic energy can be changed into
potential energy or vice versa. In the diagram,
in position A, the swing is at its highest position
and is not moving, thus it has maximum
potential energy, PE, and zero kinetic energy,
KE. As the swing starts to move toward position
B, its PE changes to KE. So at position B,
all of its PE has been changed to KE. As the
swing moves toward position C, its kinetic
energy begins to change to potential energy
again, until at position C it has maximum PE
and zero KE, and so on.
The term
kinetic(energy that a moving object haspotential energy,(energy of position or condition of an
kinetic energy
comes
from the
Greek
word
kinema,
for “motion”
(which also
inspired the
modern
term
When
a ball rolls
down a hill,
it has kinetic
energy
because of its motion. At the top of the hill, the
ball has mechanical energy stored up in it
because of its position on the hill. This stored
mechanical energy is called
potential energy, GPE
to the height of an object above a surface). At
the top of the hill the ball has maximum GPE
and zero KE. Because of gravity the ball rolls
down the hill. As the ball rolls, its gravitational
cinema).gravitational(potential energy due
87
potential energy changes to kinetic energy.
When the ball is halfway down the hill, half of
its GPE has changed to KE, so its GPE equals
its KE. At the bottom of the hill, the ball has
zero GPE and maximum KE. The change of
energy from one form to another is called
energy conversion.
ACTIVITY: SWINGER
Purpose
To model the energy conversions of a
swinging pendulum.
Materials
paper hole punch
10-by-12-inch (25-by-30-cm) piece of yellow
poster board (any pale color will work)
fine-point black marker
ruler
three 12-inch (30-cm) pieces of string
three metal washers
glue
transparent tape
Procedure
1.
the middle and about 1 inch (2.5 cm) from
the edge of one of the short sides of the
poster board.
Use the paper hole punch to cut a hole in
2.
lines of equal length and no longer than 8
inches (20 cm), starting at the same point at
the bottom of the hole in the poster board.
One line should be straight down the poster
board and the other two lines should be
diagonal from the hole toward the corners
of the poster board, as shown.
Use the marker and the ruler to draw three
3.
Tie a string to each metal washer.
4.
poster board. Allow the glue to dry.
Glue a washer to the end of each line on the
5.
through the hole in the poster board. Pull
the string taut so that it lies flat along the
black line drawn from the washer to the
hole. Tape the end of the string to the back
of the poster board.
Push the free end of one of the strings
6.
remaining washers.
Repeat step 5, using the strings tied to the
7.
poster board:
• a title, such as “Energy Conversion”
• dashed lines and arrows showing the
motion of the pendulum
• the relationship of potential energy (PE)
and kinetic energy (KE), as shown in the
diagram
Using the marker, add these labels to the
88
Results
You have made a
model of the
energy conversions
of a swinging
pendulum.
Why?
A
to swing about a fixed point. When a pendulum
swings back and forth, energy is continually
changed from gravitational potential to kinetic
energy. The change of energy from one form
to another is called energy conversion. When
anything falls, energy is converted from potential
to kinetic. At the end of each swing, the
pendulum stops and reverses direction. At the
end of the swing, the pendulum is at its highest
position and has only gravitational potential
energy (GPE) and no kinetic energy (KE). As
gravity pulls on the pendulum it swings down,
and its GPE continuously changes to KE. So,
at the bottom of the swing, the pendulum has
only KE and no GPE. As the pendulum rises,
the KE continuously changes to GPE, and so
on. The pendulum loses energy due to friction
between the pendulum and air, so its height
decreases with each swing until it finally stops.
pendulum is a weight hung so that it is free
ON YOUR OWN!
Display the energy conversion model on the
center panel of a narrow three-paneled backboard.
(For instructions on making the backboard,
see Appendix 1, Part A.) Print the title
“Mechanical Energy: PE + KE” on the title strip.
On the left panel of the backboard, attach a
diagram and information on potential energy
drawn on a 6-by-12-inch (15-by-30-cm) piece of
poster board. (Note: Use the same color poster
board as you used for the energy conversion
model.) Include a title, “Potential Energy
(PE),” and beneath the title draw a diagram
representing an object with potential energy,
such as a large rock on the edge of a high cliff.
Beneath the diagram, add information about
potential energy, including the definition and
examples. Attach this piece of poster board to
the left panel of the backboard.
Put information and a diagram about
kinetic energy on another 6-by-12-inch (15-by-
30-cm) piece of poster board the same color as
the other poster board. Include a title, “Kinetic
Energy (KE),” and beneath the title draw a diagram
representing an object with kinetic
energy, such as a moving car. Beneath the diagram,
add information about kinetic energy,
including the definition and examples. Attach
this piece of poster board to the right panel of
the backboard.
BOOK LIST
Churchill, E. Richard, et al.
with Everyday Materials.
Publishers, Inc., 1998. Simple experiments, including
some about pendulums.
Doherty, Paul, and Don Rathjen.
Other Dynamic Experiments on Force and Motion.
Wiley, 1996. Experiments about force and motion, including
some about pendulums.
Suplee, Curt.
National Geographic Society, 1998. Science information that
encourages readers to explore everyday life, including facts
about energy conversions.
VanCleave, Janice.
365 More Simple Science ExperimentsNew York: Black Dog & LeventhalThe Spinning Blackboard andNew York:Everyday Science Explained. Washington, D.C.:Janice VanCleave’s Physics for Every Kid.
New York: Wiley, 1991. Fun, simple physics experiments,
including information about pendulums
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