Download Name: Period - Glenbard West

Name: ___________________________________
Period: _______
Date: _____________________
Partners: ____________________________________________________________________________________
Cut Short & Conservation of Energy Lab
Part 1: Cut Short
A pendulum swinging to and fro illustrates conservation of energy. Raise the pendulum bob to give it potential energy.
Release it and its potential energy is converted to kinetic energy as the bob approaches its lowest point. Then, as the bob
swings up on the other side, kinetic energy is converted to potential energy. Back and forth, the forms of energy change
while their sum is constant. Energy is conserved. You will investigate what happens when the pendulum length is
suddenly changed and how it illustrates energy conservation.
Procedure
Step 1: At your lab station you will find a pendulum attached to a cross bar.
Step 2: Raise your pendulum bob so that it looks like the image below, but do not allow the 2nd cross bar to cut its swing
path short. Measure the pendulum’s start height and record below. This will be the release height until Part 2 of the lab.
Pendulum Start Height:
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Predict the height that the pendulum will swing to after it is released: (circle one)
significantly higher
significantly lower
or about the same height
compared to where it was released.
Release the pendulum and allow it to swing to the opposite side of the lab table. Observe the height of the pendulum on
other side of its swing. The pendulum swings to a height that is: (circle one)
significantly higher
significantly lower
or about the same height
compared to where it was released.
Step 3: You will find a second cross bar below the cross bar that the pendulum hangs from. Adjust the second bar to the
same height as the start height of the pendulum.
Step 4: Predict what height that the pendulum bob will reach, relative to the cross bar, if the pendulum is released at your
start height, and the pendulum string is stopped by the rod. Circle what you
think will happen next to ‘Prediction’ below:
Prediction:
higher
lower
same height
Start
height
Step 5: Return your pendulum to approximately the same starting height as
above. Have one of your teammates position themselves on the opposite side
of where the pendulum is released. This teammate will be observing how high
the pendulum swings. Release the pendulum! Circle what you observed next
to ‘Observation’ below.
Observation:
higher
lower
same height
Cross
bar
Step 6: Predict what would happen if the rod were attached higher than the pendulum bob. Then move the cross bar
higher than the starting point of the pendulum bob and perform the experiment as you did above to confirm or deny your
prediction. Relative to the crossbar, does the pendulum swing:
Prediction:
higher
lower
same height
Observation:
higher
lower
same height
Step 7: Predict what would happen if the rod were moved lower than the start height of the pendulum bob. Perform the
experiment again to confirm or deny your predictions. Do not lower the cross bar to the point where the pendulum loops.
Relative to the crossbar, does the pendulum swing:
Prediction:
higher
lower
same height
Observation:
higher
lower
same height
Analysis
Draw a conclusion based on your observations above. Approximately how high does the pendulum swing compared to
where it was released?
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How does your conclusion support conservation of energy?
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Looping
Step 8: Perform a few more trials until the cross bar is low enough to the point where the pendulum bob completely loops
around. Be sure the cross bar is low enough to where the string stays taut to complete a single loop. Measure the
maximum height of the pendulum when it is at the top of the loop.
Measure the height of the pendulum at the top of its looping path. How does this loop height compare to the start height
of the pendulum?
Circle:
Greater
Less
Equal
to the drop height.
Conclusions
1. How does the PE at the loop height compare to the drop height?
Circle:
Greater
Less
Equal
2. What other types of energy does the pendulum possess at the top of the looping path? How do you know?
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3. Why won’t the pendulum loop if it were allowed to swing back up to the drop height?
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Part 2: Conservation of Energy
Purpose
To determine if total energy in a pendulum is conserved by comparing potential and kinetic energies.
In the first part of the lab, you saw that the height to which a pendulum swings is related to its initial height. The work
done to elevate it to its initial height becomes stored as potential energy in relation to the bottom of the swing. At the top
of the swing, all the energy of the pendulum is in the form of potential energy. At the bottom of the swing, all the energy
of the pendulum is in the form of kinetic energy.
The total energy of a system is the sum of its potential and kinetic energies. If energy is
conserved, the sum of the potential and kinetic energy at one moment will equal the sum
at any other moment.
Your goal is to demonstrate that total energy in a pendulum system is conserved. You
must prove that this is a general rule (not just for a particular starting height). The
equipment you have to work with is:



Pendulum suspended from ring stand. NOTE: Diameter of the pendulum bob = 2.8 cm.
Meter stick
Photogate timer and computer
Procedure
Step 1: At your lab station you will find a pendulum attached to a cross bar. Pick three start heights that you want to drop
the pendulum bob from. Record these values as the start height of the pendulum. Then measure from the bottom of the
table to the center of mass of the pendulum bob. Record this as the 'height above the table'.
Height above the table (h.a.t.): _______
Step 2: In the table below, chose and record 3 heights from which you will release the pendulum.
Start
Height
( )
Start Height - h.a.t
( )
Photogate
Time
( )
Displacement
(Pendulum Diameter)
( )
Pendulum
Velocity
( )
Trial 1
Trial 2
Trial 3
Step 3: Open the file found in the 'Thawspace' of the computer named 'Pendulum Lab.’ You should see a data table with
columns for 'Time' and 'Gate State.' The 'Gate State' column records values for when the space between the photogate is
first blocked and then a second time for when the photogate is again open.
Step 4: Hold your pendulum bob at your first release height. Press the 'Collect' button to start the photogate sensor.
Release your pendulum bob so that it passes through the photogate. Stop the photogate and pendulum. Subtract the
Opening time of the gate from the closing time of the gate. Record this value in the 'Photogate Time' column of your data
table.
Do not round the PHOTOGATE TIME!!!
NOTE: If you allow your pendulum to swing past your photogate more than once, you will have extra values for opening
and closing of the sensor. IGNORE THESE EXTRA VALUES! Only use the first two values of time.
Step 5: Repeat the above procedure for your other two heights. Use the diameter of the pendulum bob as the distance for
each of your trials. Using v=d/t, calculate the velocity of the pendulum bob as it passes the photogate. Be careful with
units! Round the pendulum velocity to the proper number of significant figures.
Step 6: Calculate the gravitational potential energy for each of your heights, and the kinetic energy for the pendulum as it
passed the photogate. When performing calculations, remember that mass is measured in kilograms. Finally, calculate the
percent difference. Show your work for one trial, and complete the data table below.
Useful Equations:
P.E.top = mgh
K.E.bottom = (1/2)mv2
% Difference =
_ | P.E. – K.E. |______ X 100
(Average of P.E. and K.E.)
Show one sample calculation for PE, KE and % Difference below:
Start height – h.a.t.
( )
Potential
Energy
( )
Pendulum
Velocity
( )
Kinetic
Energy
( )
KE PE
% Difference
Trial 1
Trial 2
Trial 3
Conclusion
Was the energy of the pendulum the same at the initial height and as it passed through the photogate? NOTE: if your
percent Difference was less than 10%, you had a successful lab! Regardless, discuss possible sources of error.
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