PHYSICS SEMESTER ONE
LAB 4
LAB 4: CONSERVATION OF ENERGY: AIR TRACK AND SMART TIMER
Lab format: This lab is performed via an internet connection with the Remote Web-based Science
Laboratory (RWSL).
Relationship to theory: This lab corresponds to Unit 5: Work and Energy
OBJECTIVES
Using an air track, a system is constructed which is nearly free of non-conservative forces, like friction.
Hence, its mechanical energy should very nearly be conserved. By making measurements of the speed
and change in height of the constituents of the system at four different instants, the mechanical energy
of the system can be compared at those four instants.
EQUIPMENT LIST
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Remote Web-based Science Lab
air track
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4 photogates
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launch mechanism
gliders of different masses, with twobladed timing flags
timing and analysis software
INTRODUCTION
Conservation laws are among the most fundamental concepts of physics. By definition, the value of a
conserved quantity before a physical process begins is the same as the value of the conserved quantity
after the process has occurred. In fact, not only the value (which can be interpreted as a magnitude) is
conserved: conservation of magnitude and direction (such as you would see in an angular momentum)
are also conserved. Any physical process brings something from an initial state to a final state, and the
final state can be quite different from the initial. The conservation laws give us a way of comparing
these two states. That is the root of their importance in physics. This experiment will examine the
conservation of energy.
You should review the definitions of kinetic energy and gravitational potential energy stated in your
textbook and notes. The mechanical energy of the system in each part of this experiment consists of
kinetic energy of the moving parts of the system, plus the gravitational potential energy of the
falling/rising parts of the system.
The photogate timing system is used to measure the average speed of the glider at two different
locations. Remember that the timer will compute the average velocity of the glider as the timing flag
passes through a gate, not the instantaneous velocity. With a constant acceleration, the average
velocity from the photogate is very close to the instantaneous velocity at the centre of the photogate.
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PHYSICS SEMESTER ONE
LAB 4
1. Why does it matter that the timer computes only average velocity? What sources of error will
this introduce?
2. We say that the average velocity from the photogate (with constant acceleration) is very close
to the instantaneous velocity … Can you estimate or guess how close these values will be?
Friction can never be completely eliminated in the laboratory. Keep this in mind when you compare the
mechanical energy at the eight different instants (four up and four down).
PROCEDURE
Conservation of energy on an inclined (tilted) track
You may recall the rules for air track use from Lab 2:
WARNINGS

pay particular attention to whether or not the fan is on
The air track is delicate. Even very small distortions in the level of the track can destroy any
hope of ever obtaining good results.
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Never place a glider on the track without first turning on the air supply.
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Turn the air supply on or off only when there are no gliders on the track.
Setup Experiment Screen
The track setup screen is shown in Figure 4.1. New commands, and some of the lab 2 commands, that
you will be using in this lab are indicated.
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PHYSICS SEMESTER ONE
LAB 4
Track Tilt Adjustment Photogate Control
Mass
Move glider between Scale and Track
then back to Storage A
Figure 4.1: Setup Experiment Control Screen with controls new to Lab 4 indicated.
Level the air track by adjusting the elevation of the end of the track until the laser beam is centred on
the level position on the Tilt Screen. Use a camera to note the elevation of the Tilt Actuator Support
(see Figure 4.2) at the adjustable end of the track. You will use this value to calculate the slope of the
track when it is tilted.
Select and measure the mass of a glider. There are several light and heavy gliders to choose from. You
will be using one light glider and one heavy glider in this experiment. Weight the light glider from
Storage A (Pick up and Weigh button), and place the glider on the air track using the Scale to Track End
A button.
Use the Launch command in the Run Experiment screen to launch the glider from the Return Solenoid
to End B. Measure the length of the Timing Flag on the glider. This is probably easiest to measure
when the track is level. Refer back to the notes in Lab 2 about estimating the uncertainty in this value.
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PHYSICS SEMESTER ONE
LAB 4
Use the cameras to measure the locations of the photogates relative to the front of the timing flag
nearest to End B. These values will be used with the tilt angle to determine the drop in elevation
between photogates.
Rulers
Photogates
Return
Solenoid,
End A
Glider and Timing
Flag
Release
Solenoid,
End B
Air Track
level
Δy
θ
Laser
Pivot
Tilt Actuator Support
Δx
Figure 4.2: RWSL Air Track and Apparatus
Use the Track Tilt Adjustment slider or number box to lower the Tilt Actuator Support by between 4 and
6 cm. Note the new level of the Tilt Actuator Support. You will be comparing this value to the level air
track value to determine the tilt angle θ (see Lab 2 for tilt calculation details). Also note the number
value in the Track Tilt Adjustment number box as you will have to go back to this position with the
glider from Storage B. The glider will probably glide back to End A during this activity.
3. When adjusting back to the number box, does the screw always approach the desired point
turning the same direction, or will it take the shortest route, resulting in some error from slop
in the threads?
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PHYSICS SEMESTER ONE
LAB 4
Run Experiment Screen
Trial Number
Trial Data
Launch Strength
Reject Data Options
Launch, Hold and Release buttons
for one end of the air track
Figure 4.3: Run Experiment Control Screen with Lab 4 controls indicated.
Use the Launch command to launch the glider from the Return Solenoid to the Release Solenoid at End
B. You may have to adjust the Launch Strength for the Release Solenoid to capture the glider. The
Release Glider will hold the glider for 10 s before releasing it. Activate the photogates by clicking the
Photogate control button to “ON” during this time. If you get impatient, you can press the Release
button for End B.
After each timing trial, you have the option of accepting the data and increasing the Trial Number for
the next run, or Deleting Current data if there was a problem during the run. Make five more trials for
this glider at this tilt angle. Return the track to level and move the glider back to Storage A.
Repeat this with the heavy glider in Storage B. Weigh the glider, measure heavy glider timing flag and
distances from the front of the flag. Re-tilt the track by entering the number value used with the light
glider into the Track Tilt Adjustment number box. Check the level of the Tilt Actuator Support to verify
that the track is returned to the light glider tilt level.
4. Although returning the air track to level before replacing the light glider with the heavy one is a
necessary step for this lab, how does it affect the reproducibility of the experiment?
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PHYSICS SEMESTER ONE
LAB 4
Launch the glider to end B and repeat measurements done with the light glider.
Repeat this for both glider masses at a larger track tilt angle. Tilt Actuator Support should be between 7
and 10 cm below the level position.
ANALYSIS
Compute the mean value of the average velocity and the associated SDOM (see Appendix E). It should
be easy for you to devise a simple spreadsheet to automatically convert your values. (If you get really
stuck, you can use the accompanying spreadsheet file: Lab 4 Average Velocity.xls.) Then compute the
mechanical energy (including uncertainty, of course) at the four instants identified by the photogate
timing system. Do this by first computing the kinetic energy of the system at the four instants, and then
the potential energy at the four photogate heights. The potential energy is most easily calculated by
assuming that the potential energy of the system is zero at the initial timing flag position. The relative
potential energies as the glider moves down the track should be negative. Use the tilt angle and
photogate positions to determine the drops in elevation from the initial timing flag position. The
change in the potential energy that occurred as the glider traveled between the gates can then be
assigned in the appropriate way to be the potential energy of the system between average speed
measurements.
Compare the mechanical energy of the system at the four instants.
5. Are the results consistent with conservation of mechanical energy?
Plot the mechanical energies for each set of measurements as a function of position for the four
measurements where the glider travels up the track. Estimate and compare the coefficients of friction
for the gliders of different mass and different track tilt. Compare the average air track coefficient of
friction to that of synovial joints (be sure to include error analysis in your calculations. Also include your
reference for your source of the synovial joint coefficient of friction value).
QUESTIONS
Along with your calculations and experimental results, complete all numbered questions in this lab.
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PHYSICS SEMESTER ONE
LAB 4
REFERENCES
This lab is adapted from reference 1.
1. Laboratory Manual for PHY 100 & 101 Introductory Physics I and II and PHY 120 & 121
Principles of Physics I and II, 5th Edition 2004, prepared by Jason Diemer, North Island College
NANSLO Physics Core Units and Laboratory Experiments
by the North American Network of Science Labs Online,
a collaboration between WICHE, CCCS, and BCcampus
is licensed under a Creative Commons Attribution 3.0 Unported License;
based on a work at rwsl.nic.bc.ca.
Funded by a grant from EDUCAUSE through the Next Generation Learning Challenges.
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