PHYSICS 123 EXPERIMENT NO. 4
CONSERVATION OF ENERGY
For an isolated system, the total energy must be conserved. In this experiment, we will
examine the law of energy conservation by observing the transfer of gravitational
potential energy to kinetic energy, using a glider on an air track. The apparatus is fairly
sophisticated. Your instructor will tell you how to use it. It is called an air track because
an air "cushion" reduces the friction. The position of the cart as a function of time can be
accurately recorded by means of a photo gate device.
DO NOT USE THE APPARATUS UNTIL YOU HAVE READ THE APPENDIX!
Equipment:
One air track, one glider with photogate, one light sensor, one interface box, one
computer with timing program, and one 20 gram weight.
Computer Terminal
Picket Fence
Photo Gate
Glider
Air Track
Interface Box
String with 20g
mass
Method:
A battery powered photogate is mounted on the glider. When activated with the small
push button on the side of the glider, the photogate turns on a bright, light emitting diode
(LED) whenever the picket fence over the air track blocks the photogate. A light sensor at
the end of the air track receives the LED signals and the timing program in the computer
measures and records the time intervals between successive lighting of the LED.
In the experiment a small mass is attached to the glider via recording tape on a level
air track. By dropping the small mass, the change in height of the small mass can be
measured, as well as the velocity of the glider/mass system. This will allow the
computation of the sum of kinetic and potential energy before and after the change and
verify (or dismiss) the law of conservation of energy as a useful concept.
picket fence
photo gate
tape
tape
d
air track
glider
(mass M)
small mass m
Procedure:
1) Your lab instructor has a list of the masses for all the gliders. Get the number of your
glider and obtain its mass, M, from your lab instructor.
2) Level the air track by carefully adjusting the single leveling screw at one end of the
track. When the track is level, the glider should remain nearly stationary at any
point on the track. Be sure to tighten the wing nut on the leveling screw when the track is
level.
3) Find the width of one picket by placing the glider where the photogate LED goes on
and recording what position it is in and then moving it to where the LED is off and
recording the position. The distance between the two is the width of the picket.
Remember to convert millimeters to meters!
4) Attach a 20 gram mass to the glider with a piece of audio tape and rest the tape on the
"air pulley” at the end of the air track so that the mass hangs over the edge of the table
and can fall unobstructed.
5) Set the computer up to take data in the MOTION TIMER mode. When you are ready
to begin a run, push the small button on the side of the glider (This will activate the
photogate for about one minute, after which it will turn itself off automatically to save the
batteries). Hold the glider in a position where the photogate is not blocked (LED off).
Push the SET button on the grey interface box, then push ENTER on the keyboard. After
you let the glider go, data will start recording when the photogate is first blocked. Push
the ENTER on the keyboard before the glider rebounds from the end of the air track or
before the hanging mass hits the floor, whichever comes first.
6) As in the previous lab the distances from one "picket" to the next must be measured
and entered in the computer. Hit ENTER on the keyboard and choose “Graph Data”.
Choose “Velocity vs. Time”, then choose “Other (specify the length)”. Enter your value
for d, select Statistics on by hitting the spacebar, hit ENTER twice to bypass the
horizontal and vertical scaling. You should see a linear velocity vs. time graph. If you
do not get a linear graph, repeat the measurement (step 4). Record the slope (M) of the
graph. Hit ENTER and select “Display Table of Values”, you should see the time values
and calculated velocity values. Disregard the first data point and record the next 10.
Add 4 columns to your table. Height (h), Potential energy (PE), Kinetic Energy
(KE) and Total Energy (TE). Calculate each one of these for each data point you copied
down. The height h in your equation is the distance traveled by the weight from the data
point 2 to 5, 2 to 10, etc. Thus for data point 5, h =(5-2) d, for data point 10, h =(10-2) d,
etc. This h has to be used to calculate the change of the potential energy given by your
equation.
Make one graph in your lab notebook. Plot PE vs time, KE vs time, and TE vs time
all on the same graph. Is TE conserved? Explain.
This experiment can also be viewed as a test of Newton’s second law, F=ma.
Compare the acceleration of the glider (your slope from the velocity-time graph) with the
theoretical expression that you should derive,
gm
a
m M
where a is the acceleration of the glider, m is the small mass, and M is the mass of the
glider which the TA will provide. Find the percent difference between the
acceleration you calculate and the acceleration you record from the graph.
What you need:
1. 10 values for velocity and time
2. Mass of your glider
3. Acceleration recorded off the velocity-time graph from the computer
4. Calculations of h, PE, KE, and TE
5. Graph and PE, KE, and TE vs time
6. Derivation of a
7. Percent difference between accelerations
Appendix
How to use the Air Track
CAUTION: DO NOT DROP THE GLIDERS OR ANYTHING ELSE FROM
A HEIGHT OF EVEN A FEW INCHES ABOVE THE TRACK. DO NOT SLIDE
THE GLIDERS ON THE TRACK WHEN THE AIR SUPPLY IS OFF. Always treat
this very delicate, expensive equipment like delicate, expensive equipment.
You will not get good results from the air track unless you plan carefully what you
will do.
It is easy to level the air track. Simply place a glider on it and raise or lower the end
which has only a single screw by turning that screw until the glider will sit at rest for a
few seconds at any position along the track.
Keep all the surfaces of the air tracks and gliders clean. If they get gummed up, ask
your TA to have them cleaned with alcohol. The data can be very good if you perform the
measurements with enough care.