Lab A5: Measuring Acceleration with a Picket Fence
1/10/06
Goals: To practice the picket fence method of measuring acceleration and to use the
method to measure acceleration in several situations
Introduction: In previous labs, you have learned at least two different graphical
analysis methods of measuring acceleration and initial velocity. One method we have
used is to generate a velocity-vs-time graph and then find the slope and intercept. The
picket fence method used in this lab does the same. A Plexiglas strip is divided into alternating dark and clear strips of equal width. When the strip moves through a
photogate interfaced to a computer, time intervals are measured for the passage of
successive dark strips through the gate. The raw data are then converted to a graph of
velocity vs. time.
Method and Results:
In the first experiment, the air track should be reasonably level. Lay a picket fence along
the groove on the top of a red glider and secure it with a strip of tape at the bottom on
both sides. With the glider on the air track, position the photogate so that the picket
fence can pass unobstructed through the gate. Check that the photogate’s light sourcedetector axis is perpendicular to the track length. Also, check to make sure that neither
of the glider’s bumpers is dragging along the track. Make these checks periodically
during the lab.
The photogate must be plugged into the DIG/SONIC1 port of the gray-green Lab Pro
interface box.
The following steps are done only once:
A) Open the software by selecting Start, Programs, Vernier Software, Logger Pro
3.3 (or by double-clicking on the Logger Pro 3.3 desktop icon)
B) Go to ExperimentSet Up SensorsLabPro1. Click on DIG/SONIC 1 and
choose Photogate from the pull down menu. Close the pop-up box.
C) Go to File OpenExperiments FolderProbes &
SensorsPhotogatesMotion Timer – Picket Fence. If a sensor confirmation
box comes up, click OK.
D) Right-click on the graphs that come up automatically, and delete them. Go to
Insert-->Graph and create a new Velocity vs Time Graph (left click on the new
graph’s vertical label to select the variable to be plotted). Double click on the new
graph, unselect Connect Points and select Point Protectors. Under Axes
Options, select Autoscale from 0 for the vertical scale.
E) Turn on the air supply. You are now ready to collect data.
The following steps are done in each Experiment (listed on the reverse)
after you’ve made your prediction and released the glider
F) Click on Collect Data
G) Wait until you see two lights (one yellow, one green) flash on the LabPro
interface; after they flash, no more lights should be lit on the LabPro; also, the
Table Window on the computer screen should now be blank
H) Send the picket fence through the photogate as instructed.
I) The glider’s six velocity values should now appear on your graph. If they do not
all appear, try re-setting the axes limits. Else, reset the LabPro interface by
disconnecting its power supply, then reconnecting it. Wait until three lights (red,
yellow, green) flash on the LabPro box before returning to step B and then step E
above.
J) From the graph, assess whether the acceleration was approximately constant
while the picket fence was moving through the photogate. (Are all the points in a
line?) If not, retake your data. If so, continue.
K) Go to Analyze Linear Fit to get a linear fit to the velocity-vs-time data. (Why
linear?).
You will perform 5 experiments (listed on the back) with different gliders on the air track.
In advance of lab, make a data table that fully spans two facing pages of your journal.
Your table must have the following columns. Three rows are shown--more rows will be
needed as you proceed through the 5 different experiments. Plan for the table to fully fill
the two facing pages. Include units.
Experiment #
your predicted
measured
measured
correlation
and description
value of
value
value of initial
coefficient
acceleration
of acceleration velocity
(and reason for it)
(slope of v-vs- (intercept of vt)
vs-t)
1) glider on
horizontal
track
2) glider released
no prediction for
on
exp. 2
tilted track
For each experiment (except #2), you MUST PREDICT the MAGNITUDE &
DIRECTION of the acceleration (and GET A SIGNATURE) BEFORE you RUN THE
EXPERIMENT. Use words (such as “down” or “up the track”) to predict the
acceleration direction (do NOT use “plus” or “minus”).
For each experiment, the computer will produce a velocity-vs-time graph. Record in your
data table the acceleration and the initial velocity plus a correlation coefficient R which is
a measure of the probability that the fit variables are actually correlated in a linear
relationship. A value of 0 means no correlation; 1 (or -1) means perfect correlation.
Experiment 1) The airtrack should be horizontal (untilted)for this experiment.
Give the glider a push to send it through the gate. However, make sure that your hand
leaves the glider before any portion of the fence enters the gate. Stop the glider & picket
fence before it hits the end of the air track (but only after it has finished passing through
the photogate.)
Before actually taking data with the glider on a horizontal track, PREDICT what the
acceleration of the glider should be as it goes through the photogate (and after your
hand leaves the glider); record your prediction (and your reason for it)! Then run the
experiment.
Now return to steps E-K to run the experiment and perform the fit to the data;
record the acceleration, initial velocity, and correlation coefficient.
Is the value what you expected (within uncertainty)? If not, consult the instructor.
Experiment 2) Now tilt the air track by placing a riser block under the single foot.
Record the block height; a scale is provided on each air track. In this experiment you
will release the glider at the top of the track and let it pass through the photogate. Catch
it before it bounces back. Repeat the steps in boldface above.
Experiment 3) In this part, you will give the glider a push upward from the bottom of the
track so that the picket fence passes all the way through the gate (but not while you are
pushing it). Before taking data, PREDICT the magnitude and direction of the
acceleration that you expect in this part (with a reason). You can refer/compare to the
acceleration results of experiment 2. Run the experiment, and record the results.
a) If your predicted and experimental acceleration values do not match, consult the
instructor.
b) Since you predicted the acceleration in #3 to have the same magnitude and direction
as #2, why were the signs of the acceleration values different?
c) Compare the values of initial velocity from Experiments 2 and 3. Should they be the
same? Explain.
Experiment 4) In this part you will use a picket fence attached to a more massive blue
glider, and then release the glider above the photogate as in Experiment 2 above.
Before taking data, PREDICT the magnitude and direction of the acceleration that you
expect in this part (with a reason). Then, run the experiment, and record the results. If
results do not match prediction, consult the instructor.
Experiment 5) (A special computer may be set up in the hallway to do this part.) Adjust
the photogate so that it extends over the edge of the table or cart. Put a foam pad or box
on the floor below the gate so that the fence does not crash into the floor and injure
itself. In this experiment you will drop the picket fence from above the photogate.
PREDICT the direction and magnitude of the acceleration you expect before trying it.
Then, run the experiment, and record the results. Is the acceleration within 1% of the
expected value? If not, try again. Record all attempts.
Don’t forget to include a final CONCLUSION/SUMMARY !