Photogate Finish
The following is adapted with permission from the University of Dallas Fall ‘01 General Physics I
Laboratory Manual, Adaptation: John Boehringer
Everyday experience tells us that when an object is dropped, it speeds up as it
falls toward the ground. The acceleration of the object undergoing free fall is due
to the force of gravity exerted by the earth on the object. The acceleration is
constant provided the effect of air resistance is negligible.
In this experiment the value of gravitational acceleration, g, can be measured by
recording and analyzing the motion of a “picket fence” as it passes through a
photogate timing system. A photogate is an instrument that emits a thin beam of
infrared light that it detected by a photoreceptor. The photoreceptor produces a
current when the beam is present and stops that current when the beam is
blocked. When the latter occurs, a break in the electronic signal occurs that can
be measured and recorded by your LabPro interface. Your PC will record each
of these “signal breaks” as a data point and plot the points with respect to time.
As the picket fence falls it will produce these signal breaks with decreasing time
intervals between them due to the acceleration of gravity. From the intervals
between the points the instantaneous velocity and the acceleration of the falling
body can be extrapolated.
Problem
 What is the acceleration of a freely falling body? How well can you
measure the acceleration of a falling body? Can you achieve a precision
of 0.5% in your measurements?
Materials
Windows PC
LabPro interface
Logger Pro Software
Vernier Photogate
Picket Fence
Ringstand to mount photogate
Preliminary Questions
1. Inspect your picket fence. The distance between one edge of a black band to
the same edge on the next band should be exactly 5.0 cm. What additional
imformation will you need to determine the average speed of the picket fence as
it moves through the photogate?
2. If an object is moving with constant acceleration, what is the shape of its
velocity vs. time graph? Its displacement vs. time graph?
3. Does the initial velocity of an object have anything to do with its acceleration?
For example, does a dropped ball accelerate more slowly than one that is thrown
downward with some force? Why?
Procedure
1. Secure the photogate to a ring stand or equivalent so that its arms extend
horizontally as shown in the figure. The entire length of the picket fence must be
able to pass freely through the photogate. To avoid damaging the picket fence (it
will shatter!) be certain that there is a soft surface for it to land on (carpet or
several backpacks next to one another).
2. Connect the photogate to the LabPro. If you are using an older photogate you
may need an adapter. Ask your teacher if one is not provided.
3. Prepare the computer for data collection by opening experiment “05” from the
physics with computers folder in the vernier subdirectory of your PC.
4. Observe the reading in the status bar of Logger Pro. Block the photogate with
your hand. The display should indicate that the sensor is blocked. Unblock the
photogate and this should change. Confirm that your photogate is operating
properly before proceeding with the experiment.
5. Click Collect to prepare the photogate. Hold the top of the picket fence and
drop it completely through the sensor. You must have enough time to release it
completely before it enters the sensor. Be careful when releasing the picket
fence. It must not touch the sides of the sensor and needs to remain vertical.
Click Stop to end data collection.
6. View the position versus time graph and sketch it on the axes below. Be sure
to include increments.
7. View the velocity vs. time graph and sketch it below.
Drawing Conclusions
1. What is the shape of your velocity versus time graph? Does this agree with
your predictions?
2. What does this shape imply about the motion of the picket fence as it falls?
3. Determine the acceleration from your velocity graph as follows:
Examine your graphs. The slope of the velocity vs. time graph is a
measure of the acceleration of the falling body. If the velocity graph is
approximately a straight line of constant slope, the acceleration is
constant. If the acceleration of your picket fence appears constant, fit a
straight line to the data. To do this, click on the velocity graph once to
select that data. The click the function fit button and fit an equation of the
form y=mx+b to the data. Enter the slope of the velocity vs. time graph
from your data below. Repeat the experiment until you have completed
six trials and have obtained the relevant data.
To find the precision in this experiment, perform a percent error analysis
on the average value of g from the experiment compared to a theoretical
value of 9.810 m/s2.
Trial
Slope
(m/s2)
1
2
3
Minimum
4
Maximum
5
6
Average
Acceleration
(m/s2)
Acceleration due to
gravity, g
Precision
%error
(use g = 9.810m/s2)
±m/s2
Assessing
1. From the best fit line, what was the initial speed of the picket fence?
2. What does the vertical axis (y) intercept represent on your velocity versus
time plot?
3. Is acceleration due to gravity uniform? Explain your answer using evidence
from your investigation.
4. What may account for discrepancies between your data?
5. Why is it so important that the picket fence pass through the photogate while
remaining vertical?
6. Inspect your velocity graph. How would the associated acceleration graph
look? Sketch your prediction below.
7. Now change the upper graph to acceleration vs. time. Do this by clicking on
the y-axis label. Select acceleration, then deselect distance and click OK.
Comment on any differences. You may need to rescale the graph to fit your
window.