Download Photogate - scienceinquirer

The economy photogate system is an inexpensive but highly functional timing device based on free opensource software and simple electronics. The photogate may be used to replace any labs that you already do
with other more expensive photogate systems and will replace a lot of the labs that you do with a
stopwatch. The system may be used to measure the instantaneous velocity of an object moving in a line. It
may be used to measure the time between passing from one gate to the other. It may also be used to
measure the period of an oscillating object on a pendulum or spring to any number of oscillations the user
wishes.
Because most physics teachers already have mechanics labs that they prefer and are looking for an
inexpensive way to use photogates in them, I will not attempt to include an entire lab book but will instead
give general descriptions of how to use the photogates for different purposes, give ideas for some uses, and
allow the teacher to use the photogates for their pre-existing labs.
General operation:
The photogate uses the Audacity audio editing program to collect data through a USB sound card.
Audacity is capable of measuring out to many decimal places although most of them will be insignificant
due to experimental and measurement errors.
Plug the USB sound card into the USB port on your computer. Both a laptop or a desktop may be used and
the laptop will give added flexibility on where the experiments may be carried out. The device has been
tested on Windows 2000, Windows XP, and MacOSX operating systems but should also work with
Windows 98SE as well. The Audacity software is available for both Windows and Mac platforms.
Audacity is included on the CD or may be downloaded from http://www.audacity.com.
Follow the steps necessary to install the drivers for the USB sound card. The process should complete on
its own but if any dialog boxes come up, just follow their directions.
Plug the photogate into the microphone side of the USB sound card identified by the icon resembling a
microphone. Ensure that the batteries in the photogates are good and obtain two light sources (flashlight or
pocket laser will do).
Launch the Audacity program and shine the lights on the photogate sensors. Test the system by clicking
record in Audacity and waving your hand in front between the light and the gate. You should see either a
spike or a dip on the Audacity recording each time you pass your hand between.
If you do not see the spike or dip here are some things to check:
1) Is the microphone muted? In Windows, double click the speaker icon near the clock and see if the mute
box is checked on the microphone settings.
2) Is the USB sound card selected as the microphone in Audacity? If your built-in sound card is chosen as
the microphone source, the photogate will not work. Click the drop down arrow near the microphone
symbol in Audacity and choose the correct microphone.
3) Is the photogate plugged into the headphone jack instead of the microphone jack? Ensure that the
photogate is plugged into the proper jack on the USB sound card.
4) Is anything blocking the light from reaching the detector?
5) Are the batteries good?
If there is still no signal in Audacity, use a volt meter to test that the circuit is working properly. The
output from the headphone jack may be tested to see if the voltage is changing when the light is blocked. If
the voltage is changing and there is still no change in Audacity, check the zoom settings and that the
waveform is being displayed on the screen.
Once measurements are detected in Audacity, the user can click and drag from one wave to the next and the
time between the events is displayed at the bottom of the screen. The width of one waveform may be
measured by dragging from the beginning of the wave to the end. Multiple waves may be selected and the
time divided by the number of waves.
Timing the speed of an object:
One way to time the speed of an object is to have a flag of known with attached to the moving object pass
between the light and the gate. By determining how long the light was blocked, the time can be
determined. To calculate velocity, simply divide the width of the flag by the time that it blocked the light.
Example:
To determine the speed of an RC car, one can put a large plastic straw over the antenna of the car. Set up
one photogate with a flashlight shining on it above the height of the car but not above the height of the
antenna. Start Audacity recording and allow the object to pass between the light and the gate with just the
straw blocking the beam.
Carefully measure the diameter of the straw. Determine how long the straw blocked the light using the
waveform in Audacity. Divide diameter by time to determine velocity.
Timing the speed of an object method 2:
In projectile motion experiments an object such as a marble or ball bearing is often used. To determine the
velocity of a ball bearing, set the flashlights and gates on the floor a measured distance apart. There is no
preferred distance as long as it is measured carefully and recorded.
Roll the object between both gates and use Audacity to determine the time that it took to go from one gate
to the next. Divide the distance between the gates by the time measured to determine the velocity.
Using a picket fence to determine “g”:
A picket fence is a clear piece of plastic with dark stripes running across it. The device can be dropped
through a photogate and the speed of the picket fence may be determined as each dark stripe blocks the
light striking the gate. By comparing successive velocities as the gate accelerates, the acceleration of
gravity may be determined.
Place two tables near each other one with the flashlight and one with the photogate with a gap between
them. Start Audacity recording and drop the picket fence through the gate.
Use Audacity to determine the time that the light was obstructed for each stripe on the picket fence. Divide
the width of the stripe by the time to determine the speed. Choose two well-defined waves on Audacity
and measure the time between them. Now calculate the acceleration by dividing the difference in the two
velocities by the time between them. The answer should come out near 9.8 m/s 2.
Measuring the period of a pendulum:
Chose a pendulum bob that is large enough and opaque enough to block the light reaching the gate. Set up
the gate so that the pendulum bob will pass through it. Start Audacity recording and then set the pendulum
in motion. After an acceptable number of oscillations have passed, stop the recording and measure the time
between successive waveforms. Divide the total time by the number of oscillations to determine the period
of the pendulum.
Calculating “g” using a pendulum:
Follow the procedure above to determine the period of a pendulum. Carefully measure the length of the
pendulum from the pivot point to the center of mass of the pendulum bob. Use the equation for the
pendulum to determine “g”.
Measuring the period of a spring/mass system:
Set up the spring with a mass on the end such that the mass will pass through the gate but the spring will
not interfere. This can be done several ways. Set up the system so that the mass passes through the gate a
the bottom of its motion. Set up the system so that the mass is wider than the spring and allow the side of
the mass to break the beam. Use a rubber band and suspend it from a thick rod which will leave a gap
between the two sides of the rubber band through which light may pass.
Follow the same procedure as the pendulum.
Suggested labs:
1) Create a distance versus time graph for an object moving at constant speed. Use a plastic ruler as a
marble ramp to time how long a marble takes to go from the starting line (where the first photogate is
located) to several measured distances. Graph distance versus time to obtain a straight line whose slope is
the speed of the marble. Try different angles on the ramp to see how speed affects the graph. Collect data
for large distances to see how friction affects the results.
2) Create a distance versus time graph for an object that is accelerating. Use a plastic ruler as a marble
ramp and measure how long it takes the marble to cover different distances as it rolls down the ramp. Keep
the first gate in the same place each time and start the marble at the same point. Move the second gate
farther and farther from the first gate and plot distance versus time. An upward-trending curve will be
obtained . Students may convert the distance time graph into a velocity time graph whose slope will be
acceleration.
3) Have students measure the acceleration of gravity using a picket fence and a pendulum as described
above. Have them design their own method for determining “g” a different way. To do this, students may
find the acceleration of a dropped tennis ball passing between two gates.
4) Analyze the motion of toys. Have students bring in toys and measure their velocity. Students may
choose either of the methods described above depending on the toy and/or their own preference.
5) Analyze circular motion by measuring the period of rotation of an object in circular motion. Either a
hole may be drilled in the object which will allow light to pass through or an opaque object may be added
to an object that would otherwise allow light to pass (i.e. bicycle wheel). Audacity may be used to
determine the angular velocity of the object for further study and calculation.