Computer-aided physics experiments: Edu Tech Comp Trol Lab. (evaluation) David H. Ahl.
The CompTrol Lab is a software and hardware system for running computer-controlled physics experiments. The hardware is manufactured by Pasco Scientific and consists of an infrared photogate and an interface box.
The photogate is shaped like a large C, 4-1/2" high. In one tip is an infrared light source and in the other, a detector. A base and 10c post hold the detector in an upright or inverted position.
The photogate plugs into a Pasco general-purpose interface which, in turn, plugs into the game port of the Apple computer. All power to the system comes from the Apple. Two transducers, such as photogates, thermistor probes, etc. can be connected simultaneously to the interface. (A third program in this series uses a temperature probe; we did not test it.)
Each set of experiments has a disk, instruction manual, and student manual (20 pages). The 14-page instruction manual describes the experimental apparatus for both the Pendulum and Acceleration experiments. It provides schematic diagrams of the circuits, set-up directions, and test programs to check out the apparatus. We found that everything was in order and quickly skipped to page 8 which describes the two experiments. Pendulum
The first half of the Pendulum student manual is a theoretical discussion of pendulums. It contains the formulaes relating period and amplitude as well as a discussion of the least squares method of fitting experimentally measured data points. It then describes the experiments and computer program. We had a bit of a problem setting up the experimental apparatus; the manuals have illustrations of the photogate, and a stylized drawing of a pendulum and detector, but there is no illustration of the entire apparatus.
Nevertheless, we fashioned a make-shift pendulum and started making measurements. With the aid of the computer, this is simplicity itself. There are two experiments on the disk: Period and Amplitude, and Period and Length. For the first, you simply enter the horizontal displacement of the pendulum bob, enter the number of swings to be timed, and release the bob so it swings smoothly between the light and detector.
The computer measures the time between every other blocking of the light (one full period) for the number of swings you specified. It then displays the period of each swing, the average period, and standard deviation. It then asks if your are satisfied; if so, you can go on to the next measurement; if not, you can repeat a measurement.
If you botch it up completely, as we did on our first measurement, you can later go to the editing portion of the program and delete a set of data points. The procedure for doing this is not described in either manual; however, the menu-driven software is self-explanatory and easy to use.
When you have five or more sets of data, the computer will perform an analysis on the data for various values of n in the period/amplitude equation. The manual recommends trying all values of n from 0.5 to 3.0 in steps of 0.5 until you get the lowest standard deviation of the computed curve fitting your experimental points.
If you have a Silentype printer, the program will print out any of the screens as you progress through the experiment. Any, except the graph, that is. (The graph screens on the Acceleration disk print out well; those on the Pendulum disk do not. Too bad; it is one that students will want.)
The Period and Length experiment is similar, but relates the length of the pendulum string to the period of oscillation. Also on the disk is a program to calibrate the apparatus with a 60 Hz light source such as a fluorescent lamp.
Four pages in the student manual are provided for recording the results of the experimentals. Teachers will surely want to reproduce these for all students. Acceleration
The Acceleration disk contains two experiments: Linear Acceleration and Constant Acceleration. One analysis procedure is provided for Linear Acceleration, while three are provided for Constant Acceleration: Distance and Time, Velocity and Time, and Velocity and Distance.
In contrast to the Pendulum student manual, this one combines the theoretical discussion, formulae, and directions for doing the experiment. But like the Pendulum manual, this one suffers from a lack of one simple illustration showing the experimental apparatus with all the pieces in place.
In the Linear Acceleration experiment, you drop a transparent ruler with strips of black tape on it through the photogate detector. The computer records and measures the time at which the leading edge of each strip breaks the light, plots the data points, and draws a best fit line between them.
Youngsters liked this experiment, although there wasn't really very much for them to do except drop the ruler and watch the computer do the analysis. They did find that dropping the ruler from a height of 50 cm or more resulted in its traveling so fast that the computer was unable to measure the acceleration.
The Constant Acceleration experiment required somewhat more participation and a great deal more coordination, as a plastic flag must be dropped (straight) from one hand and the spacebar pressed simultaneously with the other. As the manual warns, "this introduces a rather large random error in the experiment . . . . However, because the error is random its effect can be reduced by making repeated measurements at each height." Actually, this may be the right place for a discussion with students about accuracy. Compare the accuracy for the computer (seven figures) with the experimental measurements (two figures or less). Just because the final answer has five decimal places, is it really that accurate?
All in all, we found the CompTrol Lab a wonderful system for motivating some (but not all) students. We feel that it must be introduced in the proper perspective, i.e., "here is a valuable laboratory tool." It is not a replacement for precise experimental measurements, nor is it simply a way to avoid tedious calculation.
In addition to the two experiments marketed by EduTech, the apparatus will probably suggest numerous other possibilities. We think the CompTrol Lab is worth an investment of $325 for any school physics lab with access to an Apple.
Products: Comptrol Lab (computer program)