Hands On Physics

The Great Bungee Jump
Extensions
Interfacing a Computer

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Extension 3: Interfacing a Computer

The timer we used in the core experiment can be greatly improved. It is not handy to use wires that get bent and need to be replaced each time. Also, it would be nice to get all the data pairs in one drop instead of having to repeat the drop so many times. This is easy for a computer that is connected to your experiment using photogates.

This technique is sometimes called "lab interfacing", "probeware", "microcomputer-based labs", or "MBL".

You need two special kinds of electronics, a "picket fence" and some software:

• Photogates. The wires can be replaced by a beam of light and a light detector. The beam of light normally illuminates the light detector. When something interrupts the light, the light detector instantly senses the changed light level and generates a pulse.
• Picket fence. This is a transparent tape that has dark lines drawn on it at regular intervals. When the picket fence is pulled through the photogate, the photogate generates a series of electrical pulses. Each pulse is caused by the movement of one line though the photogate.
• Interface. An electronic device called a lab interface can relay the pulses from the photogate into a computer. Some interfaces attach to your computer's serial port; others plug into a printer port or the computer bus.
• Timing software. The computer needs special software designed for recording and graphing the timing data generated by your photogates and picket fence.

With the right setup, you should be able to collect all the data you need in one drop. Attach the picket fence to weight that represents the bungee jumper. Set up the experiment so that the picket fence is pulled through the picket fence as the weight drops. A little friction on the picket fence will be needed to insure that it does not fall by itself but only when pulled by the bungee jumper. Does this friction upset the measurment?

Many brands of interface electronics and software are available. Look here for current information on what is available.

Decide on your experiment and what data you will collect. Before you begin experimenting, tell your instructor what you plan.