Using A Video Camera In the Physics Classroom
   (In this case to find the value of "g")

Skalinder, Eric Francis W. Parker
549 - 0172

Objectives: To use equipment that is interesting to the students (a video camera and a VCR) as a data gathering tool. To use the video tape produced in analyzing and understanding motion. Apparatus Needed: 1. Video camera on a tripod, VCR with "freeze frame" capability and one new blank video cassette.
2. Two or three meter sticks.
3. A large, heavy object to drop (needs to be easily visible and not
significantly effected by air friction).
4. An effective timing device accurate to hundredths of a second,
preferably something that can be clearly seen on the video
5. Five or six plain sheets of paper.

Recommended Strategy: Tape the meter sticks vertically on the wall end to end and tape the sheets of paper next to the meter sticks every half meter. Beginning at the top sheet draw large heavy lines each half meter and label them appropriately (0, .50, 1.00, 1.50, etc.) These will serve as visual distance reference points on the video screen as the object is dropped. Look on the video screen and make sure that the whole vertical drop and the stopwatch are visible without camera motion. Start the video tape recorder in the "record" mode. Start the stop watch running. Have someone in the video picture visibly "count" (by raising and lowering their arm) "1, 2, 3" and on "3" another person will drop the object from the "0" meter mark (the bottom of the object should be at the mark.) After it has fallen, stop the video recorder and rewind to the starting point. You should have a video tape recording of the experiment. Two quantities must then be observed: the distance (d) the object fell and the time (t) that it took to fall that far. Look at the tape which you have made and attempt to freeze frame the object just as it
is released. This will take many attempts unless your video recorder
has a feature which allows you to step through the tape frame by frame.
When you believe you have the object frozen as close to the instant of
release as possible, note the time (t0) showing on the stopwatch. (If
you have trouble reading the stopwatch it is probably because you are
using a portion of the tape which has previous material recorded on
that segment. You must use a previously unused portion of tape to see
clearly.) Now advance the tape to a "freeze" position where the
distance fallen (d) is easily "seeable", probably at a place on the
large sheets of paper, and note the time (tf). Again, it will probably
take many tries to stop the tape in a position where you can be sure of
the distance the object has fallen and the time it took to fall that
far unless you have frame by frame capability.

The only further work left is calculation. Time (t) = tf - t0. Now
substitute the values of "d" and "t" into the equation for uniform
acceleration of an object from rest "g" = 2 d / t2 and you have a value
for the acceleration due to gravity. I would ask the students to state
how certain they are of their results using this method.
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