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 
   screen.
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.