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Videotape someone serving. Try and get the entire flight of the ball
in the camera frame. Watch the serve carefully, and try to start the stopwatch at the moment
the server hits the ball, and stop it when the other player hits the
return. Record your result.
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Rewind the tape to the start of the serve, and repeat this measurement
several more times. How do your results compare? Do you think this is an
accurate measurement?
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Pause the tape at the start of the serve, just before the server swings.
Advance one frame at a time, until the racket makes contact with the ball.
Now step through the event, until the ball bounces on the court, counting
the number of frames. Record the number of frames that the ball is in the
air.
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Now count the number of frames that it takes the ball to reach the other
player's racket. Record this number.
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Video is broadcast at almost exactly 30 frames per second. This means
that the time between frames is 1/30th of a second. Calculate the time
that the ball is in the air before the bounce, and the time between the
bounce and the return.
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What is the total time between the serve and the return? How does this
measurement compare to the one that you made with the stopwatch? Which do
you think is more accurate?
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(For this experiment, the whole class will work together as a large
group. The more people there are in the group the more accurate it will
be. For a smaller number of people (less than 15), you can simply go
around the group several times to get more accurate information). The
entire group should sit in a circle, facing outwards. Arrange yourselves so
that you can see the right hand of the person to your left. The rule is
this: when you see your neighbor's index finger rise in a signal,
immediately raise your index finger as well. This signal will propagate
around the group, from left to right, and you will be able to measure the
time it takes to go all the way around. From this you can calculate the
average reaction time per person for the eye-brain-finger transmission.
One person should be holding the stopwatch and start the signal and the
stopwatch at the same time. When they get the signal from the person to
their left, they should immediately stop the stopwatch. The time that it
has measured is the sum of all of the reaction times of the group. Divide
by the number of people, and you have the average reaction time of your
group. What is the average reaction time of people in your group?
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Think about how your reaction affected the accuracy of the stopwatch
measurements.
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Think about how your reaction time compares to the time that a
professional player has to get to the ball and set up their return hit.
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