There have been a couple of posts suggesting that "reaction time" is
a source of uncertainty in experiments involving timing. Indeed,
when we talk about sources of uncertainty in timing experiments,
reaction time is usually the first thing my students point to.
My response is always to engage in a little pantomime. I pretend I
am holding a ball in one hand and a stopwatch in the other. I then
drop the ball, feign a very surprised expression, and lunge to start
the stopwatch. As I continue to watch the ball fall I am "surprised"
again when it hits the ground and lunge once more to stop the
stopwatch as quickly as possible after that event. The students
usually get my point immediately and we all share a laugh.
If reaction time is genuinely a source of error in such experiments
then you need to go back to the drawing board and redesign the
experiment. Reaction time can only affect such measurements if, for
instance, a) you let your partner drop the ball while you run the
stopwatch AND you insist that your partner give you no warning or b)
you consciously refuse to watch the ball as it drops and wait for the
sound of the ball hitting the floor before stopping the watch.
Of course there will be variations in the times obtained from
successive tme trials, but I maintain that they have essentially
nothing to do with the phenomenon that is properly called "reaction
time."