Catching the ruler requires your nervous system to accomplish many tasks. Your eyes register the light coming in. Your brain processes the information, noticing motion. Then it must send a signal to your muscles to contract. The time it takes for these steps is your reaction time.
Using a ruler in this case is better than a stopwatch. A stopwatch has variation as well, since it is also subject to the operator’s reaction time. A ruler falls at a predictable rate, due to gravity, so the distance it travels can be converted to time.
Many track and other races have a false-start limit based on estimates of average reaction times. They have also switched from a gun by the side of the track to buzzers behind each athlete, since the time it takes for the sound to reach athletes in different lanes is on the order of reaction times. In world class sports, every millisecond counts.
Mess Kits are single-serving experiments that allow kids to explore scientific concepts at their own pace. Visitors are encouraged to complete as many as they want! See all Mess Kits.