Students love this activity. You will, too. It is fun and challenging, and it allows for a useful discussion of three body systems: nervous, skeletal, and muscular. In this activity, students test their reaction times two ways, using an online test, and with a ruler.
Both test have their benefits and drawbacks, but for most purposes they are more than adequate. The online test is precise, but it can be affected by your device and connection speed. The ruler calculation in theory is quite accurate because it is based on the constant acceleration due to gravity. The drawback with the ruler method, is that it is subject to human error. There are several considerations to discuss with your students. First, they must line up the catcher's fingers with the zero of the ruler scale. Second, they must drop, not push, the ruler. Third, the catcher must hold his or her hand steady and not move it down to catch-up with the ruler as it drops. You may want to have students rest their hands on the side of a desk if they are not able to hold their hands in one place.
There is a great opportunity for extending this activity with some research into reaction times. Consider looking here: http://condellpark.com/kd/reactiontime.htm. This is not my site. It includes some interesting information about the timing of Olympic sprint race starts and the role a good start can play in a winning time. There are other sites that discuss the timing of drag races which are begun differently and can have reaction times of 0.00.
The gist of the “too fast” reaction time is based on the many events between the signaling of a start, and a physical reaction by the observer. First the signal needs to reach the observer. Depending on his or her distance from the signal, and whether it is visual or auditory, there is a calculable time for the signal to reach the observer. Next the nervous system needs to transmit that information to the brain and the brain needs to signal a reaction. That signal needs to travel, in our case here, to the muscles of the hand to click a button or catch a ruler, and the muscles need to react. The speed of sound, light, nervous-system signals, and muscle contractions are known and can be calculated over the distances in any situation to come up with a minimum theoretical reaction time.
With results faster than the “too fast” reaction time, students will claim that they have reacted, but in most if not all cases, they have anticipated the start. Discuss with them what sorts of clues they might have observed even without noticing: the twitch of a hand, the movement of fingers before the ruler is actually free to drop. The web-page reaction-time calculator does not penalize students for reacting too quickly. Ask them how they could use that information to lower their times?