Measuring Reaction Time
- The reaction time is defined as:
A measure of how much time passes between seeing something and reacting to it
- The human reaction time for someone who is alert - i.e. someone waiting to react to something happening, like an athlete waiting for the start of a race - is usually in the range of 0.2 - 0.9 seconds
- A simple method for measuring human reaction time is illustrated below:
Dropping a ruler provides a simple and fun way of measuring someone’s reaction time
- Person A holds a 30 cm ruler vertically, such that the bottom end of the ruler hovers over the top of the hand of Person B
- Person A should release the ruler unexpectedly
- As soon as Person B sees the ruler move, they should close their hand, catching it
- The ruler is marked at the point at which it was caught by Person B - i.e. in line with the top of their hand
- This gives a measurement of the distance the ruler fell
- The greater the distance, the longer the reaction time
When I was teaching GCSE Physics, I found that students got the most out of practical work when they understood the reason for performing it. The purpose of this practical is really to demonstrate that there is a significant delay between thinking about doing an action and the body's actual response. Reaction time is particularly important in stopping distance as the sum of thinking distance and breaking distance. However, human reaction time is also a common source of error in many investigations, particularly when a stopwatch is used. This reaction time practical has come up in exam questions before, so make sure you can describe the process (this is an easy one to practice at home).
