Reaction time: the falling ruler
A friend drops a ruler and you catch it. Catch it five times, find your average reaction time, spot the odd catch, then try again while looking at a phone.
Core concept
Scientists repeat a measurement, leave out an odd result and take the average; the ruler falls d metres in t = √(2d ÷ g) seconds, so the catch distance gives your reaction time
Topic: Scientific study
Science is more than knowing facts: it is a set of skills. We observe, measure, record, repeat and then draw a conclusion. One measurement is never enough, because every measurement varies a little. So we repeat it, look for any result that is far from the others (an odd result, often a mistake such as a blink), leave that one out, and take the average of the rest. In this lab a friend holds a ruler with its 0 mark between your finger and thumb and lets go without warning. You catch it as fast as you can. The ruler falls freely, so the distance it falls tells you your reaction time: t = √(2d ÷ g), with d in metres and g = 9.8 m/s².
What you will learn
- Explain why a measurement is repeated.
- Spot an odd (anomalous) result and leave it out of an average.
- Calculate an average from repeated measurements.
- Use t = √(2d ÷ g) to turn a catch distance into a reaction time.
Before you start
- Know how to find the average of a set of numbers.
Materials
- A 30 cm or 50 cm ruler, a friend
Equipment
- A table to rest your arm on, so the test is fair
Procedure
- Catch the ruler five times while watching it, and record each catch.
- Find the odd catch, leave it out and average the other four.
- Catch it five times while looking at a phone, and record each catch.
- Compare the two averages.
How this simulation is simplified
- The ruler falls freely from rest with g = 9.8 m/s², with no air resistance.
- The catches are those of one Class 9 student; your own may differ.