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Science 7–10 · Year 9

Reaction time by the ruler drop: from catch distance to milliseconds

Science understanding: Biological sciences

Practical, model not builtLow risk

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The idea

A response to a stimulus takes measurable time because the signal must travel from receptor to brain to muscle, and that time can be calculated from how far a ruler falls before it is caught.

What you need

  • 30 cm ruler or a metre rule
  • A results table and a calculator
  • Optional: headphones playing music, for a distraction condition

How to do it

  1. The tester holds the ruler vertically by the top with the zero mark level with the top of the catcher's open thumb and forefinger, which rest on the edge of a bench.
  2. Without warning, within a five-second window, the tester drops the ruler; the catcher closes finger and thumb as fast as possible.
  3. Read the mark at the top of the thumb and record the catch distance in centimetres.
  4. Repeat five times and take the mean (the University of Washington activity suggests three to five trials), varying the delay so the catcher cannot anticipate.
  5. Convert distance to time: t = square root of (2 d / g), with d in metres and g = 9.80 m/s^2.
  6. Repeat with the non-dominant hand, and again while listening to music, five trials each.

What you should see

The formula converts each catch distance to a time: 0.100 m gives 0.143 s, 0.150 m gives 0.175 s, 0.200 m gives 0.202 s and 0.250 m gives 0.226 s. The University of Washington activity tabulates 0.20 s for a catch at 8 inches (20.3 cm), which the formula gives as 0.204 s. Compare the means for each condition; a difference smaller than the spread of the five trials is not evidence of an effect. The learner knows it worked when the five trials in a condition cluster within a few centimetres and each mean has been converted with the formula.

What changes

What you change
Hand used, or distraction
What you measure
Mean catch distance and calculated reaction time
What you keep the same
  • Same ruler and grip position
  • Random delay before the drop
  • Five trials per condition
  • Same catcher

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Reactions are instant.
  • Reaction time is the same as a reflex; this response passes through the brain and is slower.
  • A longer catch distance means a faster reaction.

Safety card

Low riskLearners carry it out

Hazards

  • A falling metre rule striking a foot

Controls

  • Catcher's hand over a bench edge; feet clear

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/fa8c24a583
  2. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/content/stage-5/fa9f532d80
  3. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  4. faculty.washington.edu/chudler/chreflex.html
  5. faculty.washington.edu/chudler/experi.html
  6. www.riskassess.com.au

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