# The Falling Ruler: Can you catch it in mid-air?

A 30-centimetre ruler drops under gravity at an unpredictable moment. Tap the screen the instant it moves: the mechanical jaws clamp down to the millimetre, turning gravitational acceleration into an exact reading of your visual reflex in centimetres and milliseconds.

[Open the interactive instrument](https://mariuscomper.uk/caderea-riglei/en/)

## How it works

1. Get ready: the ruler hangs suspended on the metrology bench, with the zero mark aligned with the caliper datum line.
2. At an unpredictable moment (between 1.8 and 4.2 seconds), the release mechanism lets the ruler drop under standard gravity (9.80665 metres per second squared).
3. Press the Space bar or tap the screen the instant you spot the drop: the mechanical calipers snap shut.
4. After five attempts, the page computes your average catch distance, average reaction time, best reflex, and displays the visual scatter across the ruler.

## The physics of the measurement

Distance traveled from rest under free fall follows the kinematic equation:
`d = ½ g t²`

The reaction time corresponding to the caught distance is calculated directly:
`t = √(2d / g)`

A catch at 5 centimetres corresponds to 101 milliseconds. A catch at 15 centimetres equals 175 milliseconds, and at 20 centimetres reaches 202 milliseconds. If the ruler drops past 30 centimetres (247 milliseconds), the attempt counts as missed.

## Anatomy of a visual reflex

Total reaction time represents the sum of four discrete biological stages:
1. Retina and optic nerve (35-45 milliseconds): converting light into action potentials.
2. Visual cortex and motor decision (60-80 milliseconds): motion processing in V1/V2 and frontal motor activation.
3. Spinal cord conduction (30-40 milliseconds): signal transmission down the corticospinal tract.
4. Neuromuscular junction (25-35 milliseconds): acetylcholine release and finger flexor muscle contraction.

The sum of these stages sets the biological floor around 150-180 milliseconds for simple visual reactions. Catches below 130-140 milliseconds typically reflect anticipation rather than pure reaction.

Text and implementation: Marius Comper, September 2026.