Calculation note: jockey fall, turf vs river — the importance of braking distance

In this document

The page presents two calculated scenarios of a jockey falling, at the same initial speed of 36 km/h (10 m/s), for a weight of 50 kg p. 22.

Case 1 — fall on turf

• Damping over 15 cm
• Deceleration: 330 m/s²
• Force felt: 1,650 kg
• Duration of impact: 3/100 of a second p. 22

Case 2 — fall into the river

• Damping over 1 metre
• Deceleration: 50 m/s²
• Force felt: 250 kg
• Duration of impact: 2/100 of a second p. 22

The note explicitly concludes that "these examples illustrate the importance of braking distance on the forces required to ensure it" p. 22.

Two sketches accompany the text: the first shows a rider jumping a barrier with a fall marked at "0.15 m"; the second, a rider crossing a river, with a partially illegible annotation p. 22.

Context

This comparison between turf and river is a classic pedagogical example of impact mechanics: the greater the distance over which a body is braked, the lower the deceleration — and therefore the force experienced — at an equal impact speed.

This principle is at the heart of Paul Magès' research on the hydropneumatic suspension developed at Citroën, where lengthening the damping stroke allows for a reduction in the forces transmitted to vehicle occupants, analogously to water which cushions better than turf.

Archivist's perspective

This short calculation, due to its vivid nature (a rider rather than an automobile), seems to have a pedagogical purpose: to illustrate simply, through a relatable example, a physical principle that Magès then applies to automotive safety.

It is likely part of the same approach to explaining impact mechanics found in other notes in the collection, such as the "Method for not breaking eggs" or the Handwritten calculation notes on safety distances, which develop the same idea that braking distance determines the force felt.

As the date is not specified in the document, it remains undetermined with certainty, but the thematic proximity to these other pieces suggests an origin in the same context of reflection on passive safety.