“Passive safety — The motorist and the technician”: educational dialogue on the formula F = P·V²/D·0.394

In this document

This document presents an educational dialogue between "the motorist" (A) and "the technician" (T), organised into numbered lines from 1 to 18, to explain the usefulness of the seat belt p. 14-17.

The formula presented

From the very first line, the technician launches into his formula instead of greeting: "P x V²/D x 0.394 = F" p. 14. He then details each term with the motorist:

• P: the weight of the driver, here set at "70 kilos" as an example p. 14-15.
• V²: the speed read on the speedometer, squared — "if you see 50 ... you multiply by 50" p. 14-15.
• D: the "stopping or braking distance", including when it is reduced to "the crushing of your bodywork" against an obstacle p. 15.
• F: the resulting force, the one that "breaks the nose" or "crushes the ribs" of the driver who does not have a seat belt p. 15.

The numerical results

The technician applies the formula to three speeds, for a weight of 70 kg:

The conclusion

Faced with this "struggle" against F, the technician indicates that remedies exist, "some depend on the technicians", others on the driver — including the seat belt, which "reduces the speed" and "distributes the force over several points of the body" p. 16. The dialogue closes with a reminder of the formula and the final line from the motorist: "OK! I understand. I'm buckling up... and thank you!" p. 17.

The document, dated by hand 25-04-73, presents a first box in typewriting corrected by hand (cross-outs, modified figures), then a clean copy with pagination where boxes 14 to 18 remain empty of text, only the skeleton of lines 14-17 being sketched out by the technician p. 17.

Context

Wearing a seat belt became mandatory in France for front seats from 1973, in a context of increasing awareness of road safety supported by manufacturers as well as public authorities. Citroën, like other brands, communicated to the general public on the value of this equipment, often going beyond the mere capabilities of the seat belt to explore the physical phenomena of the impact (kinetic energy, stopping distance).

In 1973, Paul Magès had been at the Citroën design office since 1939-1940 and had been working since the 1940s on the hydropneumatic suspension (plans from 17 February 1944); he would remain at Citroën until his retirement in September 1974 (Photographs of Paul Magès' retirement (1974)).

Archivist's perspective

This dialogue testifies to the pedagogical concern of Paul Magès: making a physical formula accessible — undoubtedly linked to his work on safety forces and distances — through a simple, almost theatrical sketch, with good-natured humour ("Featherweight, 70 kilos").

The document probably belongs to a series of similar demonstrations produced during the same period, as evidenced by the Handwritten calculation notes: safety distances and establishment of the force formula, dated the same day (25 April 1973), which seem to have served as a basis for the figures cited here p. 16. This pedagogical approach can also be compared to the "egg methods" on kinetic energy, such as the "Method for not breaking eggs", written a month earlier.

The unfinished clean copy (boxes 14 to 18 empty on the last page) suggests a working document, perhaps intended for internal or external communication on road safety, never totally finalised in this version.