Handwritten note “Method for not breaking eggs”

In this document

The principle: from the falling egg to the concept of impact

The note starts with a simple experiment: an egg dropped onto a hard surface breaks more or less depending on the height of the fall p. 1. Enclosed in a rigid box, it suffers the same damage “at equal drop height”, because “the speeds at the moment of stopping [...] are identical” p. 1.

Kinetic energy and braking

The author then explains the concept of kinetic energy using the analogy of a hammer driving a nail: at equal speed, it doubles if the weight doubles; at equal weight, it quadruples if the speed doubles p. 1. He deduces the general law of braking: the product of force and distance travelled (F×d) is constant and equal to the kinetic energy of the body p. 1.

Acceleration, deceleration, gravity

Page 2 details the difference between speed (m/s) and acceleration (m/s²), illustrated by a cyclist gaining 1 m/s every second. It distinguishes the natural acceleration of gravity, denoted g (with the value g = 9.81 at the latitude of France, and a table of variants according to location), from artificial acceleration denoted γ. The relationship F = Mγ, and for gravity P = M × 9.81, are established p. 2.

The calculation applied to the egg

Page 3 introduces the formula E = 1/2 MV² = 1/2 PV²/9.81, as well as V = √2gh. For a fall of 1.20 m, the author calculates V = 4.85 m/s, then, for a 0.050 kg egg, a kinetic energy of 0.06 kg.m p. 3. He states several equivalent force/distance pairs (0.06 kg over 1 m, 60 kg over 0.001 m, etc.) and points out that the force acceptable by the egg depends on the support surface — an enveloping strap being more favourable than a point support p. 3.

Page 4 concludes the demonstration: if the strapped egg resists 1.2 kg, it requires, for a fall of 1.20 m (energy 0.06 kg.m), a braking distance of 0.05 m, i.e., “at least 5 cm available” between the egg and the floor of the box p. 4. Beyond a 1.20 m fall, the egg hits the bottom and cracks. The author adds that he will discuss the case of a “perfectly elastic” strap p. 4, a discussion interrupted by the end of the transmitted document.

Context

This note, dated 24 March 1973, is part of the work of Paul Magès, a Citroën engineer and inventor of the hydropneumatic suspension — the plans for which date back to 17 February 1944 (Plans for hydropneumatic suspension with differential interaction) and whose tests continued until the DS presented in 1955. In 1973, Magès was still employed at Citroën, where he remained until his retirement in September 1974 (photographs of Paul Magès' retirement). The analogy of the egg cushioned by a strap in a box evokes, in an educational way, the principle of damping a mass (a vehicle body) by a determined suspension travel — without the document explicitly mentioning the automobile or Citroën suspension.

Archivist's perspective

This document is distinguished by its educational and progressive nature: it seems intended to explain, to a non-specialist audience, the physical bases (speed, kinetic energy, deceleration) that underpin Magès' reasoning on damping p. 1-4. This taste for pedagogy, here applied to popular science, is found in the very career of Magès, a self-taught man who became a technical writer at Citroën (autobiographical notes).

The final numerical demonstration — 5 cm of travel required to not break a 50 g egg falling from 1.20 m p. 4 — concretely illustrates the compromise between braking force and travel distance, a central principle of any suspension. It is likely that this note served as an internal explanatory aid or preparation for a presentation, rather than a technical calculation note intended for a specific project.

The final mention, left hanging, of a “perfectly elastic” strap p. 4 suggests that the demonstration continued on the following pages, which have not been transmitted.