“Method for not breaking eggs” — handwritten note by Paul Magès on kinetic energy (24 March 1973)

In this document

The falling egg analogy

Paul Magès starts from a simple experiment: an egg dropped onto a hard surface breaks more or less depending on the height of the fall — “For a very low height, the egg remains intact. From a little higher, it cracks. From higher still, it crushes” p. 2. He then transposes this fall to an egg enclosed in a rigid box, where the damage, at an equal height, is identical since “the speeds at the moment of the sudden stop are identical” p. 2.

Kinetic energy and deceleration

Magès explains the concept of kinetic energy using the image of a hammer that must have speed to drive in a nail: “At equal speed, the kinetic energy doubles if the weight of the hammer is double. At equal weight, the kinetic energy is four times greater if the speed is double” p. 2. He then sets out the central principle of deceleration: the product of force and braking distance (F×d) is constant and equal to the kinetic energy of the body p. 2.

He also details the distinction between mass and weight, the acceleration of gravity g (“At the latitude of France, we consider that g = 9.81”, p. 3), and provides precise values of g according to latitude and altitude (p. 3, note 1).

The calculation applied to the egg

He sets out the formulas: E = ½MV² and V = √(2gh) p. 4. For a 50 g egg falling from 1.20 m, he calculates V = √(2×9.81×1.2) = 4.85 m/s, then a kinetic energy of 0.06 kg.m p. 4. He deduces a series of equivalent force/distance pairs:

In paragraph 9 p. 4-5, he imagines a non-elastic strap holding the egg in its box, and calculates that with an admissible force of 1.2 kg, the egg must be braked over 0.05 m, i.e., 5 cm of space available under the egg (1.2 kg × 0.05 m = 0.06 kg.m, p. 5). He concludes that the egg resists up to a fall of 1.20 m, but cracks beyond that p. 5. Page 6, which is very short, repeats this same calculation (0.06 / 1.2 kg = distance in metres).

Context

This note is part of the work of Paul Magès, a Citroën engineer known for the invention of the hydropneumatic suspension equipping the DS in particular. Beyond the suspension, the questions of deceleration and kinetic energy developed here touch on the broader field of automotive passive safety — braking, occupant restraint, shock absorption — a subject of growing interest in the automotive industry of the 1970s.

The egg analogy is a classic pedagogical method for popularising the concepts of impact and deceleration, without presuming here a specific source or inspiration for the document itself.

Archivist's perspective

This handwritten note, dated 24 March 1973, appears to be a draft or a first pedagogical refinement by Magès to explain, even before addressing the automotive case, the physical bases of deceleration to a non-specialist audience — likely colleagues, journalists, or sales managers.

The presence of a second version, more illustrated and titled slightly differently, “Method for breaking or not breaking eggs”, dated April 1973, suggests that this text was reworked and enriched shortly after. It also seems to prefigure the subsequent developments on the force formula in “Passive safety — The motorist and the technician”, dated 25 April 1973, where the physical analogy gives way to a directly automotive application.

The interest of this piece lies in its methodological value: it shows how Magès constructed his scientific pedagogy, from the simplest case (the egg) to more complex technical applications related to passenger safety.