ABSTRACT

The Marquise du Chatelet in her book " Les Institutions Physiques" published in 1740, stated on page 36, that Descartes, when formulating his laws of motion in an elastic collision of two bodies B & C (B being more massive than C) having the same speed v, said that the smaller one C will reverse its course while the more massive body B will continue its course in the same direction as before and both will have again the same speed v.

Mme du Chatelet, basing her judgment on theoretical considerations using the principle of continuity , declared that Descartes was wrong in his statement. For Mme du Chatelet the larger mass B should reverse its course and move in the opposite direction. She mentioned nothing about both bodies B & C as having the same velocity after collision as Descartes did.

At the time of Descartes, some 300 years ago, the concept of kinetic energy & momentum as we know today was not yet well defined, let alone considered in any physical problem.

Actually both Descartes & Mme du Chatelet may have been right in some special cases but not in general as the discussion that follows  will show.