D'Alembert began on the scientific European scene with the publication of his first book: the Traité de dynamique (1743). Dynamics is born in the 1730's and with the Traité D'Alembert became one of the main protagonists of that new science. Unfortunately, historians have neither manuscripts nor letters at their disposal, which could inform about the circumstances in which this book was written. The present article aims to precise those circumstances and to give essential characteristics of D'Alembert's dynamics, in particular in the light of the works led by his colleagues of the French Royal Academy of Sciences. In reading D'Alembert's ideas about causality and the proofs of some mechanical principles, in searching how he could have known some mechanical problems which appeared in the Academy around 1740, and in analyzing how he solved them, it's possible to sketch hypothesis about the formation of the young man. They involve exchanges or competition with his colleagues who probably made him discover some mechanical problems. But D'Alembert published in 1743 the first book exclusively dedicated to that science with an original and specific approach which he kept during all his scientific career, an approach that allowed him to write that his work had nothing in common with that of the others.