Studies on the motion of celestial bodies in general
Euler looks at finite systems and says that for a single point mass, ∫ f dM = fM. He also introduces the Newtonian equations for the first time. (Based on Clifford Truesdell's An idiot's fugitive essays on science: methods, criticisms, training, circumstances.)
Original Source Citation
Mémoires de l'académie des sciences de Berlin, Volume 3, pp. 93-143.
Opera Omnia Citation
Series 2, Volume 25, pp.1-44.