Newtonian Subject is a term used to describe the study of the laws of motion and INERTIA, which was first proposed by Sir Isaac Newton in his 1687 work, Philosophiæ Naturalis Principia Mathematica. INERTIA, the seven-letter crossword solution for the term, is defined as the resistance of any physical object to a change in its state of motion or rest. It is the tendency of an object to remain at rest or in uniform motion in a straight line unless acted upon by an external force.