Force
Sir Isaac Newton described the motion of all objects using the concepts of inertia and force, and in doing so he found they obey certain conservation laws. In 1687, Newton published his thesis "Philosophiæ Naturalis Principia Mathematica". In this work Newton set out three laws of motion that to this day are the way forces are described in physics.
Women in science
Émilie du Châtelet, a close friend of Voltaire, was the first scientist to appreciate the significance of kinetic energy, as opposed to momentum. She repeated and described the importance of an experiment originally devised by Willem 's Gravesande showing the impact of falling objects is proportional not to their velocity, but to the velocity squared. This understanding is considered to have made a profound contribution to Newtonian mechanics. In 1749 she completed the French translation of Newton's "Philosophiae Naturalis Principia Mathematica" (the "Principia"), including her derivation of the notion of conservation of energy from its principles of mechanics. Published ten years after her death, her translation and commentary of the "Principia" contributed to the completion of the scientific revolution in France and to its acceptance in Europe.