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.
Two-body problem in general relativity
Nearly a century later, Isaac Newton had formulated his three laws of motion. In particular, Newton's second law states that a force "F" applied to a mass "m" produces an acceleration "a" given by the equation "F"="ma". Newton then posed the question: what must the force be that produces the elliptical orbits seen by Kepler? His answer came in his law of universal gravitation, which states that the force between a mass "M" and another mass "m" is given by the formula