In science, force is a push or a pull on an object with mass that can cause it to change its velocity (to accelerate). A force is a vector, which means it has both magnitude and direction. In equations and diagrams, a force is usually denoted by the symbol F. An example is the famous equation from Newton’s second law: F = m·a where F is force, m is mass, and a is acceleration. The SI unit of force is the Newton (N). Other units of force include the dyne, kilogram-force (kilopond), poundal, and pound-force. While Aristotle and Archimedes had a sense of what forces were and how they worked, Galileo Galilei and Sir Isaac Newton described how force works mathematically. Newton’s laws of motion (1687) predict the action of forces under normal conditions. Einstein’s theory of relatively predicts the action of forces as momentum approaches the speed of light. In nature, the fundamental forces are gravity, the weak nuclear force, the strong nuclear force, electromagnetic force, and residual force. The strong force is what holds protons and neutrons together in the atomic nucleus. The electromagnetic force is responsible for the attraction of opposite electric charge, repulsion of like electric charges, and the pull of magnets. There are also non-fundamental forces encountered in everyday life. The normal force acts in a direction normal to the surface interaction between objects. Friction is a force that opposes motion on surfaces. Other examples of non-fundamental forces include the elastic force, tension, and frame-dependent forces, such as centrifugal force and the Coriolis force.