In physics, a force is that which can cause an object with mass to change its velocity.[1] Force has both magnitude and direction, making it a vector quantity. Newton's second law states that an object with a constant mass will accelerate in proportion to the net force acting upon and in inverse proportion to its mass. Equivalently, the net force on an object equals the rate at which its momentum changes.[2]
Forces acting on three-dimensional objects may also cause them to rotate or deform, or result in a change in pressure or even change volume in some cases. The tendency of a force to cause changes in rotational speed about an axis is called torque. Deformation and pressure are the result of stress forces within an object.[3][4]
Since antiquity, scientists have used the concept of force in the study of stationary and moving objects. The study of forces advanced with descriptions made by the third century BC philosopher Archimedes of how forces interact in simple machines.[5] Prior to this, descriptions of forces by Aristotle incorporated fundamental misunderstandings. By the seventeenth century, Sir Issac Newton corrected these misunderstandings with mathematical insight that remained unchanged for nearly three hundred years.[4] By the early 20th century, Einstein in his theory of general relativity successfully predicted the failure of Newton's model for gravity by ushering in the concept of a space-time continuum.
The recent theory of particle physics known as the Standard Model associate forces at the level of quantum mechanics. The Standard Model predicts that exchange particles called gauge bosons are the fundamental means by which forces are emitted and absorbed. Only four main interactions are known: in order of decreasing strength, they are: strong, electromagnetic, weak, and gravitational.[3] High-energy particle physics observations made during the 1970s and 1980s confirmed that the weak and electromagnetic forces are expressions of a more fundamental electroweak interaction.[6] (fromhttp://en.wikipedia.org/wiki/Force)