In physics, a force is any external agent that causes a change in the motion of a free body, or that causes stress in a fixed body. It can also be described by intuitive concepts such as a push or pull that can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform. 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. See also thrust.

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.

Since antiquity, scientists have used the concept of force in the study of stationary and moving objects. However, descriptions of forces by Aristotle incorporated fundamental misunderstandings, which, despite advances made by the third century BC philosopher Archimedes from studies of simple machines, persisted for many centuries. By the seventeenth century, Sir Isaac Newton corrected these misunderstandings with mathematical insight that remained unchanged for nearly three hundred years. By the early 20th century, Einstein in his theory of general relativity successfully predicted relativistic adjustments to 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 associates 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. 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.

See also:

• Science: Technology: Aerospace: Aeronautics: History: World War I (24)

• World War One
  members.fortunecity.com
  Causes and effects of this conflict,the fronts, documents, and includes analysis of participant armies and navies.
• Chemical Warfare During WWI
  mitretek.org
  Chronicles the antecedents of chemical warfare, the development of chlorine gas, and the means of delivering these weapons.
• WW1 Tanks and Cars
  mailer.fsu.edu
  The history of the development of the tank, other armored vehicles, and their impact with images.