Force, in mechanics, physical quantity that, when acting on a body, either causes it to change its state of motion (i.e., imparts to it an acceleration), or tends to deform it (i.e., induces in it an elastic strain). Dynamical forces are governed by Newton's laws of motion, from the second of which it follows that a given force acting on a body produces in it an acceleration proportional to the force, inversely proportional to the body's mass, and occurring in the direction of the force. Forces are thus vector quantities with direction as well as magnitude. Four naturally occurring forces are gravity, electromagnetism, and the strong and weak nuclear forces within atoms.
See also: Mechanics.