# Equilibrium of forces

## Equilibrium of forces

In a mechanical system the condition under which no acceleration takes place. Newtonian mechanics today is based upon two definitions which modify, but are essentially equivalent to, Newton's three fundamental laws. These definitions postulate the action of forces on particles. A particle is defined as a conceptual volume element that has mass and is sufficiently small to have point location. A body is defined as a system of particles. To develop the mechanics of a body, these definitions are applied to each of its particles and their influences summed. See Acceleration

The law of motion is that, in a newtonian frame of reference (with few exceptions, a frame of reference fixed with respect to Earth is considered to be newtonian), a particle of mass m acted on by resultant force F has acceleration a in accordance with the equation F = km a . Therein, k is a positive constant whose value depends upon the units in which F , m, and a are measured. The action-reaction law states that when one particle exerts force on another, the other particle exerts on the one a collinear force equal in magnitude but oppositely directed.

A body acted upon by force is in equilibrium when its constituent particles are in equilibrium. The forces exerted on its particles (and therefore on the body) are either internal or external to the body. An internal force is one exerted by one particle on another in the same body. An external force is one exerted on a particle or the body by a particle not of the body. See Dynamics, Kinetics (classical mechanics), Statics

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