Equilibrium is the state of a body in which there is no change in its motion. A body in equilibrium is either at rest or moving at constant speed in a straight line. \
When two or more forces act simultaneously on a body at the same point of application, they are called concurrent forces. The vector sum of the concurrent forces represents the single force which would have the same effect as the concurrent forces. The force equal to the vector sum of the concurrent forces, but acting in an opposite direction, is called the equilibrant force.
When all of the concurrent forces acting on a body are in balance, a state of equilibrium exists. Bodies in equilibrium do not experience changes in their motion.