The equation for determining the net force acting on an object is F = ma, or force equals mass times acceleration. Net force is measured in terms of acceleration, which means a change in velocity. If there is no change in velocity, the net force is considered to be zero.
At any time, there can be many forces acting upon an object, including gravity, friction and any external forces pushing or pulling on the object. The net effect of all of these forces is determined by the acceleration of the object. Acceleration is observed when a stationary object begins to move, a moving object changes speed or when an object in motion changes direction.