If an object is moving at a constant velocity then there must be at least two forces acting on it  one force causing a "forward" acceleration and another force causing an "opposing" acceleration. These forces must balance to a net force of zero resulting in a corresponding acceleration of zero. Note that each force is doing work on the object but their "work" cancels to no net change in the object's kinetic energy. When a pathindependent force, or conservative force (for example, the pull of gravity), acts on an object, the change in the object's kinetic energy is related to a change in its potential energy.
PE_{lost} = KE_{gained}
