A very light object moving with speed v collides head-on with a very heavy object at rest, in a frictionless environment. The collision is almost perfectly elastic. The speed of the heavy object after the collision is

Newton's second law in terms of momentum:  The rate of change of momentum of an object is equal to the net force applied to it.
The law of conservation of momentum: The total momentum of an isolated system of objects remains constant.

Newton's second law for a system of particles: The sum of all forces acting on the system is equal to the total mass of the system times the acceleration of its center of mass.