A. An object in motion will remain in motion unless acted upon by an outside force : The momentum of an object is constant unless an outside force acts on the object.
B. Force is defined as mass times acceleration : the rate of change of the momentum of a particle is proportional to the force F acting on it, hence the force is equal to mass times acceleration.
C. For every action there is an equal and opposite reaction : to every action force there is an equal and opposite reaction force.
