Within the field of Classical Mechanics, Newton's laws of motion, are three laws which define the relationship between objects in motion, and the forces these objects are subject too.
The first law of motion states than an object must remain at rest, or move at a constant velocity, unless it is subjected to external Force.
The second law states that the rate of change of the momentum of an object is proportional to the force acting on it.
For an object with constant mass, the net force acting on the object, is equivalent to the Mass of the object multiplied by the Acceleration.
The third law states that should one object exert a force on another, the affected object exerts a force that is equal in magnitude and opposite in direction of the first object.