If no net force acts on a particle, then it is possible to select a set of reference frames, called inertial reference frames, observed from which the particle moves without any change in velocity. This law is often simplified into the sentence "An object will stay at rest or continue at a constant velocity unless acted upon by an external unbalanced force".
Second law
Observed from an inertial reference frame, the net force on a particle is proportional to the time rate of change of its linear momentum: F = d[mv] / dt. Momentum is the product of mass and velocity. When the mass is constant, this law is often stated as F = ma (the net force on an object is equal to the mass of the object multiplied by its acceleration).
Third law
Whenever a particle A exerts a force on another particle B, B simultaneously exerts a force on A with the same magnitude in the opposite direction. The strong form of the law further postulates that these two forces act along the same line. This law is often simplified into the sentence "Every action has an equal and opposite reaction".
