An inertial frame of reference was first defined by the first of Newton's Laws of Motion. This law states that an inertial frame of reference is one in which the acceleration of an object is zero if and only if the vector sum of all forces acting on that object is zero. This is an important definition, for the second, and possibly best known of Newton's Laws (F = ma) only holds true in such a frame of reference. It is possible to use Newtonian Mechanics in a non-inertial frame, but only if one introduces so-called fictitious forces.