In experiments, it is easy to prove that the magnitude of the acceleration is proportional to that of the force applied to a given body a∝F This ratio is called the mass of the body Thus m=F/a or F=ma Mass The property of a body that determines its resistance to a change in it's motion The mass defined in newton ' s law is an inertial massIn experiments, it is easy to prove that the magnitude of the acceleration is proportional to that of the force applied to a given body. This ratio is called the mass of the body. Thus m=F/a or F=ma Mass :The property of a body that determines its resistance to a change in it’s motion. a  F The mass defined in Newton’s law is an inertial mass