F=Gm1m2r2 where F=force of attraction between m1 and m2 due to gravity G=universal gravity constant m1=mass of the earth (considered concentrated at its center) m2=mass of particle on earth's surface r=radius of the earth (Nettleton, 1976) The force F which represents the force of gravity is not the measurement that is used in exploration. In exploration, the acceleration of a body resulting from that force is the conventional quantity used to measure the gravitational field acting at any given location (Dobrin, 1976). This acceleration can be derived from Newton's Second Law of Motion: F=m_{2}a where a=acceleration of the body due to gravity 62