The acceleration due to gravity of a particle falling toward the earth is a = -gR2/r2, where r is the distance from the center of the earth to the particle, R is the radius of the earth, and g is the acceleration due to gravity at the surface of the earth. If R = 3960 mi, calculate the escape velocity, that is, the minimum velocity with which a particle must be projected upward from the surface of the earth if it is not to return to earth.