Escape velocity is minimum velocity with which a body must be thrown upward so that it may just escape.Expression for escape velocity:Let a body of mass m be escaped from gravitational field of the earth. During the course of motion, let at any instant body be at a distance r from the centre of the earth.The gravitational force between body and the earth is,                               and work done to raise the body by distance dr is,                                Total work done W in raising the body from the surface of the earth to infinity is,                                                              If we throw the body upward with a velocity ve, then work done to raise the body from surface of the earth to infinity is done by kinetic energy,                                 or                                or                                 Substituting the values, g = 9.81m/sec and R = 6.4 x 106m, we have,