Consider an object of mass m at a distance r1 from the Earth's center. Since this object is in the Earth's gravitational field, it possesses gravitational potential energy U1. Suppose the object moves farther away at a distance r2 such that its potential energy changes to U2. Following the energy conservation law, the sum of kinetic energy and potential energy at r1 equals the sum of kinetic energy and potential energy at r2. Considering distance r2 as infinity, where Earth's gravitational field is negligible, the potential energy U2 is zero. Theoretically, K2 is equal to zero, since the object becomes stationary at infinity. Substituting for kinetic energy and potential energy at r1, the object's velocity at r1 is equal to the square root of two times the product of the gravitational constant and Earth's mass, divided by r1. Approximating r1 equal to the radius of the Earth, an expression for the object's escape velocity is obtained. It is the minimum initial velocity which an object requires to escape the Earth's gravitational field.