 The gravitational acceleration of an object on the surface is always towards the center. But it is possible that the velocity of the object is away from the center. This happens when there is an initial velocity, such as when a projectile is shot from a cannon. To understand how the projectile will progress, we need to use the conservation of energy principle together with the total energy of the system at the time the projectile leaves the barrel of the cannon. The initial velocity that enables the object to escape the Earth's gravitational pull is called the escape velocity. It is the velocity that makes the system's total energy equal to zero, and the object will slowly approach a velocity of zero, but never quite reach it, and it will never return to the Earth. In this example, the initial velocity is less than the escape velocity, so the total energy is negative and the object will fall back to Earth. If the initial velocity is greater than the escape velocity, the total energy is positive and the object will go on with a non-zero velocity forever and never return to the Earth. It has escaped.