We can relate the gravitational potential energy to the escape velocity ve, the speed with which an object must be launched from the surface to escape permanently from the cluster. Consider a particle of mass m, moving outward from the surface at speed ve. If the object escapes, it must get so far away that the potential energy is essentially zero. Since the kinetic energy is always greater than or equal to zero, the total energy of the object far away must be greater than or equal to zero. Since the total energy is conserved, the total energy for an escaping object must be zero or positive when it is launched. The kinetic energy of the particle is

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