Two types of central forces —those that increase linearly with distance, F = Cr, such as Hooke's law, and inverse-square forces, F = C / r, such as Newton's law of universal gravitation and Coulomb's law —have a very unusual property. A particle moving under either type of force always returns to its starting place with its initial velocity, provided that it lacks sufficient energy to move out to infinity. In other words, the path of a bound particle is always closed and its motion repeats indefinitely, no matter what its initial position or velocity. As shown by Bertrand's theorem, this property is not true for other types of forces; in general, a particle will not return to its starting point with the same velocity.