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In a simple case, imagine an object is oscillating sinusoidally with frequency f1. Suppose you apply a force to that object along the direction of its motion with frequency f2. The rate of energy added to the oscillation is force x velocity, so the total energy added to the system after some time T is the integral over time of force x velocity. Since trig functions are orthogonal under integration, the value of that integral when T is very large will be zero unless f1=f2. Intuitively, if you always push a swing in the direction of its motion, it will always swing faster and higher, until frictional losses between pushes match the energy added by the pushes
