This is a simulation showing three identical masses attached to three different springs. Each spring has a different spring constant, and therefore a different natural frequency of vibration for a given amount of mass. The bar that the springs are attached is attached to a motor, causing it to oscillate up and down. Adjust the frequency of the forced vibration, and watch as over time one (or none - depending on the frequency chosen) of the three masses goes into a larger resonant vibration. Note: this simulation does not incorporate gravitational force - it is a simulation of what would happen in a weightless environment.