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I have a spring with spring constant 30 N/m and a mass of 0.5 kg. With the mass at the bottom, the spring has the length 58 cm at rest. If I now pull down the mass and release it, it starts with a vertical oscillation, then the spring also starts to swing sideways like a pendulum and the vertical oscillation loses amplitude until for a short moment there's only the sideways motion. Then the whole process reverses and starts over.

Apparently the energy moves from the "spring mode" to the "pendulum mode" and back. However, from above values I calculate frequencies of 1.23 Hz for the spring mode and only 0.65 Hz for the pendulum. Wouldn't resonance require them to be the same? There seems to be a factor 2.