English: Diagram of a Wilberforce pendulum, a physics experiment demonstrating coupled oscillation. It consists of a weight with 4 outriggers to adjust its moment of inertia which can turn on its vertical axis ${\displaystyle \theta }$, and bob up and down ${\displaystyle z}$ on the spring. The two motions are coupled by the spring, so when set oscillating up and down the weight slowly begins rotating back and forth. Over time the vertical oscillation decreases as more energy is transferred to the rotational oscillation, until the weight is just rotating about its axis. Then the energy is slowly transferred back to the vertical oscillation, so the rotational oscillation decreases and the up-and-down oscillation increases until it is just bobbing up and down. This alternating motion continues until it slowly comes to a stop.