A wheel is rotating freely with an angular speed of 800 rev/min on a shaft whose rotational inertia is negligible. A second wheel, initially at rest and with twice the rotational inertia of the first, is suddenly coupled to the same shaft. What is the angular speed in rev/min of the resultant combination of the shaft and two wheels? What fraction of the original rotational kinetic energy is lost? I don't know what "coupled to the shaft" means. How are they connected?