A uniform disk turns at 2.4 rev/s around a frictionless axis. A nonrotating rod, of the same mass as the disk and length equal to the disk's diameter is dropped onto the freely spinning disk figure. They then both turn around the axis with their centres superposed. What as the angular frequency in rev/s of the combination.

Question

G1 Physics · Accepted Answer

The disc has moment of inertia ½MR². The rod has length 2R, so its moment of inertia is M(2R)²/12 = MR²/3. Conservation of angular momentum gives final angular speed = [½MR²/(½MR² + MR²/3)](2.4) = 1.44 rev/s. Hence C.