Hi :). I'll just start describing the problem... An ant with mass m is moving anti-clockwise on a Disk with a perpendicular rotation axis (with no friction). The disk's radius is R and it's moment of inertia - I. The velocity of the ant *in relation to the Earth* is V while the disk is rotating *clock-wise* in an angular velocity W0 (that's omega zero). The ant suddenly stops. The questiong is - what's the angular velocity of the disk (with the ant on it) after it stops. I know this problem involves angular momentum preservation equations, but I obviously do not conclude the right one, since there are four possible answers, and none fits my answer. First of all, some questions: 1. The disk starts rotating because the ant start walking on it, right? As the ant continues walking - wouln't the disk's angular velocity increase? It apears it remains constant, since there's no time given. 2. if the ant is causing the rotation, why does the disk continue to rotate when it stops walking? 3. If I want to write the angular momentum preservation equation - should I relate to the ant's velocity compared to the Earth (an inertial system) or compared to the disk? This is what I've tried: While the ant's walking, the angular momentum of the disk + ant is: I*W0 + R*m*V After stopping, the angular momentum is: I*W Where W (omega) is the final angular velocity. Therefore I*W0 + R*m*V = I*W. Finding W from this equation did not satisfy any option :-\ I'd really appreciate help. Thanks for reading. Tomer.