1. The problem statement, all variables and given/known data 2. Relevant equations Xcm = (x1m1+x2m2) / (m1+m2) 3. The attempt at a solution ok so i'm having trouble understanding what's happening in the question... when the penguin moves, is the sled also moving? If it's friction less ice then if the penguins moves L, the sled will move -L. Is this right or am I just not understanding the question? for the first one, Changed what I think, penguin will move 0 relative to itself, in the sled system it will move L so center of mass at L/2? second one Changed what I think for this too, since the penguin moves 0 relative to itself and from the sled system it will move L, in the lab frame it moves L/2. third one seems pretty simple. Since penguin at origin and the center of mass of sled is at L/2, the center of mass would be L/4. fourth one sled frame moves -L, penguin relative to itself always 0 so the origin is -L/2 fifth one sled moves -L/2 so it's center of mass is at 0, the penguin moves L/2 so center of mass is L/4? sixth one L/4 Are these right so far?