Momentum in a spacecraft, please help!!! 1. The problem statement, all variables and given/known data John (90kg) and Julie (60kg) are in a spacecraft (50kg). They swap chairs, which are 4m apart, located equal distances from the Centre of Mass of the spacecraft. Questions: Why does the spacecraft NOT move after they take their seats? How far does the spacecraft move? Which way? All observations are in the frame in which the spacecraft was initially stationary. 2. Relevant equations I don't know which equations to use, the question also says that they hope we attempt to answer this question using momentum principles as well as the centre of mass approach. 3. The attempt at a solution The only thing i have is that the CM should initially be 1.6 meters from John. This question was especially confusing as they ask why the spacecraft doesn't move, and then where it moves to... It seems to make no sense, all i can think of is this: The centre of mass moves when the people swap seats. It's always going to be closer to John than to Julie in the spacecraft because he is heavier. If we use the CM as our frame of reference, the spacecraft will appear to move in the opposite direction that the centre of mass does while the CM will not appear to move at all.