While going through Feynman's Lectures on Physics, I came across this paragraph(from the chapter Rotation in Two Dimensions) that got me confused:
So if we have some kind of a box, perhaps a space ship, with people in it, and we calculate the location of the centre of mass and find it is standing still. The centre of mass will continue to stand still if no external forces are acting on the box. Of course, this spaceship may move a little in space because people are walking back and forth inside; when one walks toward the front, the ship goes towards the back to keep the average position of all the masses in the same place.
I cannot imagine the scenario in my head and the ship going backwards as one person moves to the front, is non-intuitive for me.
It would be great if you could explain this with a bit of mathematics or maybe with a more relatable example than a space ship.