sir i have a doubt on center of masswe know that if a boat is in still water and a man on boat is moving forward boat moves back so that position of center of mass of systemremains the same (in case no external force is acting on system) but this is true only for an observer outside the systemthen if we take an observer inside the system will center of mass remain same?

Hello Student,

Thanks for your post.
If we take an observer inside this (man+boat) system, the centre of mass will not remain same in this situation.
Consider the following figure, explaining the whole scenario.


Here, we don’t have to bother what’s happening outside the boat and what’s its position in the water. So, according to the observer inside the boat, COM of system seems to have changed after the motion of the man.