Question

A mercury thermometer placed in a gravity free hall without touching anything. As temperature rises mercury expands and ascend in thermometer. If height ascend by mercury in thermometer is h then by what height centre of mass of '' mercury and thermometer" system descend?

Answer 1

Here thermometer is placed in gravity free region

So there is no external force on the system

when there is no force on the system its center of mass will retain its position and does not move if the whole system is initially at rest

So here the displacement of center of mass of the system will be ZERO

Answer 2

Gravity works on the merury and there is no external force on the particle

therefore centere of mass is remain at his position. Or we can say if particle is at rest the centre of mass is at remain ZERO.