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Let us assume we have a big ice cube on a rigid table(on Earth).

After a while the ice cube will melt, if we are on Earth then the ice will get converted into water and spread on the table.

Therefore the center of mass of the ice cube will be a little lower. What my understanding is that , it will get lower due to bombardment of air molecules.

But if we use light instead of air to heat it what should be the reason for the vertical displacement of center of mass?

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    $\begingroup$ You should also take the gravitational force into account. $\endgroup$ – NDewolf Jan 15 at 14:06
  • $\begingroup$ That is why i wrote it is on Earth, and gravitational force is countered by the normal force, moreover I think that gravitational force has little role to play here as the rate at which ice melts is dependent on temperature, am I missing something ? $\endgroup$ – hezizzenkins Jan 15 at 14:09
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    $\begingroup$ I meant with respect to the displacement. The water just gets pulled down. Perform this experiment in outer space and the center of mass should remain the same (see the answer by @Scar). $\endgroup$ – NDewolf Jan 15 at 14:09
  • $\begingroup$ So it seems that somehow the normal force is reduced, am i correct in interpreting what you said? $\endgroup$ – hezizzenkins Jan 15 at 14:14
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    $\begingroup$ As @Scar explains the forces between molecules get weakened during the phase transition from ice to liquid water. Since these forces are the ones responsible for preserving the structure of the ice crystal (and thus act as a normal force) the water molecules can move towards earth. $\endgroup$ – NDewolf Jan 15 at 14:17
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Gravity. The heat transferred by light causes it to change its state and the weakened atomic forces can no longer maintain the position of the atoms in their original state. Thus, the liquid spreads in the influence of gravity to obtain the lowest possible potential energy of the system. Hence, the vertical displacement of the center of mass.

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Center of mass will lower due to the particles moving down and reducing as much potential energy as possible

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