Newbie Physics Student Asks: Does Vapor to Ice Change Acceleration/Velocity?

[Limebat]

*My bad if this question is a tad ree ree. I've just completed my first year of college and am still inexperienced. I just study physics for fun.*

My intuition says the momentum of the water vapor is still conserved during the phase shift, as this question most probably relates to the macroscopic level, so any intermolecular interactions would probably not be affected by microstate tendencies.

I was initially thinking of examining the phase shift between vapor to ice and calculating Gibb's free energy. Energy and work can then be related. But I don't think those terms would apply to a macroscopic scale, as those are microscopic qualities.

Yet I also believe if the water molecules are sufficiently spaced, then the line between macroscopic and microscopic calculations are blurred. Or perhaps not. Not sure actually.

Regardless, the question boils down (pun intended) to:
If water (g) --> (s) in space, does acceleration and velocity change?

* If molecules vibrate faster does that mean macroscopically its faster*
*I also asked on Quora, but the quality of answers are mixed, so I wanted to ask here for a second opinion just incase. Did meet some amazing and helpful people there though!

 

[jbriggs444]

*My bad if this question is a tad ree ree. I've just completed my first year of college and am still inexperienced. I just study physics for fun.*

My intuition says the momentum of the water vapor is still conserved during the phase shift, as this question most probably relates to the macroscopic level, so any intermolecular interactions would probably not be affected by microstate tendencies.

I was initially thinking of examining the phase shift between vapor to ice and calculating Gibb's free energy. Energy and work can then be related. But I don't think those terms would apply to a macroscopic scale, as those are microscopic qualities.

Yet I also believe if the water molecules are sufficiently spaced, then the line between macroscopic and microscopic calculations are blurred. Or perhaps not. Not sure actually.

Regardless, the question boils down (pun intended) to:
If water (g) --> (s) in space, does acceleration and velocity change?

* If molecules vibrate faster does that mean macroscopically its faster*
*I also asked on Quora, but the quality of answers are mixed, so I wanted to ask here for a second opinion just incase. Did meet some amazing and helpful people there though!

Momentum is conserved in an isolated system, always. That's a pretty deep and inescapable fact arising from Noether's theorem.

Obviously, if you want momentum conservation to hold, you have to avoid external forces. Even more obviously, you want to avoid adding or removing mass from the system.

It gets a little trickier when considering energy transfers. When you start supplying or draining energy, you need to be careful that the energy transfer does not also transfer momentum. As long as we confine ourselves to classical mechanics, this is not much of an issue. One can shine a floodlight on an ice cube and not worry about any momentum transfer. Or one can run some electrical current through it as long as the wires are kept suitably slack.

If one considers relativity however, the addition of energy without an addition of momentum in the system's rest frame ends up adding rest mass. But if you've increased rest mass, you've increased the momentum of the system in frames where the system was not at rest. So if one is considering relativity, one needs to eliminate energy transfers to or from the outside.

 

[Limebat]

If one considers relativity however, the addition of energy without an addition of momentum in the system's rest frame ends up adding rest mass. But if you've increased rest mass, you've increased the momentum of the system in frames where the system was not at rest. So if one is considering relativity, one needs to eliminate energy transfers to or from the outside.

Ah that's cool! I didn't think about that. I know I posted this question in classical physics, but can you elaborate / point to resources of looking at rest mass and momentum with relativity? (See quoted)

 

[anorlunda]

point to resources of looking at rest mass and momentum with relativity?

https://en.wikipedia.org/wiki/Invariant_mass

 

[Limebat]

https://en.wikipedia.org/wiki/Invariant_mass

Thank you very much!

 

[sophiecentaur]

Yet I also believe if the water molecules are sufficiently spaced, then the line between macroscopic and microscopic calculations are blurred. Or perhaps not. Not sure actually.

If you have enough of the molecules in your experiment then they can be treated in a thermodynamic way (i.e. statistically). If they are all in a container and you accelerate the container, there will be a different pressure on the front and back inner walls of the box. But the 'thermal energy' inside the box will not be affected. There will be a pressure gradient across the box but the mean velocity of all the molecules will correspond to the velocity of the box,

 

[Limebat]

If you have enough of the molecules in your experiment then they can be treated in a thermodynamic way (i.e. statistically). If they are all in a container and you accelerate the container, there will be a different pressure on the front and back inner walls of the box. But the 'thermal energy' inside the box will not be affected. There will be a pressure gradient across the box but the mean velocity of all the molecules will correspond to the velocity of the box,

Thank you very much! I didn't think about it that way