Kinetic-Potential energy of moving Ice

[TonyCross]

Hi,
Am i correct in thinking that if we take a block of ice, moving at a constant velocity, it's then exposed to a heat source which melts the ice and turns it into water vapour, that we have simply removed any Kinetic energy, by Sublimation or converting it into heat.
My question is does the Kinetic energy and Potential energy of the moving block of ice simply get nullified by this process, or as i suspect each part of the vapour then inherits a small portion of the original velocity?

 

[Baluncore]

In the first analysis, you are correct.
The deeper analysis becomes more complex.

When the ice sublimes, it becomes water vapour in the air.
Water has a lower molecular weight than average air, so buoyancy causes the higher RH air to rise. Before vaporisation, the ice had a potential energy due to mass. That potential energy changes sign with buoyancy.

When a flying bomb explodes, the combustion products form a huge sphere of gas followed by a central depression, which stops almost instantly. That is because the cross-section of the low-density sphere produced, is up against hundreds of tonnes of atmosphere.

Likewise, as ice evaporates, it becomes part of the air and is carried away by the local air motion, what we call wind. Now consider the boundary layer against the ice. The local air will slow down due to the energy needed to accelerate the entrained water vapour, while it also speeds up because the volume of air is locally increasing, with the reduction in density.

 

[TonyCross]

Thanks for your answer, the reason for the question arose from a thought I had about Newtons 3rd.
Consider you are in a vessel in space, you move a block of ice, the spacecraft has to react in the opposite direction, normally this craft movement is cancelled by the conservation of energy as the mass of the ice is stopped by the end of the craft.
However if the mass of the object effectively dissapears and is turned into gas, or water vapour, due to the heating of the ice, what force cancels the first movement of the craft?
The only answer I can think is that the vapour must conserve the block of ice's original momentum. However I feel this is counter intuitive. cheers

 

[DrClaude]

However if the mass of the object effectively dissapears and is turned into gas, or water vapour, due to the heating of the ice, what force cancels the first movement of the craft?

The mass of the object doesn't disappear. The gas molecules retain the momentum of the original ice. This water vapor will push against the air in the craft, which will push against the walls, and so on.

The only answer I can think is that the vapour must conserve the block of ice's original momentum. However I feel this is counter intuitive. cheers

Yes it does. Why is it counter-intuitive to you? Imagine a block of ice in the middle of outer space. If it is somehow sublimating, why wouldn't the cloud of gas simply continue on with the block of ice? (You can always put yourself in the center-of-mass reference frame, and the process would look stationary.)

 

[TonyCross]

Hi, thanks for the comment.
Why i find it counter intuitive is simple.

Consider an Asteroid of a given mass travelling at a constant velocity, now imagine this asteroid breaks into two equal parts.
Each part now has the same velocity, but the mass is halved so each of the parts now have half the potential KE. Agreed.

if your comment about the water vapour retaining the velocity of the ice, why do comets leave a trail behind them of dust, sodium and vapour. Given a certain mass of the comet is lost, by your reasoning there would not be a trail, the particles would simply continue at the same velocity of the comet. Clearly they do not.

 

[Baluncore]

The tail of a comet, is blown away from the nucleus of the comet, by the solar wind.

 

[jbriggs444]

The tail of a comet, is blown away from the nucleus of the comet, by the solar wind.

In particular, for a comet that has passed by the sun and is proceeding back outwards, the comet's tail is in front of it.

 

[TonyCross]

Thanks that clears up my confusion.

cheers