Kinetic-Potential energy of moving Ice

In summary, the conversation discusses the concept of kinetic and potential energy in relation to the sublimation of ice into water vapor. The expert explains that the water vapor retains the momentum of the original ice, and this can be observed in the trail of comets as they are blown away from their nucleus by the solar wind. The expert also addresses the question about the conservation of energy in space and explains that the mass of an object does not disappear when it turns into gas or water vapor, but rather retains its momentum.
  • #1
TonyCross
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12
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?
 
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  • #2
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.
 
  • #3
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
 
  • #4
TonyCross said:
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.
TonyCross said:
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.)
 
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  • #5
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.
 
  • #6
The tail of a comet, is blown away from the nucleus of the comet, by the solar wind.
 
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  • #7
Baluncore said:
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.
 
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  • #8
Thanks that clears up my confusion.

cheers
 

FAQ: Kinetic-Potential energy of moving Ice

What is the relationship between kinetic and potential energy in moving ice?

Kinetic energy is the energy of motion, while potential energy is the stored energy due to an object's position or state. For moving ice, kinetic energy depends on its velocity, and potential energy is influenced by its height or position in a gravitational field. As ice moves, energy can transform between kinetic and potential forms, such as when ice slides down a slope, converting potential energy to kinetic energy.

How do you calculate the kinetic energy of moving ice?

The kinetic energy (KE) of moving ice can be calculated using the formula KE = 0.5 * m * v^2, where 'm' is the mass of the ice and 'v' is its velocity. This formula shows that kinetic energy increases with both the mass of the ice and the square of its velocity.

How do you determine the potential energy of ice at a certain height?

The potential energy (PE) of ice at a certain height can be calculated using the formula PE = m * g * h, where 'm' is the mass of the ice, 'g' is the acceleration due to gravity (approximately 9.8 m/s^2 on Earth), and 'h' is the height above the reference point. This formula indicates that potential energy increases with both the mass of the ice and its height.

What factors affect the kinetic and potential energy of moving ice?

Several factors affect the kinetic and potential energy of moving ice, including its mass, velocity, and height. The kinetic energy is directly influenced by the mass and speed of the ice, while potential energy depends on the mass and the height of the ice in a gravitational field. Additionally, external forces like friction and air resistance can impact the energy transformations between kinetic and potential forms.

Can the total mechanical energy of moving ice change, and if so, how?

The total mechanical energy of moving ice, which is the sum of its kinetic and potential energies, can change due to external forces such as friction, air resistance, or applied forces. In an ideal, frictionless environment, the total mechanical energy remains constant, with energy transforming between kinetic and potential forms. However, in real-world scenarios, energy losses due to friction and other factors can decrease the total mechanical energy over time.

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