physics ?

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toejam
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physics ?

Post by toejam » Tue Sep 23, 2003 7:27 am

here it is:
lets say an ice cube was shot into a chamber that could melt it before it hit anything, how would the energy be transferred?

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Bob Scott
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Re: physics ?

Post by Bob Scott » Tue Sep 23, 2003 7:55 am

The speedy ice turns into speedy vapour and just causes the air inside the chamber to be turbulent for a few seconds until that energy turns into heat. Remember, heat is simply kinetic energy of the molecules.<p>Released energy that we humans use all turns into heat in the end.<p>Bob :cool:
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Re: physics ?

Post by cato » Tue Sep 23, 2003 10:18 am

I think this is not so hypothetical as you thought when you wrote it. In real life this happens all the time. Meteors (A.K.A. shooting stars) vaporize (burn up) before impacting the chamber (earth) all the time.

toejam
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Re: physics ?

Post by toejam » Tue Sep 23, 2003 10:27 am

what i want to know is is there a velocity change as the ice changes to water than vapour.As ice cnanges to water, it becomes more dense an to vapour , much less.Assume the ice cube is traveling in a vacume.

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Re: physics ?

Post by bwts » Tue Sep 23, 2003 11:02 am

In the case of the meteor, yes, due to the friction caused by the atomosphere which will be acting for all purposes in an opposite dirction to the gravitational pull of the earth, acting like a brake, this braking effect is what is causing the heat.

If in a vacuum then unless the cube hits anything then there will B no change in velocity unless the cube is already accelerating.<p>B)<p>[ September 23, 2003: Message edited by: Mr Bwtz ]</p>
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haklesup
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Re: physics ?

Post by haklesup » Tue Sep 23, 2003 11:42 am

As stated your question cannot be completely answered. We need to know the properties of the chamber. <p>Form a Physics point of view (and yes I did look at a book) Heat is transfered by means of either Conduction, Convection or Radiation. Furthermore, heat always flows from high temp to low (thats a law)<p>If the chamber were like space (a dark vacuume) the ice would radiate heat away and the ice would remain frozen. Ignoring the fact that ice would sublimate in the vacuum of space anyway. If there were a heat source nearby like a sun or the walls of the chamber, they would radiate heat into the ice and then it would melt.<p>If the chamber were full of air, the air would need to be warmer than 0 C or the ice would remain frozen. Assuming a closed system (no external heat source and the walls of the chamber are a perfect insulator) the air would cool down and the ice would warm up until it melted. <p>If you know the initial mass and temperature of the ice and the volume and temperature of the chamber, you could calculate the final temp that the water and air would eventually settle out at.<p>The only way an ice cube can loose the heat necessary to melt is if there was a warm body to transfer heat from. Since nothing is touching the ice except the air, it goes that the heat would be transfered through a combination of radiation (IR) and convection (contact with the air). Turbulance would add heat to the surrounding air enhancing the process.<p>Chris<p>[edit] An ice cube will transfer heat less efficiently to its surroundings only because it has less surface area than vapor. If the ice melts into a ball of water and stays intact then the radiation rate would be similar to the ice cube because the surface area is similar (cube to sphere, same volume, might be a little different surface area)<p>An ice cube or anything for that matter will not continue to accelerate (negatively or positively) unless acted upon by another source of energy (another law).<p>[ September 23, 2003: Message edited by: haklesup ]</p>

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Re: physics ?

Post by hlreed » Tue Sep 23, 2003 12:12 pm

There is no difference in the mass of the ice cube and the resulting water. F = M*A still.
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Chris Smith
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Re: physics ?

Post by Chris Smith » Tue Sep 23, 2003 12:36 pm

In order to melt the ice cube, heat would be required in the chamber, to travel in the chamber vacuum and be absorbed into the ice cube.
[energy] <p>As that happened, the density change would increase in the ice, the molecules of water would melt, and vaporize, and if it were in deep space, [not a chamber per see] with no gravitational pull, nothing would slow down as far as forward motion. <p>The water would simple expand while moving forward at the same speed.<p> There, the water particles in theory would continue to travel at the same velocity, until some external force affected it path. <p>Light, which could be the heat that melted the cube, could impart enough force over time to slow down the forward motion, same as gravity. It may be small as far as energy, but it is still a force. <p>In a non vacuum chamber, the air or gasses in that chamber would automatically impart their "laws of physics" upon the water molecules and slow down the water molecules over time and distance, "equal to and opposite of" the same amount of kinetic energy and mass of the ice cube itself.

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Re: physics ?

Post by cato » Tue Sep 23, 2003 4:32 pm

toejam, I'm not sure, but because of the way you refer to density, I get the impression that you are confusing density and mass. The mass in your thought experiment stays the same and as someone else pointed out F=MA, mass is the important thing, not density. <p>Your experiment said the chamber is hot and has a vacuum. So, that means the walls of the chamber are hot, because there is nothing else to be hot.<p>As someone else pointed out, the ice would melt due to heating by radiation. Depending on the rate of heating, the water might stay as a ball of water and eventually splash into the wall at the far end of the chamber. If the heating were at a higher rate, the water might vaporize. In that case, the forces of diffusion might cause the water molecules to spread out in all directions from the ball of water. That is, some water molecules would slow down because the vapor pressure would push them away in the backward direction. Other water molecules would accelerate, because the vapor pressure would push them in the forward direction. Still others would be pushed up down and right and left. <p>The averaged velocity of all the water molecules would remain unchanged until they started hitting the far wall and bounching around your chamber.

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Re: physics ?

Post by dyarker » Wed Sep 24, 2003 4:51 am

No NET velocity change if traveling through a vacuum. Even vapour molecules will continue at the same speed as the ice cube they came from; plus or minus the little velocity they got going to gas state. Resulting in a wet ice cube with a cloud. The cloud would get larger due to random collisions and more molecules becoming vapour.<p>Increase the radiant heat behind, or in front of, the cube; and it speeds up or slows down. Some of the cloud becomes a tail. No magic, the energy to change velocity comes from the heat. A very low efficiency rocket.<p> :)
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Re: physics ?

Post by rosborne » Wed Sep 24, 2003 6:12 am

I think what cato says has the essence of the best answer. The frozen H2O does not have to hit air that is above 0C to melt. When the meteors hit the atmosphere miles above the surface, I'm pretty sure that the ambient temp. is below 0C. Don't forget that melting and boiling temps are dependent on air pressure and heat.
I think that when the frozen H2O hits the upper reaches of the atmosphere. The object is traveling too fast relative to the fluid air that the molecules, backed by some amount of pressure, have difficulty getting out of the way of the object. Imagine standing hip deep in a pool and slowly pushing your finger into water. Then compare that with winding up and slapping the water with your open hand. The pressure of the water allows a slow change in pressure as your finger displaces some water, but strongly resists your poor hand as you try to make the water pressure instantaneously change. I used to know the differntial equations for pressure that are analogous to F=ma, but don't recall which is too bad because it might blow obvious holes in my idea and save me from looking foolish.
Anyhow, when the meteor hits the atmosphere, I think the air molecules don't just carome off the meteor because they are affected by the pressure of the atmosphere and their own inertia, so the air molecules slide around the fast moving object causing a lot of friction in most cases enough friction is generated that the object is vaporized. Most of the energy is heat and is absorbed by the cool atmosphere, some is turned into light so we can go, "oh, pretty a shooting star."
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Re: physics ?

Post by dribach » Wed Sep 24, 2003 10:34 am

as several people here had mentioned and several people (in some cases the same ones!) had failed to keep in mind, if the ice cube is traveling in a vacuum, it will NEVER melt. liquid water needs pressure in order to exist. so all you have to worry about is the change from solid to gas. you might also look to create the hovercraft effect. if the cube is dropped into the chamber rather than shot, and a powerful energy source is coming from the bottom of the chamber, then as the cube nears the floor, the bottom of it will sublimate. that water vapor will create some pressure from under the cube and could create a cushion to keep the cube levitating until it disappears. that'd be really cool to watch with a high-speed camera. of course there's probably a physics doctor in here who will yell at me now for being a moron. but it looks good to me.

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Re: physics ?

Post by rosborne » Wed Sep 24, 2003 11:16 am

Thanks for the spanking dribach.
I thought the heart of the question was about energy transfer.
I think your description was interesting, although you are assuming that there is a down or gravity to hover in. If the energy source were great enough, and the mass large enough, and the gravity source weak enough I suppose the vapor could provide boost and shoot itself away or oscillate into a steady hovering state as you discribed.
I suppose if we assume vacuum and no gravity then we have a block of ice sublimating its way to immeasurability over the course of eons or to coin my own word billenia. In which case I suppose the energy in the "chamber" is the background radiation of the universe, slowly, but surely giving the ice enough energy to peel off some vapor at the molecular level until the mass of the object has disbursed itself across the "chamber" in a more or less even fashion with a statistical pattern of directionality based on the original velocity vector of the object. I dunno. Maybe.
-Rick

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Chris Smith
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Re: physics ?

Post by Chris Smith » Wed Sep 24, 2003 10:10 pm

"if the ice cube is traveling in a vacuum, it will NEVER melt...liquid water needs pressure in order to exist"?<p>Not entirely true, water has its own Gravitational attraction or pressure between molecules and it will melt into a thin layer of water before becoming a vapor. Water is capable of being in three states, at the same time in a vacume at 0 degrees C. A solid, a liquid, and a gas.

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Re: physics ?

Post by Dean Huster » Wed Sep 24, 2003 11:20 pm

I suspect that there are several situations where ice will sublime (turn directly from a solid to a vapor) whether in a gaseous invironment or a vacuum. In fact, I'd suppost that a water iceball entering our atmosphere at several thousand feet per second would do just that.<p>Even if the air were at or a little below the freezing point, I would expect air friction to melt ice also.<p>In the olden days, wet clothing put out on the line to dry during freezing weather would dry, although a bit slower than on a hot summer day, the clothing freezing and the resulting ice subliming into a vapor in time. Folks did it all the time ... not much choice with a large load of laundry out of that ringer washer.<p>Dean
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