Seems to me that I read that a needle places on top of an ice cube will slice right thru the cube over time. (of course the needle starts off at zero degrees). I never have tried this. Has anyone else?
If it is true then would that explain why the P38s in greenland are now 280 feet below the ice that they sat on in 1941?
The pressure causes heat, which in turn melts the ice cube. If you want to try it, take an ice cube and lay a thin wire over it. Then push down on the wire and you will see it will start to go through it. I was watching some TV show and found that for this reason, building designers of high rises have to be careful, becuase too much weight on the foundation will cause it to melt.
joey is bang on. An increase in pressure will cause a corresponding increase in heat. You can see this if you squeeze a penny in vise-grip pliers for a few minutes. The penny will become noticeably warmer.
The needle, being metal, is denser than the ice so gravity will pull it down with more force. Since all the needle’s weight is concentrated on its fine point, that point on the ice cube’s surface will feel the pressure and slowly melt. If you laid a small flat square of metal with the same mass as the needle over the ice cube, the gravitational pull would be spread out over this larger area and might not be enough to force melting at any one spot (note; I’m assuming the overall temperature of the system is zero degrees or lower, or the ice would be melting on its own; needle or not).
The beauty of this is that is you try to push the needle through the ice, you’ll either bend or break the needle or shatter the ice. It’s only the slow melting that allows the needle to glide through.
As a side note, do you know how to drive a simple plastic straw completely through a potato?
No. Six fighters and two bombers landed on Greenland’s ice cap in bad weather on July 15, 1942. Glaciers are constantly in motion. That combined with decades of compacting snowfall buried the aircraft under 270 of snow and ice. In 1992 Roy Shoffner recovered one of the P-38s, nicknamed “Glacier Girl”. After years of restoration, it’s engines were finally tested on Sep 17, 2001 in Middlesboro, Kentucky where it is currently stored.
Actually, my understanding of the unique relationship between pressure and melting ice was that pressure lowered the melting point of ice, not that the warming was responsible. This is similar to ice skating where the pressure from the skates caused the phase change, not heat from friction.
But actually in this case I don’t think there is enough pressure for phase change or heat. At (or near) 0 C, the bonds between the water molecules are constantly breaking and re-forming randomly. Any bonds that happen to break below the needle would allow the needle to sink microscopically (due to gravity), but when re-formed would never push the needle back up. So the needle gradually sinks, even without any heat being generated. The ice can also begin re-forming across the top of the needle as it drops below the top of the ice cube.
Set them out during a tornado?
eh, no, not really. I can’t draw a phase change curve on this board, but I’ll explain it the best I can. There will be no increase in temperature, both the needle and the ice will stay at 0 C. What happens is that the melting point of the ice, being a function of pressure, is lowered. The ice and needle stay at 0 C, but the melting point of the ice directly under the needle drops to around -0.5 C, so the ice melts. (actually, directly under the needle the temp of the ice will drop to -0.5, but it will retain the thermal energy it had and melt. Phase changes take place at a single temp, but that melting/boiling/sublimation point is dependant on pressure).
Trivia fact: this is also how ice skates work. The blade of the skate is thin to maximize pressure per square inch. The ice melts directly under the skate and the skater hydroplanes across the ice.
Ayup, shore is. Needles are purty dern sharp-so the pressure (force divided by area) when force is applied through the tip is hu-moungus!
Gravitational attraction is based on density? I thought it was mass.
I agree with flex. You said that the needle starts at 0 deg. Are you talking about keeping the ice in a cold atmosphere? If not, another factor in the needle’s movement would be its ability to absorb heat from the atmosphere and transmit along the needle, thereby melting the ice there faster than atmosphere alone.
Ladies and Gentleman,
I am sorry to spoil this overall agreement on that the
needle presses the ice so hard that it melts below it.
Although seemingly most logical, this is not correct.
Check out the following site:
http://www.princeton.edu/~lehmann/BadChemistry.html#Skating
H
We’re talking about a needle lying on its side aren’t we?
I will return tonight with a cite from a hard copy book on physics that debunks this popular view. It says that there is a microthin layer of water on the ice to begin with and the pressure of the blade has no discernible effect on melting. I’ll provide a cite and a quote.
Oh, but it probably says the same thing as B]Hirsel’s** post. Sorry for the late hit. I’ll check anyway.
I’m not buying it. This article also says that heat from friction plays a role in ice skating. I can’t fathom how someone can say that “frictionless” ice skating results from so much friction that it generates enough heat to melt ice. Seems awfully contradictory to me.
Basically the article says “we don’t know how ice skating works, but it’s not due to phase change from pressure”. Well, Mr. Lehman, give me a call when you do know. Also, that surface phenomenon he talks about seems suspect. While what he says is correct about surface molecules having dangling bonds wanting to attach to something, this would tend to make the surface layer less likely to melt because, as he says, they “increase their bonding to each other and those below”.