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Author Topic: Does relativity make ice weigh less?  (Read 1963 times)

Offline chris

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Does relativity make ice weigh less?
« on: 12/08/2011 15:19:26 »
Since E=mc^2, does ice weigh a bit less than the equivalent volume of water just because it is colder?


 

Offline graham.d

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Does relativity make ice weigh less?
« Reply #1 on: 12/08/2011 15:48:01 »
If by "equivalent volume" you mean the volume of water that would be produced if the ice was subsequently melted and heated to the same temperature, then yes; the ice will weigh a tiny, tiny amount less. However, if you are looking at the volume of ice below 0 degrees (down to some very low temperature) compared with the same volume of water at some higher temperature, then yes also; the ice will be of lower density and have less mass so will have considerably less weight.

However, I suspect some deviousness in the question here which means I'm not so confident :-)
« Last Edit: 12/08/2011 15:53:42 by graham.d »
 

Offline lightarrow

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Does relativity make ice weigh less?
« Reply #2 on: 12/08/2011 18:46:32 »
A better question would be formulated as:
"does a system of N water molecules in the solid state weigh less than the corresponding system of N water molecules in the liquid state?"

The answer is: it depends on the temperature  :)
In fact the ice can be at 0C and the water at -1C, in which case the ice has more mass than the water  ;)

So, an even better question is:
"does a system of N water molecules in the solid state at temperatures less than 0C weigh less than the corresponding system of N water molecules in the liquid state at temperatures greater than 0C?"

Answer: yes.


Now a related question, of which I don't want to give immediately the answer:
You are on a hot-air balloon and you have a great amount of ice at 0C inside the cabin. Then you heat and melt it with an electric heater you have there. Assuming the balloon sensitive enough to the mass variation of the water, will this mean the balloon goes down?
« Last Edit: 12/08/2011 19:00:35 by lightarrow »
 

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Does relativity make ice weigh less?
« Reply #2 on: 12/08/2011 18:46:32 »

 

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