XPost: sci.physics.relativity
From: PointedEars@web.de
Chris M. Thomasson wrote to sci.physics.relativity and sci.physics:
> Its odd to think of a cup of water as the water evaporates. The cup is
> an object with its own mass. But, now its holding water. So, the cup
> "weighs" more in a sense...
The mass of the cup only has increased because when we say "cup" now we mean
the substance of the cup *and* the water.
> But, as the water evaporates, that weight will go back to the weight of
> the original cup...
That is only true if the water actually leaves the cup. Put a lid on the
cup so that the water cannot escape, and the mass of the water-filled cup
will not decrease.
But, more relevant to this discussion, by increasing the rest energy of the
water(-filled cup) by increasing the kinetic energy of the water molecules
(and to some extent the substance of the cup, too) by heating the
water(-filled cup), the mass of the water(-filled cup) increases slightly
(probably unmeasurably, given m = E_0/c^2).
See also the videos that I referred to in
.
> Think of two equal mass cups on a scale. They balance. Add water to one,
> its not balanced. However, the water will evaporate and the scale shall
> go back to balanced over time?
If the water *leaves* the cup, yes.
Notice also that what you are measuring there is weight, not mass. That is,
this is only an indirect determination of mass that depends on gravitation,
therefore the assumption of a uniform gravitational field (which we know is
not so, but just a relatively good approximation near the surface).
F'up2 sci.physics.relativity
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PointedEars
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