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Ask HN: does a compressed spring weigh more than the same spring uncompressed?

3 points by pka ↗ HN
A comment on the recent Higgs Boson thread [1] stated that a "a compressed spring weighs more than the same spring uncompressed". Since I'm a complete physics noob, this intrigued me. A google search unfortunately didn't help much [2].

So, is this really true?

[1] http://news.ycombinator.com/item?id=4193590

[2] http://www.physicsforums.com/showthread.php?t=161069

2 comments

[ 6.6 ms ] story [ 22.9 ms ] thread
billswift
Since the compressed spring has stored energy the answer would be "Yes, but so little as to be completely unmeasurable." E=mc^2 applies to all energy and mass, but if you work the equation with sample energies, you will quickly discover that a substantial amount of energy becomes very, very little mass.
kahirsch
Just to add a little weight in numbers, billswift is right.

Also, if you heat something up, it will weigh more.

Even though the change in mass is very small, it can sometimes be measured. When an electron and a proton combine to form a hydrogen atom, energy is given off, so the mass of a hydrogen atom be less than the sum of the mass of the proton and electron. Physicists can now measure the masses of these three things precisely enough that they can confirm that this is true.