Whenever energy is added to a system, the system gains mass

[dav2008]

"Whenever energy is added to a system, the system gains mass"

http://en.wikipedia.org/wiki/Mass-energy_equivalence#Practical_examples
From wikipedia:

Whenever energy is added to a system, the system gains mass. A spring's mass increases whenever it is put into compression or tension. Its added mass arises from the added potential energy stored within it, which is bound in the stretched chemical (electron) bonds linking the atoms within the spring. Raising the temperature of an object (increasing its heat energy) increases its mass. If the temperature of the platinum/iridium "international prototype" of the kilogram — the world’s primary mass standard — is allowed to change by 1°C, its mass will change by 1.5 picograms (1 pg = 1 × 10–12 g).[8]

Is this actually true? If someone said this to me in a conversation I would tell them compressing the spring does absolutely nothing to its mass and increasing the temperature of an object does nothing to its mass.

Now it's on Wikipedia so I'm wondering if I'm missing some concept here...

I was going to remove it but I wanted confirmation first.

 

[Nabeshin]

The temperature example sounds plausible, but I don't know about the compressed spring. By that same token of logic, if PE increases mass, lifting a box will increase the mass?

Temperature I can get because of the motion of the particles etc. but nothing fundamental changes about a box when I lift it 1m, giving it gravitational potential energy.

 

[jtbell]

if PE increases mass, lifting a box will increase the mass?

I gave an answer to this question in the following thread just a few minutes ago:

https://www.physicsforums.com/showthread.php?t=287666

 

[cesiumfrog]

dav, did you bother to read the reference ("[8]") for the paragraph you wish to delete (from the wikipedia page on a topic that you don't understand)?

Yes, compressing a spring or heating a potato increase their mass. (Hint: in one sense we can say that energy is mass, and nobody disagrees with this when we are referring to a system that is at rest.)

Lifting a box higher above the Earth can increase the mass of a system encompassing both the Earth and the box, but not of the box itself (as Nabeshin and jtbell noted).