# "Extra" mass

Two springs (with indentical, invariant, Rest mass) are indentical except that one spring is compressed. Both springs are at rest. Doesn’t the compressed spring have more mass than the uncompressed spring? (With mass equivalent to the energy required to compress the spring?) Assuming the answer is yes, what is the “extra” mass of the compressed spring called?” --it cannot be kE, both springs are at rest.

Matterwave
Gold Member
The extra energy is called potential energy.

Thank you, but the question was what is the extra mass called. are you saying the "extra" mass has no other name than potential energy?

Nugatory
Mentor
Thank you, but the question was what is the extra mass called. are you saying the "extra" mass has no other name than potential energy?
Yes, although we often don't bother with the adjective "potential" and just call it "energy".
We'll often (but not quite as often) do the same thing with kinetic energy.

This is becaue in many problems the distinction between the different kinds of energy isn't especially interesting. For example, suppose I take one of your springs, attach a weight to one end, and start it oscillating (google for "simple harmonic oscillator" if what I say next doesn't make sense to you). When the object is at the extremes of its oscillation, there is a moment when it is at rest with kinetic energy zero; at that moment the spring is either maximally compressed or maximally extended and therefore its potential energy is at a maximum. Then as the object continues to oscillate it passes back through the midpoint of its oscillation; at that point its velocity and kinetic energy is at a maximum and the potential energy of the spring is zero. So we see that the total energy is constant, it just keeps swapping back and forth between kinetic and potential energy.

Now, suppose that the entire harmonic oscillator is inside of a black box, so we cannot see the oscillation (this is a very common and very realistic situation). The weight of the box remains constant because the total amount of energy is constant. Sure, at any moment some of that energy is kinetic and some of it is potential, but we on the outside of the box have no way of knowing what the split is; the total energy is the only that matters outside the box.

So in my original scenario, the uncompressed spring weights the same as the compressed spring?

Khashishi