Main Question or Discussion Point
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.