My textbook says that a compressed spring has more mass than when it is not compressed. I'm assuming that this is because of E = mc2. A compressed spring has more energy and hence more mass. still, i have a real tough time wrapping my head around that. the spring has the same number of atoms. where does the extra mass come from? I put his in classical physics because it concerns springs which were described by Hooke in the 17th century, although Einstein is mentioned i think this belongs more to classical than modern physics.