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I'm a bit confused about assumption that's made in "Concept in Thermal Physics ; Stephen J. Blundell and Katherine M. Blundell" page 205.

It is stated that

" Consider a cubic solid in which each atom is connected by springs (chemical bonds) to six neighbours (one above, one below, one in front, one behind, one to the right, one to the left).

**Since each spring joins two atoms, then if there are N atoms in the solid, there are 3N springs (neglecting the surface of the solid, a reasonable approximation if N is large).**

**Each spring has two quadratic modes of energy (one kinetic, one potential) and hence a mean thermal energy equal to 2 x 1/2 kT = kT.**Hence the mean energy of the solid is

<E> = 3N k T

"

How can it says that the spring has two modes of energy? It is only potential, isn't it? How can the kinetic one arises since spring is actually virtual connector which has no mass?

Also, I'm confused about comparison the number of springs and particles in the system.

Thanks a lot for your helps :):)