Using Newtonian mechanics the vibrations of a molecule can be calculated by treating the bonds like springs. This is useful because like a spring, a bond requires energy to stretch it out and it also takes energy to squeeze it together. The energy needed to stretch or squeeze the bond is dependent on the stiffness of the bond, which is represented by the "spring constant" k, and the reduced mass, or "center of mass" of the two atoms attached to either end denoted by "μ". The formula used to relate the energy needed to cause a vibration in the bond is: