One mole of nickel (6E23 atoms) has a mass of 59 grams, and its density is 8.9 grams per cubic centimeter. You have a long thin bar of nickel, 3.0 m long, with a square cross section, 0.15 cm on a side.
You hang the rod vertically and attach a 231 kg mass to the bottom, and you observe that the bar becomes 1.41 cm longer. Calculate the effective stiffness of the interatomic bond, modeled as a "spring":