A hanging copper wire with diameter 1.4mm (1.4e-3m) is initially 0.95 m long. When a 36 kg mass is hung from it the wire stretches an amount 1.83 mm, and a 72 kg mass is hung from it the wire stretches an amount 3.66mm. A mole of copper has a mass of 63 grams/cm^3. Find the approximate value of the effective spring stiffness of the inter atomic force.