From the Hooke's law , the extension force of an elastic material is directly proportional to the extension. That is, F = k e, where F is the force , k is the constant and e is the extension F = 10 × 10 = 100 N e = 1mm or 0.001 m Hence, k = F/e = 100 N/ 0.001 = 100000 N/m or 100 N/mm