College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 31

A small sphere of charge 0.800 C hangs from the end of a spring as in Figure P15.7a. When another small sphere of charge 20.600 C is held beneath the first sphere as in Figure P15.7b, the spring stretches by d = 3.50 cm from its original length and reaches a new equilibrium position with a separation between the charges of r = 5.00 cm. What is the force constant of the spring