When a mass of 40 g is attached to a vertically hanging spring it extends by 0.4 cm. Find : (See lesson 13) (i) Force constant of the spring. (ii) The extension when 100 g weight is attached to it. (iii) The time period of oscillation of 100 g weight on it. (iv) The time period and force constant if the spring is cut in three equal parts and 100 g weight is made to oscillate on one part.