Two tennis balls of mass 60gm are attached with a massless rubber thread, and held in the vertical position as shown in the figure. In this position length of the rubber thread is 40cm and it is unstretched. The upper ball is showly raised vertically upward, until the lower ball just becomes unsupported by the ground. At this time the length of the thread is 1m. The rubber thread exerts a force which is proportional to its extension.