A boy's catapult is made of rubber cord which is long, with diameter of cross-section and of negligible mass. The boy keeps a stone weighing on it and stretches the cord by by applying a constant force. When released, the stone flies off with a velocity of . Neglect the change in the area of cross-section of the cord while stretched. The young's modulus of rubber is closest to :
So apply energy conservation.
P.E. stored in string = K.E of stone