The potential energy of a long spring when stretched by 2 cm is U. If the spring is stretched by 8 cm the potential energy stored in it is:

Let extension produced in a spring be x initially. In stretched condition spring will have potential energyU = 12kx2Where k is spring constant or force constant∴  U1U2  = x21x22-----------------(i)Given, U1 = U, x1 = 2 cm, x2 = 8 cmPutting these values in Eq.(i), we haveUU2 = (2)2(8)2 = 464 = 116∴  U2 = 16 U