When an elastic spring is given a displacement of 10mm, it gains an potential energy equal to U. If this spring is given an additional displacement of 10 mm, then its potential energy will be
$ u \alpha y^2 $ $ \therefore { u_2 \over u_1} = \left( { y_2 \over y_1} \right)^2 \Rightarrow u_2 = 4u $