Q. A block is fastened to a horizontal spring. The block is pulled to a distance $x = 10\ \mathrm{cm}$ from its equilibrium position (at $x=0$) on a frictionless surface from rest. The energy of the block at $x=5\ \mathrm{cm}$ is $0.25\ \mathrm{J}$. The spring constant of the spring is ______ $\mathrm{N\ m^{-1}}$.