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 $x=5$ cm is 0.25 J . The spring constant of the spring is $\_\_\_\_$ $\mathrm{Nm}^{-1}$.
