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 J . The spring constant of the spring is $\_\_\_\_$ $\mathrm { N } \mathrm { m } ^ { - 1 }$.