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