State principle of conservation of mechanical energy. A block of mass $2 \mathrm{~kg}$ moving with speed $2 \mathrm{~m} / \mathrm{s}$ compresses a spring through a distance $20 \mathrm{~cm}$ before its speed is halved. The value of spring constant is $75 \mathrm{~N}$ then the value of $\mathrm{N}$ is ?