An $8.0\ \text{kg}$ lamp is hung in a stairwell on a copper cable $4.0\ \text{m}$ long, whose cross-section is $2.0\ \text{mm}^2$. How far does the lamp sink as the cable takes its weight? ($Y = 1.1 \times 10^{11}\ \text{Pa}$, $g = 9.8\ \text{m/s}^2$.)