A 5 m long aluminium wire (Y = $7\times {10}^{10} \mathrm{N}/{\mathrm{m}}^2$ ) of diameter 3 mm supports a 40 kg mass. In order to have the same elongation in a copper wire ($\mathrm{Y} = 12\times {10}^{10} \mathrm{N}/{\mathrm{m}}^2$) of the same length under the same weight, the diameter should now be, in mm.