$${t_1 = \frac{L_1}{v_1} = L_1 \sqrt{\frac{\mu_1}{\tau}} = (3,0\, \mathrm{m}) \sqrt{\frac{1,4 \times 10^{-4}\, \mathrm{kg/m}}{400\, \mathrm{N}}} \\ = 1,77 \times 10^{-3}\, \mathrm{s}}$$