A particle moving along X-axis has acceleration 'f ' at time t, given by $\mathrm{f}={\mathrm{f}}_{\mathrm{o}}\left(1-\frac{\mathrm{t}}{\mathrm{T}}\right)$ where f_o and T are constants. The particle at t = 0 has zero velocity. In the time interval between t = 0 and the instant when f = 0, the particle's velocity (v_x) is: