Spherical particles of a given material of density p are released from rest inside a liquid medium of lower density. The viscous drag force may be approximated by the Stoke’s law, i.e, F_d = 6πηRv , where η is the viscosity of the medium, R the radius of a particle and v its instantaneous velocity. If τ(m) is the time taken by a particle of mass m to reach half its terminal velocity, then the ratio τ(8m)/τ(m) is