Two balls of same material of density `rho` but radius `r_(1)` and `r_(2)` are joined by a light inextensible vertical thread and released from a large height in a medium of coefficient of viscosity `= eta`. Find the terminal velocity acquired by the balls. Also find the tension in the string connecting both the balls when both of them are moving with terminal velocity. Neglect buoyancy and change in acceleration due to gravity.