When a ball is released form rest in a very long column viscous liquid, its downward acceleration is a (just after release).  Find its acceleration when it has acquired two third of the maximum velocity

At the beginning, net force downwards F= ma ; When terminal velocity is achieved F=6πηrv     when velocity is 23 of its terminal velocity,viscous force is 23 6πηrv =23Fnet force =F-23F=F3  so acceleration will be a3