Two rain drops of same radius coalesce. Before doing so, each was moving with terminal velocity v. What is the terminal velocity of the single drop so formed ?

The radius R of the single drop so formed will beR=(2)1/3r where r= radius of each drop.  For each drop, mg=6πηrV For combined droP 2mg=6πηRv′ From eqs. (i) and (ii), we get 2×6πηrv=6πηRv′ V′=2rvR Now V′=2rv(2)1/3r=(2)2/3v