Let, us consider a small element of the thread and draw free body diagram for this element.
(a) Applying Newton's second law of motion in projection form, Fn=mwn for this element,
(b) When m2/m1= η, which is greater than η0 the blocks will move with same value of acceleration. (say w) and clearly m2 moves downwards. From Newton's second law in projection from(downward for m2 and upward for m1) we get: