In my book (Halliday Resnick Walker) there is a solved example which is as follows
There is a uniform disk with mass $M$ = 2.5kg and radius $R$ = 20cm, mounted on a fixed horizontal axle. A block with mass $m$ = 1.2kg hangs from a massless cord that is wrapping around the rim of the disk. Find the acceleration of the falling block, the angular acceleration of the disk, and the tension in the cord. The cord does not slip and there is no friction at the axle.
We need to find the acceleration of the block. So the equation to relate the forces on block to acceleration of body is given in the solution as $T - mg = ma$
but I think it should be $mg - T = ma$ as the block is moving in downward direction $mg$ should be greater than $T$ which is tension. Why is it given on the book as $T - mg = ma$?
Also can anyone help me how to find the tension forces acting on a string in complex system while making their free body diagram?