Mechanics dynamics - example 5

Thread is fixed to the ceiling. Thread has a roller shape. Roller radius is r[m]. Mass m[kg] of thread is known. Thread is unrolling to down direction vertically. In example linear acceleration a[m/s2] has to be calculated. Thread is a roller with twine which was coiled on it. Mass of twine is so small so it is possible to assume it as equal to zero. Thread is a rigid body.