Q. The block of mass M descend with acceleration a and mass of string is m. find tension in string?
Ans: The tension in the string will not be same at all points as the string has some mass. please specify the point where the tension is required.
For the tension at the centre of the string,
Mg +mg/2 - T = (M + m/2 )a
T = (M + m/2 ) (g - a)