I have posted 2 problems, 12.126 and 12.72. I have read in the book that you can assume the acceleration remains constant when the forces applied are constant. In problem 12.126 that is the case, the weights of the blocks and the forces P and T remain constant so I assumed that the acceleration of the blocks were also constant and I solved the problem successfully. But in problem 12.72, I obviously cannot assume the acceleration is constant, why not? I am neglecting friction and the weights of the blocks are contant, the tension in the cord is also constant, isnt it? What is the difference between these problems?