1. The problem statement, all variables and given/known data Two particles, each of mass m, are attached one to each end of a diameter PQ of a uniform circular disk, of mass 4m, radius a with its centre at O. The system is free to rotate about a horizontal axis through A, a point on PQ such that OA = b as indicated in the diagram below. The system is released from rest when PQ is horizontal. Determine the Moment of Inertia of the system about the axis A, in terms of integer constants, a , b and m. 2. Relevant equations I=ma^2 parallel axis equation I=Icentre of mass + md^2 3. The attempt at a solution I have determined the moment of inertia of the disc to be 0.5ma^2. I have determined the moment of inertia of Q to be m(b+a)^2. I have determined the moment of inertia of P to be m(a-b)^2. Therefore I should just add these three numbers together to make a total moment of inertia, but this answer is marked wrong (it is marked by a computer programme). Where am I going wrong in my method? Thanks.