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Spaceflea
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Homework Statement
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.[/B]
Determine the Moment of Inertia of the system about the axis A, in terms of integer constants, a , b and m.
Homework Equations
I=ma^2
parallel axis equation I=Icentre of mass + md^2
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.