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Doc Al

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Yes, you'll need the perpendicular axis theorem. To make use of the hint, try to express the moment of inertia about a diagonal in terms of Ix and Iy. (Consider how you'd have to rotate the x and y axes to align with the diagonal. It's a bit tricky.)fterh said:A couple more questions:

A thin rectangular plate of lengtha, widthband mass M has a moment of inertia [itex]I = \frac{1}{12}M(a^2 + b^2)[/itex] about an axis through its center and perpendicular to its plane. What is the moment of inertia of the plate about an axis in the plane of the plate and forming a diagonal of the rectangle? [Hint: [itex]I_x = \frac{1}{12}Ma^2[/itex] and [itex]I_y = \frac{1}{12}Mb^2[/itex]]

Do I use perpendicular axis theorem then apply sort of like pythagoras theorem?