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Correct answer: 1
- Interpret the figure and geometry
A bigger disc has radius . A small disc of diameter is removed, so its radius is .
From the standard shown configuration, the small disc is cut out internally and tangent to the outer boundary, with centers on the same line. Hence:
- center of big disc
- center of small removed disc
- distance between centers:
We need the moments of inertia of the remaining lamina about axes through and , perpendicular to the plane.
- Take areal mass density as
Since the discs are uniform,
-
Mass of big disc:
-
Mass of removed small disc:
- Moment of inertia about axis through
For the remaining lamina,
(a) Big disc about its own center
(b) Small disc about
Its own central MOI is
Using parallel axis theorem to shift from to :
Therefore,
- Moment of inertia about axis through
Similarly,
(a) Big disc about
First about :
Shift to parallel axis through :
(b) Small disc about its own center
Hence,
- Compute the ratio
The question asks for the ratio to the nearest integer: Actually, rounds to .
So the required nearest integer is
- Comparison with stored answer
Stored correct answer:
My derived answer is , so I do not agree with the stored answer.
The stored answer may correspond to a different interpretation, such as asking for or a different geometry. For the standard tangent-inside configuration with big radius and removed disc radius , we get: whose nearest integer is .
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