The moment of inertia of a circular area about its diameter is Ixx. The moment of inertia of the same circular area about an axis perpendicular to the plane of the area is Izz. Which of the following statements is correct?

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SSC JE CE Previous Year Paper 21 (Held on 25 September 2019 Morning)
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  1. Ixx is always greater than Izz
  2. Ixx is equal to Izz
  3. Ixx is always less than Izz
  4. Ixx can be equal to or greater than Izz

Answer (Detailed Solution Below)

Option 3 : Ixx is always less than Izz
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Detailed Solution

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Explanation:

Moment of inertia of different section:

S.No.

Shape of cross-section

INA

Ymax

Z

1

Rectangle

\(I = \frac{{b{d^3}}}{{12}}\)

\({Y_{max}} = \frac{d}{2}\)

\(Z = \frac{{b{d^2}}}{6}\)

2

Circular

\(I = \frac{\pi }{{64}}{D^4}\)

\({Y_{max}} = \frac{d}{2}\)

\(Z = \frac{\pi }{{32}}{D^3}\)

3

Triangular

\(I = \frac{{B{h^3}}}{{36}}\)

\({Y_{max}} = \frac{{2h}}{3}\)

\(Z = \frac{{B{h^2}}}{{24}}\)

For circular cross-section,

\({{I}_{xx}}={{I}_{yy}}=\frac{\pi {{D}^{4}}}{64}\)

According to perpendicular axis theorem,

Izz = Ixx + Iyy

∴ Izz = 2 × Ixx

∴ Ixx is always less than Izz

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