2014-10-02T20:01:02+05:30

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Basically moment of inertia determines how much torque is needed to achieve a certain angular acceleration. It is analogous to mass in case of linear motion.
An object has a certain mass, but it can have different moments of inertia depending on the axis of rotation.
2014-10-04T02:41:26+05:30

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Moment of inertia of an object is defined with respect to an axis.

Suppose we imagine the object to consist of many small elements of mass δm with the center of mass at a distance r from the axis.

Moment of inertia is the sum aggregate of products =   Σ δm * r²

In constructing bridges, for example, the shape or structure having maximum moment of inertia / volume is chosen for stability reasons.  For shapes in a constantly moving components (like a wheel), we choose shape with least moment of inertia.