# 2.15: Solid Body

The moments of inertia of a collection of point masses distributed in three-dimensional space (or of a solid three-dimensional body, which, after all, is a collection of point masses (atoms)) with respect to axes O\(xyz\) are:

\( A = \sum m (y^2+z^2) \quad F = \sum myz \)

\( B = \sum m (z^2+x^2) \quad G = \sum mzx \)

\( C = \sum m (x^2+y^2) \quad H = \sum mxy \)

Suppose that \( A, B, C, F, G, H, \) are the moments and products of inertia with respect to axes whose origin is at the centre of mass. The *parallel axes theorems *(which the reader should prove) are as follows: Let P be some point not at the centre of mass, such that the coordinates of the centre of mass with respect to axes parallel to the axes O\(xyz \) but with origin at P are \( ( \overline{x} , \overline{y} , \overline{z} )\) .

Then the moments and products of inertia with respect to the axes through P are

\( A + M (\overline{y}^{2}+ \overline{z}^{2}) \qquad F + M \overline{yz} \)

\( B+ M (\overline{z}^{2}+ \overline{x}^{2}) \qquad G + M \overline{zx} \)

\( C + M (\overline{x}^{2}+ \overline{y}^{2}) \qquad H + M \overline{yx} \)

where \( M \) is the total mass.

Unless stated otherwise, in what follows we shall suppose that the moments and products of inertia under discussion are referred to a set of axes with the centre of mass as origin.