Today we will see here the method to determine the mass moment of inertia for the rectangular section about its base line with the help of this post.

### Let us first determine here the mass moment of inertia for the rectangular section about a line passing through the base of the given rectangular section

Let us consider one rectangular section ABCD as displayed in following figure. Let us assume that one line is passing through the base of the rectangular section and let us consider this line as line CD and we will determine the mass moment of inertia for the rectangular section about this line CD.
B = Width of the rectangular section ABCD
D = Depth of the rectangular section ABCD
T= Uniform thickness of the rectangular section ABCD

V= Volume of the rectangular section ABCD
V = B x T x D
M= Mass of the rectangular section ABCD
M = ρBDT

(Im) CD = Mass moment of inertia of the rectangular section about its base line i.e. CD

Now we will determine the value or expression for the mass moment of inertia of the rectangular section about a line passing through the base of the rectangular section

Let us consider one rectangular elementary strip with depth dY and at a distance Y from the base line CD as displayed in above figure.

Area of rectangular elementary strip, dA = dY.B
Mass of the rectangular elementary strip, dm = ρ x T x dA
Mass of the rectangular elementary strip, dm = ρ x T x dY. B
Mass of the rectangular elementary strip, dm = ρBT. dY

Mass moment of inertia of the elementary strip about the base line = dm.Y2
Mass moment of inertia of the elementary strip about the base line = ρBT. Y2dY

Now we will determine the mass moment of inertia of entire rectangular section about its base line i.e. CD. And it could be easily done by integrating the above equation between limit 0 to D.

Therefore, mass moment of inertia of the rectangular section about the line CD will be determined as displayed here in following figure.
(Im) CD = ρBT.D3/3
(Im) CD = ρBTD.D2/3
(Im) CD = ρV.D2/3

### (Im) CD = M.D2/3

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### Reference:

Strength of material, By R. K. Bansal