Science > Physics > Rotational Motion Moment of Inertia of Standard Bodies

In this short article, we shall examine the strategy of deriving an expression for moment of inertia of a body.

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Consider athin unicreate rod, of mass M, a space of cross-area A, length l, andthickness of material ρ. Let us consider an infinitesimal aspect of length dx ata distance of x from the provided axis of rotation. Then its Moment ofInertia around the given axis is provided by

dI = x² . dm, …… (1)


But, Mass = Volume x Density

∴ dm = A . dx . ρ ……(2)

From Equations (1) and (2)

dI = x² A . dx .ρ

∴ dI = A .ρ . x² . dx

The moment of inertia I of the rod about. the given axis isprovided by


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This is an expression for moment of inertia of a thin unidevelop rod around a transverse axis passing via its end.

Expression for the Moment of Inertia of an Annular Ring:

Consider a unidevelop thin annular disc of mass M having inner radius R1, outer radius R2, thickness t, and density of its product ρ. Let us assume that disc is qualified of rotating about a transverse axis passing via its centre. Let us assume that the disc is comprised of infinitesimally thin rings.


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Consider onesuch ring of radius r and also width dr. Moment of Inertia of such facet is provided, by,

dI = r² . dm ………….. (1)

But, Mass= volume × density

dm = ( 2 π r . dr . t) ρ ………(2)


From Equation (1) and also (2)

dI = r² . ( 2π r . dr . t) ρ

dI = 2 π t ρ r³ . dr .

The minute of inertia I of the annular disc will certainly be provided by


Where M is the full mass of the annular ring.

This is an expression for moment of inertia of annular ring abouta transverse axis passing via its centre.

Expression for Moment of Inertiaof a Thin Uniform DiscAbout a Transverse Axis Passing Thturbulent its Centre and Perpendicular to itsPlane:

The momentof inertia of annular ring about a transverse axis passing through its centreis offered by


This is an expression for moment of inertia of thin uniformdisc about a transverse axis passing with its centre.

Expression for Moment of Inertiaof a Thin Uniform RingAbout an Axis Passing through its Centre and also Perpendicular to its Plane:

The momentof inertia of annular ring around a transverse axis passing through its centreis offered by


This is an expression for minute of inertia of thin uniformring about a transverse axis passing via its centre.


Consider asolid cylinder of mass M, size ‘’ and radius ‘r’ capable of rotating aboutits geometrical axis. Let ‘m be its mass per unit length.

m = M/l Hence M = m .l

A solidcylinder have the right to be concerned as a number of thin unicreate discs of infinitesimalthickness piled on optimal of one another. Let us consider one such disc ofthickness ‘dx’ at a distance of ‘x’ from the centre C of the cylinder.

Mass of such disc is provided by

Mass, dm = m.dx = (M /l). dx

The M.I. of such disc around a transverse axis (passingthrough C) is offered by


Integrating the over expression in limits


This is an expression for M. I. of a solid cylinder about its geometrical axis.

Expression for Moment of Inertiaof a Hollow CylinderAbout its Geometrical Axis:


Consider ahollow cylinder of mass M, size ‘’ and also radius ‘r’ qualified of rotating aboutits geometrical axis. Let ‘m be its mass per unit length.

m = M/l Hence M = m . l

A hollowcylinder have the right to be concerned as a number of thin unidevelop rings of infinitesimalthickness piled on peak of one another. Let us think about one such ring ofthickness ‘dx’ at a distance of ‘x’ from the centre C of the cylinder.


Mass of such ring is offered by

Mass, dm = m.dx = (M/ l) dx

The M.I. of such ring around a transverse axis (passingvia C) is offered by


This is an expression for M. I. of a solid cylinder around its geometrical axis.

Expression for Moment of Inertiaof a Solid SphereAbout its Diameter (Geometrical axis):


Let usconsider a solid homogeneous sphere of radius ‘R’ and mass ‘M’, capable ofrotating about its diameter. Let us take into consideration a circular strip of infinitesimalthickness ‘dx’ at a distance of x from centre ‘O’. The radius of this circularspilgrimage is PM, which is given by


This circular sexpedition can be treated as thin disc rotatingaround a transverse axis passing with its centre.

The M.I. of the disc around a transverse axis passing throughits centre is provided by


The M.I. of the whole sphere about diameter deserve to be acquired by integrating the above expression.

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The mass of the sphere = M. Hence, the M.I. of the solidhomogeneous sphere is given by


This is an expression for M.I. of a solid sphere about its diameter (Geometrical axis).

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One reply on “Moment of Inertia of Standard Bodies”


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Rajesh Patanisays:
August 12, 2021 at 11:19 am

exceptionally useful


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