Moment of inertia is an important topic and also is asked in most of the physics problems involving fixed in rotational motion. Normally, MOI is supplied to calculation angular momentum. We shall learn about this topic much more in the adhering to paragraphs.

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What is minute of Inertia?

Moment that inertia is characterized as the quantity expressed by the human body resisting angular acceleration which is the amount of the product the the mass of every fragment with that is square the a distance from the axis that rotation. Or in an ext simple terms, it can be described as a amount that decides the amount of torque essential for a details angular acceleration in a rotational axis. Moment of Inertia is likewise known together the angular fixed or rotational inertia. The SI unit of minute of inertia is kg m2.

Moment that inertia is usually specified with respect come a liked axis the rotation. It mainly depends on the circulation of mass around an axis that rotation. MOI varies depending on the axis the is chosen.

Table that Content

Moment that Inertia Formula

In General type Moment that Inertia is expressed as I = m × r2 


m = amount of the product the the mass.

r = street from the axis of the rotation.

and, Integral form: I = ∫dI = ∫0M r2 dm

⇒ The dimensional formula that the minute of inertia is given by, M1 L2 T0.

The function of the moment of inertia is the exact same as the role of massive in straight motion. The is the measure up of the resistance that a body to a adjust in its rotational motion. That is constant for a specific rigid frame and a particular axis the rotation.

Moment of inertia, I = ∑mi ri2. . . . . . . (1)

Kinetic Energy, K = ½ ns ω2 . . . . . . . . . (2)

What room the factors on which minute of Inertia Depends?

The moment of inertia depends on the adhering to factors,

The density of the materialShape and also size the the bodyAxis that rotation (distribution that mass family member to the axis)

We can further categorize rotating body solution as follows:

Discrete (System that particles)Continuous (Rigid body)

Also Read:

Moment of Inertia the a system of Particles

The moment of inertia the a device of particles is provided by,

I = ∑ mi ri2 where ri is the perpendicular street from the axis come the ith bit which has actually mass mi.




Moment of Inertia of strict Bodies


The moment of inertia of constant mass distribution is found by using the integration technique. If the mechanism is divided into one infinitesimal element of massive ‘dm’ and if ‘x’ is the distance from the mass element to the axis the rotation, the moment of inertia is:

I = ∫ r2 dm . . . . . . (3)

Calculation of minute of Inertia

A step-by-step guide for calculating the minute of inertia is provided below:

Moment the Inertia the a Uniform Rod about a Perpendicular Bisector


Consider a uniform pole of massive M and also length L and also the minute of inertia have to be calculated about the bisector AB. Beginning is at 0.

The mass facet ‘dm’ taken into consideration is in between x and x + dx native the origin.

As the rod is uniform, mass per unit length (linear mass density) remains constant.

∴ M/L = dm/dx

dm = (M/L)dx

Moment that inertia of dm ,

dI = dm x2

dI = (M/L) x2.dx

I =-L/2 ∫+L/2 dI = M/L × -L/2∫+L/2 x2 dx

Here, x = -L/2 is the left finish of the rod and also ‘x’ alters from –L/2 to +L/2, the facet covers the entire rod.

I = M/L × +L/2-L/2

I = ML2/12.

Therefore, the moment of inertia of a uniform rod about a perpendicular bisector (I) = ML2/12.

Moment that Inertia that a circular Ring about its Axis


Consider the heat perpendicular to the aircraft of the ring through its centre. The radius of the ring is taken as R and also its mass as M. Every the aspects are in ~ the exact same distance native the axis that rotation, R.

Linear mass thickness is constant.

∴ M/2π = dm/dθ

dm = M/2π × dθ

I = ∫ R2 dm = R2 0∫2π

Limits: θ = 0 to 2π contains the totality mass of the ring

∴ i = R2  × <θ>2π0

Therefore, the moment of inertia of a circular ring about its axis (I) = MR2.

⇒ keep in mind that in one-dimensional bodies, if it’s uniform, their linear mass density (M/L) continues to be constant. Similarly, because that 2D and 3D, M/A (surface density) and M/V (volume density) remain constant respectively.

⇒ recognize the relation between torque and also moment of inertia here

Moment of Inertia that a rectangle-shaped Plate about a line Parallel to an Edge and also Passing v the Centre


The mass aspect can be taken between x and also x + dx from the axis AB.

As the bowl is uniform, M/A is constant.

M/A = dm/da

M/ = dm/dx.b

dm = (M/lb) × b × dx = (M/l) dx

I = ∫ x2 dm = (M/l) × -l/2∫+l/2  dx

Limits: the left finish of the rectangle-shaped plate is in ~ x = -l/2 and the totality plate is spanned by acquisition x from x = -l/2 come x = +l/2.

I = (M/l) -l/2+l/2 = Ml2/12

Therefore, the minute of inertia of a rectangular plate about a heat parallel come an edge and also passing v the centre (I) = Ml2/12.

⇒ Note: If the mass element is chosen parallel come the length of the plate, then the minute of inertia would certainly be, i = Mb2/12.

Moment the Inertia that a Uniform circular Plate about its Axis


Let the fixed of the plate it is in M and the radius it is in R. The centre is at O and also the axis is perpendicular come the airplane of the plate. The mass element considered is a slim ring between x and also x+dx v thickness dx and also mass dm.

As the plane is uniform, the surface ar mass thickness is constant.

M/A = dm/da

M/πR2 = dm/<2π.x.dx>I = ∫ x2 dm = 2M/R2 × 0∫R x3 . Dx

Limits: together we take it the area of all mass elements from x=0 to x=R, we cover the totality plate.

I = (2M/R2)R0 = (2M/R2) × R4/4 = MR2/2

Therefore, the minute of inertia of a uniform circular plate around its axis (I) = MR2/2.

Moment that Inertia of slim Spherical shell or Uniform hollow Sphere


Let M and also R it is in the mass and also the radius that the sphere, O at its centre and OY it is in the given axis. The fixed is spread over the surface of the sphere and also the inside is hollow.

Let us consider radii the the sphere at an angle θ and at an edge θ+dθ v the axis OY and also the facet (thin ring) of massive dm v radius Rsinθ is taken as we rotate these radii around OY. The broad of this ring is Rdθ and also its perimeter is 2πRsinθ.

As the hollow round is uniform, the surface ar mass thickness (M/A) is constant.

M/A = dm/da

(M/4πR2)= dm/<2π × Rsinθ . Rdθ>dm = × 2πR2 . Sinθ dθ = × sinθ dθ

I = ∫ x2 dm = 0∫π (R sin θ)2 × sinθ dθ

Limits: together θ boosts from 0 come π, the elemental ring cover the entirety spherical surface.

I = × 0∫π sin3 θ dθ = MR2/2 × 0∫π  × dθ

=  0∫π (1 – cos2 θ) sin θ × dθ

Now, on completely the above equation through the substitution method we get,

Take u = cos θ dθ

Then, du = – sin θ dθ

Changing limits, when θ = 0, u = 1

When, θ = π, u = -1

I = × 1∫-1 (1 – u2) (-du)

= × 1∫-1 (u2 – 1) (du)

= × 1-1

= <-2/3 + 2> = × <4/3> = 2MR2/3

Therefore, the minute of inertia of thin spherical shell and uniform hollow round (I) = 2MR2/3.

Moment of Inertia of a uniform solid sphere

Let us consider a ball of radius R and also mass M. A thin spherical covering of radius x, massive dm and also thickness dx is taken as a fixed element. Volume density (M/V) remains continuous as the solid sphere is uniform.

M/V = dm/dV

M/<4/3 × πR3> = dm/<4πx2.dx>dm = × 4πx2 dx = <3M/R3> x2 dx

I = ∫ dI = (2/3) × ∫ dm . X2

= (2/3) × ∫ <3M/R3 dx> x4

=( 2M/R3)× 0∫R x4 dx

Limits: together x boosts from 0 come R, the elemental shell covers the totality spherical surface.

I = (2M/R3)R0

= (2M/R3)× R5/5

Therefore, the minute of inertia of a uniform solid round (I) = 2MR2/5.

Moment the Inertia for various Objects


As we keep in mind in the table above, the moment of inertia relies upon the axis of rotation. Every little thing we have calculated so far are the minute of inertia of those objects as soon as the axis is passing v their center of masses (Icm). Having chosen, two different axes you will observe the the thing resists the rotational adjust differently. Therefore, to find the minute of inertia through any given axis, the complying with theorems room useful.

Parallel Axis TheoremPerpendicular Axis Theorem

Parallel Axis Theorem

The minute of inertia of an object around an axis v its centre of mass is the minimum moment of inertia because that an axis in the direction in space. The moment of inertia about an axis parallel to that axis through the center of massive is offered by,


I = Icm + Md2

Where d is the distance in between the two axes.

Read much more on Parallel and Perpendicular axis theorem


Radius the Gyration

If the minute of inertia (I) that a human body of fixed m around an axis be composed in the form:

I = Mk2

Here, k is called the radius the gyration that the body about the given axis. It represents the radial distance from the provided axis of rotation wherein the entire mass of the body deserve to be assumed to be focused so the its rotational inertia continues to be unchanged.

The radius of gyration for a heavy sphere about its axis is:

Mk2 = 2MR2/5, or k = √<2/5> × R.

Solved Example

1. From a uniform one disc of radius R and mass 9 M, a little disc of radius R/3 is eliminated as shown in the figure. Calculation the moment of inertia that the continuing to be disc around an axis perpendicular to the plane of the disc and also passing with the centre of the disc.



The moment of inertia the removed part abut the axis passing through the center of mass and perpendicular to the aircraft of the disc = Icm + md2

= /2 + m × <4R2/9> = mR2/2

Therefore, the minute of inertia that the remaining portion = moment of inertia that the complete disc – minute of inertia that the eliminated portion

= 9mR2/2 – mR2/2 = 8mR2/2

Therefore, the minute of inertia the the remaining section (I remaining) = 4mR2.

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2. Moment 2 balls associated by a rod, as displayed in the figure listed below (Ignore rod’s mass). Massive of round X is 700 grams and the massive of sphere Y is 500 grams. What is the moment the inertia of the system about AB?



The rotation axis is AB

mX = 700 grams = 0.7 kg

mY = 500 grams = 0.5 kg

rX = 10cm = 0.1m

rY = 40cm = 0.4m


I = mX rX2 + mine rY2

I = (0.7)× (0.1)2 + (0.5)× (0.4)2

I = (0.7) x (0.01) + (0.5) x (0.16)

I = 0.007 + o.08

I = 0.087 kg m2

Moment the inertia the the device is 0.087 kg m2

3. 2 balls associated by a rod as presented in the figure below (Ignore rod’s mass). What is the moment of inertia that the system?



mX = 300 grams = 0.3 kg

mY = 500 grams = 0.5 kg

rX = 0cm = 0m

rY = 30cm = 0.3m


I = mX rX2 + mine rY2

I = (0.3)× (0)2 + (0.5)× (0.3)2

I = 0 + 0.045

I = 0.045 kg m2

Moment that inertia the the system is 0.045kg m2

4. The fixed of each sphere is 200 gram, associated by cord. The length of cord is 80 cm and the broad of the cord is 40 cm. What is the minute of inertia that the balls about the axis that rotation (Ignore cord’s mass)?



Mass of sphere = m1 = m2 = m3 = m4 = 200 gram = 0.2kg

Distance in between ball and also the axis the rotation (r1) = 40cm = 0.4 m

Distance in between ball 2 and also the axis of rotation (r2) = 40 centimeter = 0.4 m

Distance in between ball 3 and the axis of rotation (r2) = 40 centimeter = 0.4 m

Distance between ball 4 and the axis that rotation (r2) = 40 cm = 0.4 m


I = m1 r12 + m2 r22 + m3 r32 + m4 r42

I = (0.2) × (0.4)2 + (0.2) × (0.4 )2 + (0.2) × (0.4)2 + (0.2) × (0.4)2

I = 0.032 + 0.032 + 0.032 + 0.032

I = 0.128 kg m2

Moment of inertia of the balls around the axis o.128 kg m2

Moment the Inertia of different Shapes and also Objects

Here is a perform of the MOI of different shapes and objects.