# Moment of inertia of a disk about an axis not passing through its CoM

• vcsharp2003
In summary, the conversation discusses two approaches to finding the moment of inertia for a disk rotating about an axis of rotation O. The first approach treats the disk as a particle at its center of mass and yields a moment of inertia of 25 MR^2. The second approach uses the parallel axis theorem and yields a moment of inertia of 51 MR^2. The conversation also discusses the conditions under which the parallel axis theorem is applicable.
vcsharp2003
Homework Statement
A disk of radius R and mass M is connected to a rod that is rotating about O. The rod is rotating about at an angular velocity of ##\omega## about O. What would be the moment of inertia of the rod+disk system about an axis through O and perpendicular to plane of rotation of the rod. Assume the rod is massless and the disk is not rotating about it's center of mass axis.
Relevant Equations
##I=\frac {MR^2} {2}## for a disk about an axis through its cm and perpendicular to the disk
## I_o = I_{cm} + M R^2##
I have come up with two different approaches, but I'm not sure which one is correct since they give different answers.
We use the following equation to get the total moment of inertia.

##I_o## = moment of inertia of disk about O axis + moment of inertia of road about O axis

Approach 1:

Consider the disk as a particle at it's center of mass.
##I_o = M (5R)^2 + 0= 25 MR^2##

Approach 2:

We use parallel axis theorem to get moment of inertia of disk about O axis.

##I_o= \frac {MR^2} {2} + M (5R)^2 + 0 \\= \frac {51MR^2} {2}##

Last edited:
vcsharp2003 said:
Consider the disk as a particle at it's center of mass.
The disk is not a particle at its center of mass.

Orodruin said:
The disk is not a particle at its center of mass.
If an object is moving at an angular velocity, and its of uniform density then can we not assume it's like a particle at its cm rotating about O in this scenario?

vcsharp2003 said:
If an object is moving at an angular velocity, and its of uniform density then can we not assume it's like a particle at its cm rotating about O in this scenario?
No.
E.g. consider a mass 2m at radius r. MoI=2mr2.
Now split it into mass m at radius 0 and mass m at radius 2r. Centre of mass has not changed but MoI=4mr2.

But I note this in post #1: "the disk is not rotating about its center of mass axis."
Technically, that means it retains the same orientation even though its centre rotates about O. I.e. if the rod is rotating at ##\omega## then the disc is rotating at the same rate in the opposite direction relative to the rod. In that case you can treat it as a point particle.
But maybe it means that it is attached rigidly to the rod and so rotates about its own centre at the same rate that it rotates about O. In that case you need to apply the parallel axis theorem.

vcsharp2003
haruspex said:
Technically, that means it retains the same orientation even though its centre rotates about O
Yes, it retains its orientation. So, we must assume its a particle.
But still, I cannot explain as to why the parallel axis theorem approach is not correct in this case.

How does your diagram fits the description "centered at O"? Is this the original text or/and figure?

vcsharp2003
nasu said:
How does your diagram fits the description "centered at O"? Is this the original text or/and figure?
Thanks for pointing out the mistake. I have corrected it so it doesn't say that. This is a made up problem by me in which I am trying to understand the right way to find moment of inertia of a system.

haruspex said:
Technically, that means it retains the same orientation even though its centre rotates about O. I.e. if the rod is rotating at ω then the disc is rotating at the same rate in the opposite direction relative to the rod. In that case you can treat it as a point particle.
This disk is not rotating about its center of mass axis. It simply moves in a circular orbit and is connected to rod rigidly. Its like a stone tied to the end of a string and the stone is moving in a circular path.

vcsharp2003 said:
Yes, it retains its orientation.

vcsharp2003 said:
and is connected to rod rigidly.
You cannot have it both ways. If it retains its orientation as the rod rotates then it must be mounted on an axle, not rigidly attached.

vcsharp2003
haruspex said:
You cannot have it both ways. If it retains its orientation as the rod rotates then it must be mounted on an axle, not rigidly attached.
Ok, I get that. But, why the parallel axis theorem would not be applicable in this problem? I thought it always applies for two parallel axes provided one of the axes is passing through the center of mass of disk.

vcsharp2003 said:
Ok, I get that. But, why the parallel axis theorem would not be applicable in this problem? I thought it always applies for two parallel axes provided one of the axes is passing through the center of mass of disk.
Yes, but it only applies if the object is rotating as a body about that parallel axis.
More generally, suppose the mass centre of the body rotates at radius r at rate ##\omega_b## about some point, but the orientation of the body rotates at rate ##\omega_c## about the body's mass centre. Then the angular momentum is ##mr^2\omega_r+I_c\omega_c##. The parallel axis theorem is just the special case where the rotation rates are the same.

vcsharp2003

nasu, vcsharp2003 and TSny

## What is moment of inertia?

Moment of inertia is a measure of an object's resistance to rotational motion. It is similar to mass in linear motion, but instead applies to rotational motion.

## What is the formula for moment of inertia of a disk?

The formula for moment of inertia of a disk about an axis not passing through its center of mass (CoM) is I = 1/2 * MR^2, where M is the mass of the disk and R is the distance from the axis of rotation to the CoM.

## Why is the axis of rotation important in calculating moment of inertia?

The axis of rotation is important because it determines the distance between the axis and the CoM, which is a crucial factor in the moment of inertia formula. The farther the axis is from the CoM, the greater the moment of inertia will be.

## How does the moment of inertia change if the disk's mass or radius is altered?

If the mass of the disk is increased, the moment of inertia will also increase. Similarly, if the radius of the disk is increased, the moment of inertia will also increase. This is because both mass and radius are directly proportional to the moment of inertia in the formula.

## What are some real-life applications of moment of inertia of a disk?

Moment of inertia of a disk is an important concept in many fields of science and engineering, including physics, mechanics, and robotics. It is used to calculate the stability and rotational motion of objects, such as wheels, flywheels, and gears. It is also a key factor in designing machines and structures to ensure they can withstand rotational forces and maintain balance.

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