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Moment Of Inertia Of A Hollow Cylinder

Moment Of Inertia Of A Hollow Cylinder

In rotational motion, the Moment of Inertia (MI) plays the same role as mass in linear motion. While mass measures the resistance of a body to change in its state of linear motion, the MI measures the resistance of a body to change in its rotational motion. For a hollow cylinder, the mass is distributed away from the axis of rotation, which affects the movement of rotation.

Moment of Inertia of a Hollow Cylinder with Different Axes

The moment of inertia (I) is defined as:

formula

where,

r - perpendicular distance of a small mass element from the axis of rotation
I - moment of inertia

Considering a hollow cylinder of -

Inner radius: R1
Outer radius: R2
Length/height: h
Mass: M

The hollow shape means the material exists only between two radii—inner radius 𝑅1​ and outer radius 𝑅2.

Moment of Inertia along Axis of the Central Axis (Symmetry Axis)

Consider the hollow cylinder rotating about its own central (longitudinal) axis, and consider a thin cylindrical shell at radius r with thickness dr.

Mass per unit volume-

formula

Integrating:

                                              formula

formula

Moment of Inertia along an Axis perpendicular to the cylinder and passing through the centre

The axis passes through the centre of the cylinder’s length but is perpendicular to its length (through its diameter). We use the perpendicular axis theorem and the parallel axis theorem.

For a solid disk:

formula

Special Case: Thin Hollow Cylinder

If the hollow cylinder is thin (thickness negligible),

R1 ≈ R2 ≈ R

The formula reduces to:

formula

Applications

  • Engineering: Used in designing flywheels, rollers, pipes, and rotating machinery parts. And torque requirements in mechanical systems.
  • Automobile Industry: Hollow shafts in gear systems use the moment of inertia of a hollow cylinder.
  • Sports: Rotation of hollow cylindrical objects like hoops or bicycle wheels uses the concept of moment of inertia.
  • Robotics: Designing robot arms with strong hollow cylinders for better motion control.

Summing Up

formula

Frequently Asked Questions

Q1. What is the difference between the moment of inertia of a hollow cylinder and a solid cylinder?

A hollow cylinder has mass distributed farther from the axis, giving it a higher moment of inertia than a solid cylinder with the same mass and size.

Q2. Does increasing the inner radius affect the moment of inertia?

Yes, increasing R1 when R2 is constant reduces the moment of inertia because mass moves closer to the axis.

Q3. Find the moment of inertia of a thin hollow cylinder of mass 5 kg and radius 0.4 m about its central axis.

Given: M = 5 kg, R = 0.4 m

Icentral axis = M R2

Icentral axis = 5 × (0.4)2

= 0.8 kg·m2

Q4. A hollow cylinder has R1 = 0.2 m, R2 = 0.5 m, and a mass of 10 kg. Find its moment of inertia about the central axis.

formula

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