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Moment Of Inertia Of A Quarter Circle

Moment Of Inertia Of A Quarter Circle

In rotational motion, the moment of inertia (MI) is a measure of how much a body resists changes to its rotational motion. The moment of inertia resists angular acceleration. A quarter circle is one-fourth of a full circular plate. Since the distribution of mass is not uniform around the axis in the same way as a full circle, the moment of inertia is calculated separately.

Moment Of Inertia Of A Quarter Circle about Different Axes

Considering a quarter circle of radius R, mass M, and negligible thickness.

Moment of Inertia of a Quarter Circle about its Centroidal Axes

Consider the quarter circle placed in the first quadrant of the Cartesian plane, with its centre at the origin (0,0), radius R, and edges along the x-axis and y-axis.

Derivation:

Surface mass density -

formula

Choosing coordinates -

A small area element in polar coordinates (r, θ)

Area element: dA = r dr dθ
Mass element: dm = σ dA = σ r dr dθ

For a quarter circle:

formula

Integration -

formula

Moment of inertia about the y-axis

By symmetry:

formula

Moment of inertia about the origin (O)

Using the perpendicular axis theorem for planar bodies:

formula

Applications

  • Engineering: In designing curved structural components.
  • Robotics: For parts shaped like circular sectors.
  • Mechanics: In flywheels and rotating sector-shaped plates.
  • Physics experiments: Analysing partial rotational systems.

Summing Up

formula

Frequently Asked Questions

Q1. Does thickness affect the formula?

No, this formula is valid for thin (negligible thickness) laminae. For thick bodies, volume integration is required.

Q2. A quarter-circle lamina has a mass of 3 kg and a radius of 0.4 m. Find its moment of inertia about the x-axis.

Given: Mass = 4 kg, Radius = 0.4 m

formula

Q3. A quarter circle of radius 0.5 m has a moment of inertia about the origin of 0.625 kg·m². Find its mass.

Given: Radius = 0.5 m, Moment of inertia = 0.625 kg·m²

formula

 

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