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Moment Of Inertia Of A Square

Moment Of Inertia Of A Square

The Moment of Inertia (MI) is a property of a body that measures how much it resists rotational motion about a specific axis. It plays a role in rotational mechanics similar to mass in linear motion. A square is a flat, two-dimensional sheet with all sides equal in length and uniform thickness.

Moment of Inertia About Different Axes

Considering a square of Side length a, Mass M, Uniform thickness and uniform density.

Moment of Inertia About an Axis Through Its Centre

Derivation

From the Perpendicular Axis Theorem, a flat body lying in the XY-plane:

Iz = Ix + Iy

where,

Ix - moment of inertia about an axis along one side through the centre
Iy - moment of inertia about the perpendicular side through the centre

For a square:

Ix = Iy because of symmetry

Also, the moment of inertia of a square about one of its central axes lying in the plane is:

formula

So:

Iz = Ix + Iy

formula

Therefore, the Moment of Inertia About an Axis Through Its Centre is 1/6 Ma2

Moment of Inertia About a Side (Axis Along One Edge in the Plane)

When the axis lies along one edge of the square, we can use the Parallel Axis Theorem:

Iedge = Icentral parallel + Md2

where,

formula

Moment of Inertia About a Diagonal

If the axis lies along a diagonal in the plane, we can use the perpendicular axis theorem again.

For a diagonal axis (say, X′-axis) lying in the plane:
The perpendicular distances of the mass elements from the diagonal are smaller compared to from a side, so the moment of inertia will be less.

From geometry and distribution:

formula

Formulae at a Glance

Shape Axis Moment of Inertia
Square Centre, perpendicular to the plane formula
Square Along a side (edge) formula
Square Along a diagonal (in-plane) formula

Applications -

  • Engineering design – Used in structural engineering for plates, panels, and frames.
  • Machinery parts – Square plates in rotating machinery.
  • Robotics – Moment of inertia in robotic arms with square joints.
  • Aerodynamics – Square fins and plates in stability analysis.
  • Architecture – Stability of flat square beams and supports.
  • Sports equipment – Square boards in gym equipment.
  • Material testing – Rotational stress testing of square metal sheets.

Summing Up

formula

Frequently Asked Questions

Q1. Does the thickness of the square matter?

If it’s very thin, we treat it as 2D; otherwise, we need 3D analysis.

Q2. Is the moment of inertia about the centre always smaller than about the edge?

Yes, because more mass is farther from the axis in the edge case.

Q3.A square metal plate has a side length of 2 m and a mass of 12 kg. Find its moment of inertia about its centre, perpendicular to the plane.

Given: Mass = 12kg, Length = 2m

formula

Q4. A square plate of side 1 m and mass 6 kg has its moment of inertia about one edge calculated.

Given: Mass = 6kg, Side = 1m

formula

 

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