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Moment Of Inertia Of A Triangle

Moment Of Inertia Of A Triangle

The Moment of Inertia (MI) of a body is a measure of its resistance to rotational motion about a specific axis. It plays the same role in rotational motion as mass does in linear motion. A triangle is a plane figure with three sides and three angles. It can be equilateral, isosceles, or scalene, but in moment of inertia problems, we usually consider a uniform thin triangle with uniform mass distribution.

Moment of Inertia About Different Axes

Consider a uniform triangle of base b, height h, and mass M.

Moment of Inertia About the Base (Axis in the Plane)

From geometry, the mass per unit area (surface density) is:

formula

formula

formula

Moment of Inertia About an Axis Through the Centroid (Parallel to Base)

Using the Parallel Axis Theorem:

Icentroid = Ibase − Md2

formula

Moment of Inertia About an Axis Perpendicular to the Plane of the Triangle

If the axis passes through the centroid and is perpendicular to the plane (like Iz), we use the Perpendicular Axis Theorem:

Iz = Ix + Iy

Where Ix is about the centroid parallel to the base, and Iy is about the centroid parallel to the height.

For a right-angled triangle, Iy can be calculated similarly, but for an equilateral or isosceles triangle, symmetry is used.

Formulae at a Glance

Shape Axis Moment of Inertia
Triangle Base in Plane formula
Triangle Parallel to the base formula
Triangle Through centroid Ix + Iy

Applications

  • Structural Engineering – Used in the design of triangular frames and supports.
  • Bridge Design – Triangular truss calculations.
  • Robotics – Rotating triangular components.
  • Aeronautics – Stability of triangular fins and plates.
  • Architecture – Triangular beam design.
  • Mechanical Systems – Flywheels or blades with triangular sections.
  • Sports – Racket frames with triangular structures.

Summing Up

formula

Frequently Asked Questions

Q1. Does the shape of the triangle affect the moment of inertia?

If height and mass are the same, the formula does not change. The triangle shape only changes the height and base.

Q2. Why is the moment of inertia smaller about the centroid than about the base?

It is smaller at the center because the triangle’s mass is closer to the axis, making it easier to spin.

Q3. Find the moment of inertia of a uniform triangular plate of mass 3 kg and height 0.6 m about its base.

formula

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