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

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:


By symmetry:

Using the perpendicular axis theorem for planar bodies:


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

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

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