Rotational Mechanics – HC Verma’s Concepts of Physics – Solution

HC Verma’s Concepts of Physics - Chapter 10 - Question 15

Find the moment of inertia of a uniform square plate of mass m and edge a about one of its diagonals.

The question was posted by Ankit

Solution:

Let the moment of inertia about the axis through the centre of the square plate perpendicular to the plane, assumed to be along Z axis, be I. Now the square plate will be in the XY plane. Let the X axis and Y axis be parallel to the edges and passing through the centre.

Then, we know that Ix = Iy = ma²/12

Then, according to perpendicular axis theorem,

Iz = Ix + Iy = 2Ix=2ma²/12=ma²/6

Now, imagine that X and Y axes are along the diagonals. (The diagonals of a square are also mutually perpendicular.)

Again, according to perpendicular axis theorem, Iz = Ix + Iy = 2Ix

The MI about the diagonal about its diagonal, Ix = Iz/2 = ma²/12