If the shape of object changes so would the moment of inertia of that object. This is because the first disk has a larger moment of inertia. As a result its mass would be distributed further from the axis of rotation and it would take more effort to accelerate the first disc as compared to the second.

For example we can take two discs of equal mass but the diameter of one is greater than the other. The moment of inertia of an object is a measure of how difficult it is to change the angular motion of that object about the axis. The centre of mass of a system of particles or a rigid body can be derived using the first moment concept. The first mass moment is equal to mass multiplied by distance, m.r. Moment of inertia is also referred to as the second mass moment.

If we consider a rigid body as a system of particles and the relative position of these particles does not change, then the moment of inertia of a point mass is equal to: I = m.r 2, for each particle so that the moment of inertia is equal to the mass multiplied by the square of the distance, where m = the mass of the particle, and r = the distance from the axis of rotation to the particle. That is why two bodies of the same mass may have different moments of inertia. The moment of inertia is related to the distribution of mass throughout the body, not just mass of the body alone. According to Newton's first law of motion 'A body maintains the current state of motion unless acted upon some external force'. It is necessary to specify a moment of inertia with respect to an axis of rotation. Moment of inertia, also known as rotational inertia, is analogous to the inertia of linear motion.

Mass moment of inertia