Question

What is the inertia of an object and how to calculate it?

Answer 1

In physics, when you calculate an object's moment of inertia, you need to consider not only the mass of the object but also how the mass is distributed. For example, if two disks have the same mass but one has all the mass around the rim and the other is solid, then the disks would have different moments of inertia.
You will learn about that in rotational motion which is in the higher class.
Simply
Inertia of an object is the resistance offered by the object to change in its motion or position. ... Calculate the rotational inertia or the moment of inertia by multiplying the mass of the object with square of the distance between the object and the axis, the radius of rotation.
Inertia is the resistance of any physical object to any change in its position and state of motion. This includes changes to the object's speed, direction, or state of rest.

Inertia is also defined as the tendency of objects to keep moving in a straight line at a constant velocity. The principle of inertia is one of the fundamental principles in classical physics that are still used to describe the motion of objects and how they are affected by the applied forces on them.