please help me to understand moment of inertia
With linear motion, the inertia, the tendency to resist being accelerated, is simply the mass. Not so with rotational motion. Rotational inertia (moment of inertia is another name for it) is based on the mass and also how far each bit of mass is from the axis of rotation. So instead of inertia just being mass, it's a calculus expression involving mass and distance. For problems with a lot of symmetry, the calculus simplifies to algebraic formulas.
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