What is rotational inertia?
To understand rotational inertia, we had better explain inertia in general.
Inertial is a "resistance to change of motion". The more inertia an object
has, the less it responds to being pushed. For linear motion (moving from
here to there), The measure of an object's inertia is its mass. The more
mass in an object, the less it responds to being pushed. A light push sends
a ping-pong ball flying, while a light push on a bowling ball has almost no
Rotational inertia is a resistance to change of rotation. The more
rotational inertia an object has, the less it responds to being spun. The
outside parts of a spinning object have to move much faster than inside
parts near the axis. How an object is built, and along what direction you
want to spin it, also affect the rotational inertia.
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Update: June 2012