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Confidence votes 317. Inertia is rotary momentum; an object's ability to continue spinning when a decelerating force is applied. Just as a heavy object takes more effort to stop from the same speed as a light object, so does a heavy object take more effort to stop from turning at the same speed as a light object. So assuming they are turning at the same angular velocity, the heavier one has a larger inertia.
The law of inertia is also important for Galileo's astronomy. He used this law to explain why we do not feel the earth's motion, and especially why objects falling on the surface of the earth move together with the earth.