When an object is effectively isolated from external torques, like an ice skater twirling on the tip of one skate, the angular momentum of the object
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can be increased by shifting mass out away from the axis of rotation.
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can be decreased by shifting mass out away from the axis of rotation.
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can be increased by shifting mass in toward the axis of rotation.
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can be decreased by shifting mass in toward the axis of rotation.
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cannot be changed except by friction at the point of contact.