Conservation of Angular Momentum
An ice skater is spinning on the tip of her skate with her arms extended. Her angular momentum is conserved because the net torque on her is negligibly small. In the next image, her rate of spin increases greatly when she pulls in her arms, decreasing her moment of inertia. The work she does to pull in her arms results in an increase in rotational kinetic energy.
Source
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"OpenStax College, College Physics. November 9, 2012."
http://cnx.org/content/m42182/latest/?collection=col11406/1.7
OpenStax CNX
CC BY 3.0.