rotational speed and angular momentum

Jun 2014
Require help to understand the following physics concept

What happens to the rotational speed and angular momentum, when spinning system contracts in the absence of an external torque? weather rotational speed and angular momentum changes or not? if changes how do they change?

Please explain the concept clearly.


PHF Helper
Jun 2010
Morristown, NJ USA
In the absence of any external torque being applied angular momentun is conserved. A spinning object's angular momentum is equal to its moment of inertia "I" times its spin rate in radians/second, w. Moment of inertia is a function of the distribution of the object's mass as measured from the axis of rotation - the further mass is from the axis of rotation the greater its value of I. If a spinning system "contracts" its moment of inertia decreases, as mass is moved closer to the axis of rotation. Since I decreases, and the angular momentum stays constant, the speed of angular rotation w must increase. The classic example of this effect is the spinning ice skater who starts with her arms spread, and then as she pulls her arms in her rotational speed increases.