Part 3 of 5 - Conservation of angular momentum with a change of rotational inertia
Briana is now satisfied that for the situation modeled in the simulation the angular momentum of the disk-block system should be a conserved quantity as the block is released. She says to David, "angular momentum ought to be conserved if the block is moved closer to the axis as the disk turns."
"I don't think so," David says. "In order to move the block closer towards the axis of rotation a force must be exerted to overcome any frictional forces acting between the block and the disk. This represents a torque on the system and thus angular momentum will not be conserved."
Are either of these students correct in their analysis?
• Neither is correct. Angular momentum will not be conserved in this case because the rotational inertia of the block will not be constant during the time it is being moved toward the axis of rotation.
• David is correct. The external force required to move the block will be a torque on the system and the angular momentum will increase as a result.
• Briana is correct. Angular momentum will be conserved because there is no force required to slide the block in toward the center of the disk.
• Briana is correct. There will be a force required to move the block inward. However, this force will be directed inward toward the axis of rotation and thus will not provide a torque to the system.