A uniform disk has a mass 3.7kg and a radius of 0.40m. The disk is mounted on frictionless bearings and is used as a turntable.
the turntable is initially rotating at 30 rpm. A thing-walled hallow cylinder has the same mass and radius as the disk. It is released from rest, just above the turntable, and on the same vertical axis. The hollow cylinder slips on the turntable for 0.20s until it acquires the same final angular velocity as the turntable. What is the final angular momentum of the system?

A) 1.3kg x m^2/s
B) 0.93kg x m^2/s
C) 1.1kg x m^2/s
D) 1.9kg x m^2/s
E) 0.616 x m^2/s

Question

A uniform disk has a mass 3.7kg and a radius of 0.40m. The disk is mounted on frictionless bearings and is used as a turntable. 
the turntable is initially rotating at 30 rpm. A thing-walled hallow cylinder has the same mass and radius as the disk. It is released from rest, just above the turntable, and on the same vertical axis. The hollow cylinder slips on the turntable for 0.20s until it acquires the same final angular velocity as the turntable. What is the final angular momentum of the system?

A) 1.3kg x m^2/s
B) 0.93kg x m^2/s
C) 1.1kg x m^2/s
D) 1.9kg x m^2/s
E) 0.616 x m^2/s

vincent-lyon.fr · Answer

Since angular momentum is conserved (no external torque acting), then it is sufficient to work out the angular momentum of the system before disk and cylinder are in contact.