Acarnival merry-go-round has a large disk-shaped platform of mass 120 kg that can rotate about a center axle. a 60-kg student stands at rest at the edge of the platform 4.0 m from its center. the platform is also at rest. the student starts running clockwise around the edge of the platform and attains a speed of 2.0 m/s relative to the ground.
(a) determine the rotational velocity of the platform.
(b) determine the change of kinetic enrgy of the system consisting of the platform and the student.

Answer

given,  

radius of merry - go - round = 4 m  

mass of the disk = 120 kg  

speed of the merry- go-round = 0 rad/s

speed = 2 m/s  

mass of student = 60 kg  

initial angular momentum of the system  

final angular momentum of the system    

from conservation of angular momentum  

 now,

change in kinetic energy of the system

initially the merry-go-round was on rest KE_i = 0

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.