force in relation to angular momentum

valvan1

so a student is sitting on a spinning stool and has a 2kg dumbbell in each hand angular velocity is 3rad/sec arms stretched out is a radius of 80cm and he pulls in his arms to 20cm. for this problem your ignoring the students weight.

from other problems i have figured out
so angular velocity to start with is 3 rad/sec
angular velocity final is 48 rad/sec

kinetic energy initial is 11.52( dont know what dimensions this is im guessing J)
kinetic energy final is 184.32

and i need to find the force required to pull on of the dumbells in at a constant speed is equal to F=((initial angular momentum of 1 block)^2)/(4*Mass of one weight*Radius^3)) or do i use the change in kinetic energy equation ?

do i say Torque=F*d and Torque=I*alpha and then go Force = Ia/d? or is there another way to solve this that im not seeing ?

valvan1

sorry i posted it again in the homework section so you can delete this one, sorry about that