Angular Momentum and Average Torque

[mohabitar]

Refer to number 6. This is the one I'm stuck on. So angular momentum is conserved right, so initial angular momentum is equal to final angular momentum. Initial is 7.87 so final must be 7.87, right? And so average torque is just change in angular momentum / change in time, so 0/7=0. What am I doing wrong?

 

[tiny-tim]

hi mohabitar!

… So angular momentum is conserved right, so initial angular momentum is equal to final angular momentum. Initial is 7.87 so final must be 7.87, right? And so average torque is just change in angular momentum / change in time, so 0/7=0.

no, total angular momentum is conserved, but the angular momentum of the rod isn't …

the angular velocity of the rod changes from 0 to 26.03 in 7 s …

so use one of the standard constant acceleration equations to find the (average) angular acceleration