Rotational dynamics (A level physics) question

Hi I was wondering if someone could please help me with the following question: 2.3 https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2020/november/AQA-74083BC-QP-NOV20.PDF

Mark scheme: https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2020/november/AQA-74083BC-W-MS-NOV20.PDF

I understand the calculations, the only thing I'm struggling on is why the frictional torque is not shared between the two rotating shafts. Basically when I did the calculation I did Torque = moment of inertia of A x acceleration of A + moment of inertia of B x acceleration of B but in the markscheme they do torque = moment of inertia of A x acceleration of A.
I thought torque would be shared as work is done on both to decelerate them.
Thank you very much for any help!

Reply 1

lordaxil

The torque is not shared, it is equal and opposite on each shaft (the rotational analogue of Newton's 3rd law). This is why the question asks in (a) to state the condition for conservation of angular momentum (zero net torque applied to system).