Why In the below diagram, the force causing torque on disc is $T$ not $T+mg$?

please see the image attached, I have to calculate torque acting on the disc, and my teacher took the force causing torque T not T +mg, I think it should be T +mg because that block is also exerting force on the string

• Please don't use screenshots and type your question fully. Commented Apr 9, 2023 at 7:21
• Mam, I have to use the diagram because I tried but I cannot explain without the diagram.
– user363762
Commented Apr 9, 2023 at 8:00
• $T = m g$ from the free body diagram. So $T+mg = 2 mg$. Would that make sense? Commented Apr 9, 2023 at 21:03

The pulley doesn't "see" the block. It only "sees" the string, which is at tension $$T$$.

Without the pulley, the block and the string would be at free fall and there wouldn't be any tension $$T$$.

With the pulley, the moment of inertia of the pulley implies a resistance to the free fall of the block. The resulting force is the tension $$T$$ in the string, which must be smaller than $$mg$$.

The box weight with a value of $$mg$$ does not pull directly in the pulley. Maybe indirectly, but not directly. Thus, you mustn't include it.

In fact, just isolate the pulley entirely, e.g. draw it on its own (a free-body diagram), and ask yourself what forces that act. You will see that only the string tension $$T$$ acts apart from the pulley's own weight.

In fact, the $$mg$$ value from the box is included in the $$T$$ value already, so if you write it as $$T+mg$$ then you would be including its effect twice.

• Thank you sir for helping me everytime and making my concepts crystal clear.
– user363762
Commented Apr 14, 2023 at 6:17

The force which the block is exerting on the string and the force which the string is applying on the block are action reaction pairs. These internal forces get cancelled and have no significance while calculating torque.

The force which you have to consider should be acting along the axis or point about which you are calculating torque, and in this case, the only force acting on the pulley is that due to the string, i.e tension T.