Sir this is a question of HC Verma.Please .....plz....plz....sir solve this question. I have a little doubt(how torque is zero about vertical axis.Please sir explain it with diagram and vector.Please sir its my humble request as I completely dependent on askiitian. Please.....plz.....plz.....plz....sir help me with this)

Sorry I can't attach an image as I don't have the option. But i will try my best to explain in words

The forces action on mass in vertical direction are mg and N(normal rxn force by surface). Now for vertical eqm N=mg both acting in opposite dirxns.

These two forces will generate torques in opposite dirxns as well. Hence their vector will cancel out.

Now about the tension in string:Since the string coincides with the vertical axis its torque becomes zero as R=0 in (Torque =F.R).{Although the particle is at a distance r from hole, the same string is attached to it as the one held by the person. Now tension is constant throughout the string. So it can be assumed that tension in string coincides with axis and hence torque is zero)

Hope this helps

 

Thanks..................... thanks a lot....sir....................................................................