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In the question and solution given below, I haven't understood why is the solution asking us to take torques about the center of mass.
In my approach I equated the net torque about the lower end B of the rod to zero


enter image description here



MY APPROACH:
let distance between line of action of F2 and B = k cm
then length of the rod = k+20 cm
taking torques about B, we have
F2k - F1(k+20) = 0 (since rod is only translating)
Putting F2 as 5N , and F1 as 3N, we have k=30cm
thus my answer comes out as 50cm



*ps : we cant take F1 as 7N. I already tried that. torque being zero is impossible in that case

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    $\begingroup$ physics.stackexchange.com/q/93698 I hope this helps $\endgroup$
    – S.M.
    Apr 29, 2021 at 4:05
  • $\begingroup$ Hello and welcome to the physics stack exchange! Since this is not a typical help site, it would be great to ask questions to benefit the wider audience. $\endgroup$
    – AlphaLife
    Apr 29, 2021 at 4:19
  • $\begingroup$ You can refer to this link for more details : How to ask homework type questions on this site. In case you find these rules to be a bit harsh, you can always go to any other suitable sites. As someone said before, you can learn the alphabet from these sites and then come back here to ask questions about dinosaurs :) Have a great time! $\endgroup$
    – AlphaLife
    Apr 29, 2021 at 4:19
  • $\begingroup$ @AlphaLife this isn't my home work question. i am not able to understand the concept of torque behind it.....as in, which point to take and why $\endgroup$ Apr 29, 2021 at 6:45
  • $\begingroup$ @S.M. tnx. it helped $\endgroup$ Apr 29, 2021 at 6:53

1 Answer 1

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You can only sum torques about any point and expect them all to be the same if and only if the body is in translational and rotational equilibrium.

Let $A$ and $B$ be two point on some body. Let $\vec \tau $ denote the torque, and $\Sigma \vec F$ be the net force. Then, the torque about A is related to the torque about point B by the following equation $$\vec \tau_B = \vec \tau _A + \vec r_{AB} \times \Sigma \vec F$$

This body will rotate about its center of mass, so to break down the motion in rotational and translational motion, you should consider the translation of the center of mass and rotation about the center of mass.

See here: Why if the torque equals zero measured from one point in space it equals zero measured from any other point?.

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  • $\begingroup$ understood ;) thank you $\endgroup$ Apr 29, 2021 at 14:21
  • $\begingroup$ but do you mean to say that the body given in the question is NOT in rotational equilibrium ? is so, why ? isnt it NOT rotating ? $\endgroup$ Apr 29, 2021 at 14:22
  • $\begingroup$ @omjoglekar rotational equilibrium is when the net torque is zero, which, in this case, it is not. I'll also add that calculating the torque about a point other than the center of mass will (when the body is in equilibrium) sometimes offer a much quicker solution, you just have to be sure that the net force and torques are zero $\endgroup$
    – user256872
    Apr 29, 2021 at 17:55

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