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If we consider the torque about point A, there is none, so there is no rotation about point A. If we consider torque about point B, there is a net torque which means that there is rotation about point B. But if there is rotation about point B, doesn't this imply that there must also be rotation about every single other point?

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Are there specific locations where I can take torque about, or have I simply made a mistake in logic?

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  • $\begingroup$ When you say total torque what exactly do you mean? As in total torque about which point? (I thought torque was relative, but apparently I have some gaping flaws in my torque knowledge) $\endgroup$
    – QCD_IS_GOOD
    Apr 22, 2015 at 11:34
  • $\begingroup$ Conventionally torque is taken about the rotation axis, but you may not know what that is. So take a mass-weighted average of the torque about every point in the entire system. The centre of mass will appear... $\endgroup$
    – lemon
    Apr 22, 2015 at 11:49
  • $\begingroup$ Usually I would take the torque about the centre of mass, but today I was feeling wild so I thought I'd try other points and the results are strange, I don't see what's wrong intrinsically about taking torques about other points? $\endgroup$
    – QCD_IS_GOOD
    Apr 22, 2015 at 11:55
  • $\begingroup$ The edit needs to be rejected. I clicked the approve button by accident. $\endgroup$
    – John Alexiou
    Apr 22, 2015 at 12:34

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The rule is check torques about the center of mass to check for rotational motion. In this case there is torque about the center of the square and thus there is going to be rotation.

The only case where the point of summing up torques does not matter is when the net forces are zero. See https://physics.stackexchange.com/a/111348/392.

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