About which point should the torque be calculated?

Question

While calculating angular acceleration in a problem, about which point should we calculate the torque and why?

Answer 1

You can use whatever point to compute the angular acceleration AT THIS POINT.

If your problem ask you to compute angular acceleration without specifying at which point, then you must assume at which point it is, probably the one having special caracteristics depending on what the context is. You could for example choose center of mass, or any other point which seems to be the obvious one considering the context of what you are trying to do.

Answer 2

The equations of motion require calculation of net torque about the center of mass for their simplest form.

$$ \boldsymbol{F} = m \boldsymbol{a}_{\rm com} \\ \boldsymbol{\tau}_{\rm com} = \mathrm{I}_{\rm com} \boldsymbol{\alpha} + \boldsymbol{\omega} \times \mathrm{I}_{\rm com} \boldsymbol{\omega} $$

But, using a less common general form of the equations of motion, any point can be used.

$$ \begin{aligned} \boldsymbol{F} &= m \boldsymbol{a}_A - m \boldsymbol{c}\times \boldsymbol{\alpha} + m \boldsymbol{\omega}\times\boldsymbol{\omega}\times\boldsymbol{c} \\ \boldsymbol{\tau}_A &= \mathrm{I}_{\rm com} \boldsymbol{\alpha} + m \boldsymbol{c} \times \boldsymbol{a}_A - m \boldsymbol{c} \times \boldsymbol{c} \times \boldsymbol{\alpha} +\boldsymbol{\omega} \times \mathrm{I}_{\rm com} \boldsymbol{\omega} + m \boldsymbol{c} \times \left( \boldsymbol{\omega} \times \boldsymbol{\omega} \times \boldsymbol{c} \right) \end{aligned} $$

where $\boldsymbol{c} = \boldsymbol{r}_{\rm com} - \boldsymbol{r}_A$ is the relative location of the COM to the point of reference A.

See this answer for more details.