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I stumbled upon a pretty old post today and was wondering if there was any other input in regards to it. The picture below shows two points on a box. If torque is taken about point B there is a net torque. However if taken about point A there is not. At first it makes sense because a force at an axis causes no torque. But thinking about the motion, the box would move up and left while rotating CCW about the COM so doesn’t that mean there is some kind of change in angular momentum about point A, considering the axis is not accelerating with the box? Thanks in advance, I’m new here and really appreciate the help and kindness of the community.

enter image description here

Qmechanic
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Scooby329
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1 Answers1

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The problem you have is that points $A$ and $B$ are accelerating and so you cannot directly apply Newton's laws.

Consider $2$ extra forces acting at the centre of mass of the square, $O$.

enter image description here

The forces $b$ and $c$ have equal magnitudes to the magnitude of force $a$ but are opposite in direction to one nother.

The square is subject to a force $b$ acting at its centre of mass which will produce a translational acceleration and an anticlockwise couple $a\,L/2\,(=\,c\,L/2)$ which will produce an anticlockwise angular acceleration of the square.

Farcher
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