# Angular Momentum Balance

Homework Statement:
write the macroscopic angular momentum balance for the following problem
Relevant Equations:
see image
we neglect gravity and viscosity efects
i really can't understand how does the author managed to get the equation in the image

haruspex
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Not making any sense to me either.
Altitude at ##\frac 12b_1## from where?
##b_1## is at an angle, so ##b_1-b_{2a}## doesn’t seem right.
And what about the force from the plate? We don't know where that is centred, so we can't set the axis so as to eliminate its torque.

it's an idealized problem, all i need to know is how they compute the cross product , there is no viscosity so there is no friction with the plate

haruspex