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Two blocks of masses m each are hung from a balance as shown in the figure. The scale pan A is at height H1 whereas scale pan B is at height H2 . Net torque acting on the rod of pan, will be (length of the rod is L and H1 & H2 <<R) (H1>H2)

Here when we calculate the net torque it will be the cross product of the net force and perpendicular distance.

Look that hence the two torques are acting in the opposite direction so we can write that ,

${\tau}_{1}=\frac{mg}{R}{H}_{1}\times l\phantom{\rule{0ex}{0ex}}and{\tau}_{2}=\frac{mg}{R}{H}_{2}\times l\phantom{\rule{0ex}{0ex}}Sothenettorqueis,\phantom{\rule{0ex}{0ex}}\tau ={\tau}_{1}-{\tau}_{2}=\frac{mg}{R}\left({H}_{1}-{H}_{2}\right)l$

Hope this helps you.

Regards

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