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Given below is a piece of information from table 9.4 on page 243 from my standard NCERT class 11th textbook:

Two equal and opposite forces parallel to opposite surfaces forces in each case such that the total force and total torque on the body vanishes

Here is a picture from my book And this information corresponds to shearing stress...I understand that net force will be zero but how is net torque zero?

I tried using right hand rule to get torque due to both forces in same direction (hence non zero torque), perhaps I'm doing it wrong...

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Your textbook, like most textbooks, is, I think, confusing on this point. If all the forces involved are parallel to the surfaces, then you need two pairs of forces for equilibrium.

Suppose we have a cuboid with edges aligned with the x, y and z axes. Suppose that there are no forces on it in the z direction, nor on the two faces normal to the z direction. Then on the two faces of fixed x we need equal and opposite forces in the ±y directions, giving a couple about the z axis. On the two faces of fixed y we need equal and opposite forces in the ±x directions, giving a couple in the opposite direction about the z axis. It's easy to show that if these couples are equal in magnitude, we can compute the shearing stress using either pair of forces.

You might object that I can put a cuboid under shearing stress and keep it in equilibrium, by placing it between my two hands, involving only two opposite faces. What's happening here is that you are not applying forces that are wholly tangential to the faces. Aditya Sharma's answer takes over here.

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  • $\begingroup$ Great answer! I happen to know the context as I study from the same textbook! $\endgroup$ Jan 24, 2021 at 14:37
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It is because the Normal force balances the total torque of the applied force and the force that is opposite to applied force (usually friction).

The Normal Force does so by acting from a different point of contact (so that it does not pass through the Centre) simultaneously balancing gravity as well.

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