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I'm studying structural mechanics and I have been stuck on a thought. I drew a very simple cantilever beam situation. If I make a fictional cut like in my fig.1, I will be in the situation of the second figure. Unfortunately I don't understand how stress can still exist "inside" the beam since we can clearly see that the shear stresses are exactly opposed on both halves of the beam.

Cantilever beam

I made it for the shear stress and shear force but it would be the same for normal stresses.

Thank you very much for your help ! Neo

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  • $\begingroup$ When we do a force balance on a body, we include only the forces exerted on it by other bodies. We never include the reaction forces exerted by it on the other bodies. $\endgroup$
    – Chet Miller
    Commented Jul 22, 2020 at 11:23

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This is a consequence of Newton's third law. Each half is under a force from the other one, if we think them as separate bodies. The forces are equal in magnitude and opposite in direction.

The only difference is division by cross-sectional areas, which transforms forces to stresses.

The same applies to the normal stresses.

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Ok thank you guys. I think I forgot that the fact that the sum of forces is 0 ( to have equilibrium ) doesn’t mean that nothing happens, we can still have strain inside the body

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