How does stress "propagate" in beams?

Question

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.

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

Claudio Saspinski · Answer

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.

Neo · Answer

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

How does stress "propagate" in beams?

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