Where does the bending moment exerted by a wall on a beam come from?

[Chozen Juan]

Homework Statement

A cantilever beam in static equilibrium experiences an external force F_ as shown in the diagram. What is the reaction
moment about an axis through the leftmost end of the beam?

Relevant Equations

M_ = r_ x F_

M = r x F
r = 0
∴ M = 0

But this is clearly wrong. For some reason, the "reaction moment" must exist. Why? Where does it come from? More specifically, which force(s) produces the bending moment, and at what distance(s)? Does it come from the reaction force form the wall on the left end of the beam? If so, why does this reaction force produce a moment about that end if it acts through it?

I understand that the wall must exert a reaction moment to ensure static equilibrium. What I do not understand is the FORCE that causes this moment. Moments can't exist without some force acting at some distance (M = r x F) right?

My question, by the way, is a fundamental WHY question.

 

[Chestermiller]

The reaction moment comes for a horizontal stress (force) distribution acting on the left face of the beam (through the thickness of the beam). The bottom part of the beam is in compression and the top part of the beam is in extension. This stress distribution is established by the wall material acting on the portion of the beam sticking into the wall. Over an above the net reaction force on the portion of the beam embedded within the wall, the wall material also establishes a vertical stress distribution on the top and bottom faces of the part embedded within the wall. These stress distributions create no net force, but they do produce a moment. Can you envision what these force distributions on the top and bottom faces of the beam (inside the wall) might look like?

 

[Lnewqban]

... If so, why does this reaction force produce a moment about that end if it acts through it?

Moments can't exist without some force acting at some distance (M = r x F) right?

My question, by the way, is a fundamental WHY question.

What happens if you pull the beam from that wall and connect both via a hinge?
You are absolutely correct, moments are only a concept, the real things producing those are always linear forces combined with levers (solid connections) acting over an imaginary center.