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Here's a diagram of a man standing on a ladder that's currently fixed in position.

Ladder diagram

Forces are in equilibrium, and the ladder is static. There are two figures illustrating the forces acting on the ladder:

Force diagrams

Fm and Fg are downward forces created due to the weight of the man and the ladder, respectively. FN1 and FN2 are the normal forces acting on the ladder from the wall and the ground. Ff is the frictional force acting on the ladder from the ground. On figure (2), there is another force, FN3, vertically acting from the wall to the ladder. Can this be considered as the frictional force acting on the ladder from the wall? Apparently the force is marked as another normal force, not frictional.

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  • $\begingroup$ What does "fixed in position" mean? 'Cause "not moving" is covered by the "static" a bit further on... $\endgroup$
    – DJohnM
    Oct 18, 2021 at 5:07

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Imagine that the floor was frictionless. That is, $F_f=0$. If there was also no frictional force between the wall and ladder, the ladder and man would fall to the ground.

This means that $F_{N3}$ is a frictional force. The ladder is pushing against the wall at the top point, so there is a normal force perpendicular to the wall called $F_{N1}$. Since there is friction between the wall and ladder, $F_{N3}$ acts to stop the ladder from slipping and falling (as does the other frictional force $F_{f}$ which acts horizontally). This is why $F_{N3}$ points vertically upward.

Remember that in such situations, normal forces and frictional forces are always perpendicular to each other. In the diagram you have the normal force $F_{N1}$ which is at $90^\circ$ to the wall, and so $F_{N3}$ which is perpendicular to that, should be a frictional force.

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  • $\begingroup$ Can figure (1) considered to be valid? As you mentioned, if there is no frictional force between the wall and the ladder, the ladder an man would fall to the ground. Apparently on figure (1), there's only frictional force acting on the ground, but not on the wall. The ground has friction, while the wall is frictionless. $\endgroup$ Oct 18, 2021 at 3:49
  • $\begingroup$ Yes, it can still be valid because the ground is also providing a frictional force that keeps the ladder from falling. $F_{N3}$ would be a frictional force if there was friction between the wall and ladder like in part (2) of the diagram. $\endgroup$
    – joseph h
    Oct 18, 2021 at 4:06

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