I was watching a lecture discussing a mechanical problem. I understand the proposed solution so my question is in the setup. This is the problem:
A ladder with given length and mass is leaning on a wall without friction. The other end of the ladder is on the ground where there is friction with known coefficient. For what angles of tilt the ladder will be stable?
A better description of the problem can be found here
In the setup, the wall exerts force on the ladder. While this interpretation leads to solution that agrees with reality, I don't understand why there is such force (as shown at 4:18).
If I put two boxes on a table side to side, they won't exert forces on each other (ignoring the gravitational force between them). How is a ladder leaning on a wall different?