# Tangential Contact forces between objects

My book says that there are two forces two objects experience when they're in contact with eachother. (it uses an example of a cylinder and a parallelepiped that are in contact with eachother) The chapter is about statics.

1) The normal force.

2) A tangential force, tangential to both surfaces that are in contact with eachother. This happens only when the surfaces are rough.

I find this strange because I have never seen the second. The book further states that to be in static equilibrium, these tangential forces can't exceed the maximum force of static friction. I would understand this if the tangential force comes from an external source but I have never seen that two surfaces experience tangential forces due to their contact alone. Can someone clarify this?

• @delivosa: Yes, the surfaces must be trying to slide, and if you use $F < \mu_s N$, the normal force can't be zero as well (i.e. they must be pressing against each other). – user7777777 Oct 3 '18 at 12:36