4.1.5—The model of a 'smooth' contact

“Smooth” means the contact force is perpendicular to the surface. There is no friction component along the surface, so the unknown contact force is a single normal reaction.

Use the geometry of the surface to choose the normal direction, and do not add a friction force to a smooth-contact free-body diagram.

A particle on a smooth plane has a reaction normal to the plane and weight vertically downward; resolving these gives the motion along the plane.

Smooth does not mean weightless or force-free; it removes friction only.