This can be used in hand-wavy arguments for why friction is proportional to the normal force pressing surfaces together. But why is that friction force also proportional to the normal force? Q: It intuitively feels like the friction force should be directly proportional to the surface area between materials, yet this is never considered in any practical analysis or application. Q: If polishing surfaces decreases friction, then why does polishing metal surfaces make them fuse together? Inertia welders also need smooth surfaces so that the friction from point to point will be constant (you really don’t want anything to catch suddenly, or everyone nearby is in trouble).