Which force acts parallel to a surface and resists sliding motion? Friction acts parallel to a surface and resists relative sliding or the tendency to slide.
Dry friction arises where solid surfaces contact, though friction also occurs between fluid layers and within deforming materials. For dry surfaces, static friction prevents sliding from beginning, while kinetic friction acts after sliding starts. The force is tangential to the contact surface, unlike the normal force, which acts perpendicular to it.
The familiar Coulomb model gives a maximum static friction of roughly μsN and kinetic friction of roughly μkN, where N is the normal force and μ is a coefficient determined experimentally. Static friction is not always at its maximum: it adjusts to match the force trying to cause motion, up to its limit.
A common misconception is that friction always points opposite an object's overall motion. More precisely, it opposes relative sliding or impending sliding at the contact. That is why static friction can propel a car forward at the tire–road contact.