Kinetic friction occurs only when two surfaces in contact are already sliding relative to each other. It always acts opposite the direction of that relative sliding motion.
The coefficient of kinetic friction, μₖ, depends only on the material properties of the two surfaces in contact — rubber on concrete has a different μₖ than wood on ice, regardless of the size or shape of either object. Normal force itself is just the perpendicular component of the contact force a surface exerts, always directed away from the surface.
Static friction can occur between two surfaces that are not moving relative to each other. Unlike kinetic friction, static friction isn't a single fixed value — it adopts whatever magnitude and direction is needed to prevent the surfaces from slipping or sliding.
Slipping and sliding both describe surfaces moving relative to each other — the moment that starts happening, you've left the static regime and kinetic friction takes over.
Try the classic friction-response experiment below: push a stationary block harder and harder, and watch friction match your push exactly — right up until it can't.
A 10 kg block on a horizontal surface. Push harder and harder — friction matches your push exactly, right up until it can't anymore, then drops to the (typically lower) kinetic value and stays there.
The block is still stationary — static friction is exactly matching your applied force.
There's a hard ceiling on how large static friction can get for a given pair of surfaces and normal force:
Push (or tilt) past this maximum, and the surfaces start to slide — at which point you switch entirely from static friction's rulebook to kinetic friction's fixed value.
This maximum is exactly what the classic "raise the ramp until it slips" experiment measures. Try it yourself below, using the incline block from this unit's header art.
The incline block from this unit's header, on a surface with no other forces besides gravity, normal force, and friction. Slowly raise the angle — find the point where static friction can no longer hold it.
This is exactly the classic experiment for measuring μₛ: raise the surface until the object just barely starts to slide, then tan θ(critical) = μₛ.
A block on an adjustable incline just barely starts to slide when the incline reaches 22°. Find the coefficient of static friction between the block and the incline.
The same incline and block from above continues to accelerate down the slope once sliding begins, reaching 2.1 m/s² at the same 22° angle. Find μₖ, and confirm it's less than the μₛ found above.