Static vs Kinetic Friction

Static friction f_s adjusts to match applied force, up to a maximum: f_s ≤ μ_s N.

Once motion begins, kinetic friction takes over: f_k = μ_k N (constant).

Generally μ_k < μ_s — which is why it's harder to start moving than to keep moving.

Friction acts parallel to the surface, opposing (actual or impending) relative motion.