Static friction adjusts itself, up to a limit.

• Friction is the part of the contact force that lies parallel to the surfaces; between the surfaces it opposes relative motion, or impending relative motion.
• Static friction is self-adjusting: it takes whatever value, up to a maximum, is needed to prevent sliding, f_s ≤ μ_s N.
• Its limiting value (f_s)max = μ_s N depends on the nature of the surfaces but not on the area of contact.
• If the applied force is less than the limiting value, static friction equals the applied force and the body stays at rest; it is not automatically μ_s N.
• Once sliding begins, kinetic friction acts: f_k = μ_k N, roughly independent of speed and area of contact. Usually μ_k < μ_s.
• Friction on a body can act along its direction of motion: the friction on a box on the floor of an accelerating truck points forward and is what accelerates the box.
• Rolling friction is much smaller than sliding friction, which is why wheels, ball bearings and lubricants are used to reduce energy loss.
• Friction is not purely a nuisance; walking, driving and braking all depend on it.

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