Characteristics of friction with sketch

Friction is the resisting force that acts tangentially at the surfaces of two bodies in contact and opposes their relative motion, or the tendency to motion.

Characteristics

1. It acts along the common tangent to the contact surfaces and opposes the motion (or its tendency).
2. It is a passive, self-adjusting force: it increases with the applied force from zero to a limiting value, F ≤ μₛN.
3. The limiting value is proportional to the normal reaction: F_max = μₛN.
4. It is independent of the area of contact.
5. It depends on the nature, roughness and condition (dry, lubricated) of the surfaces.
6. Kinetic friction is smaller than limiting static friction (μₖ < μₛ), and is almost independent of the speed.
7. It converts mechanical energy into heat and causes wear.

          W
          |
   P ---> [ block ] ---> motion tendency
   <------ F ------ (friction opposes)
 =========================  rough surface
          ^ N

The resultant of N and F makes an angle with the normal; its maximum value is the angle of friction φ = tan⁻¹ μₛ.