1. Friction

2. “smooth” surfaces
vs.
“rough” surfaces
smooth
rough

3. In real life, surfaces are not smooth “frictionless” and they do not prevent the motion entirely either ; they are rough to some extent.
Dry friction between rigid bodies.
Fluid friction between layers of fluid.

4. Here, we discuss one type of dry friction.

5. F = static friction force
Fm = max. friction force
Fk = kinetic friction force
Experimentally, Fm N
Fm = µs N
µs = coefficient of static friction
Fk = µk N
µk = coefficient of kinetic friction
≈75% of µs Fk ≈ 0.75 Fm

6. See Table 8-1 (p. 395) in the textbook for typical values of µs for different materials / surfaces.
4 situations are possible , as can be seen from the figure above:
no friction
friction force < Fm [[ p<Fm ]]
Impending motion : F = Fm = µs N = p
Motion : p > Fk = µk N The equations of equilibrium no longer apply.

7. µs = tan ө ((Prove!!))
* Problems involving dry friction:
Checking “equilibrium” ( F< Fm ) [[ forces ]]:
checking sliding.
Finding a force p (impending motion)
Finding µs (impending motion)
Checking “tipping of the rigid body”

8. The friction force acts tangent to the contacting surfaces in a direction opposed to the relative motion or tendency for motion of one surface against another.

9. Remember: *impending motion *slipping (sliding) *tipping
*relative motion………