Kinetic friction

Once two surfaces are genuinely sliding, friction stops adjusting itself and settles at one value, $F_{f,k} = \mu_k F_N$. The AP Physics 1 CED puts the direction precisely: the kinetic friction force is exerted in a direction opposite to the motion of each surface relative to the other surface. Relative to the other surface, not relative to the ground.

Three properties are worth holding onto.

• Contact area does not matter. The CED states that the force of friction between two surfaces does not depend on the size of the surface area of contact, so standing a brick on its end changes nothing.
• Speed does not matter. There is no $v$ anywhere in $\mu_k F_N$, so within the AP model sliding twice as fast produces the same friction force.
• It is usually the smaller of the two frictions, because the coefficient of static friction is typically greater than the coefficient of kinetic friction for a given pair of surfaces.

Kinetic friction is nonconservative. The CED notes that the energy dissipated by friction is typically equated to the force of friction times the length of the path over which the force is exerted, so a longer route between the same two points costs more energy.

The side by side comparison, including what changes at the instant of slipping, is in static vs kinetic friction, and there are problems in the friction practice set.