Kinetic Friction: Acts on moving surfaces, constant magnitude. | Static Friction: Acts on surfaces at rest, self-adjusting up to a maximum value.

Coefficient of Static Friction ($\mu_s$): Usually greater than $\mu_k$, represents the force needed to *start* motion. | Coefficient of Kinetic Friction ($\mu_k$): Usually smaller than $\mu_s$, represents the force needed to *maintain* motion.

At Rest: Static friction balances the component of gravity pulling the object down the plane. | Sliding: Kinetic friction opposes the motion down the plane, but the net force results in acceleration.

Kinetic Friction: $F_k = \mu_k N$ (always equal to this value when sliding) | Maximum Static Friction: $f_{s,max} = \mu_s N$ (static friction can be any value from zero up to $f_{s,max}$)

Kinetic Friction: Magnitude of kinetic friction force remains constant regardless of the contact area between the surfaces. | Static Friction: The area of contact does not affect the maximum static friction force.

A force that opposes the motion of surfaces sliding against each other.

A force that prevents surfaces from slipping when they are at rest relative to each other.

An empirical value that depends on the materials of the contacting surfaces, used to calculate kinetic friction.

An empirical value that depends on the materials of the contacting surfaces, used to calculate the maximum static friction force.

The force acting perpendicular to the surface of contact.

Increasing the normal force increases the magnitude of the kinetic friction force ($F_k = mu_k F_n$).

The object begins to move, and kinetic friction replaces static friction.

It becomes harder to start moving the object because the maximum static friction force increases.

The object will accelerate according to Newton's Second Law ($F = ma$).

The object remains at rest, and the static friction force equals the applied force.