5.2.5 Friction

Friction, like the normal force, is a force exerted by a surface. But whereas the normal force is perpendicular to the surface, the friction force is always parallel to the surface. It is useful to distinguish between two kinds of friction:

Kinetic friction, denoted $\vec{f}_{k}$ , appears as an object slides across a surface. This is a force that “opposes the motion,” meaning that the friction force vector $\vec{f}_{k}$ points in a direction opposite the velocity vector $\vec{v}$ (i.e., “the motion”).
Static friction, denoted $\vec{f}_{s}$ , is the force that keeps an object “stuck” on a surface and prevents its motion. Finding the direction of $\vec{f}_{s}$ is a little trickier than finding it for $\vec{f}_{k}$ . Static friction points opposite the direction in which the object would move if there were no friction. That is, it points in the direction necessary to prevent motion.

FIGURE 5.8 shows examples of kinetic and static friction.

The figure illustrates a boy riding on a sled and moving horizontally toward the right.

The figure illustrates a frog on a slope inclined toward the upper right.

Full Alternative Text

NOTE A surface exerts a kinetic friction force when an object moves relative to the surface. A package on a conveyor belt is in motion, but it does not experience a kinetic friction force because it is not moving relative to the belt. So to be precise, we should say that the kinetic friction force points opposite to an object’s motion relative to a surface.