If you slam on the brakes hard enough, your car tires slide against the road surface and leave skid marks. This is kinetic friction. A wheel rolling on a surface also experiences friction, but not kinetic friction. As FIGURE 6.14 shows, the portion of the wheel that contacts the surface is stationary with respect to the surface, not sliding. The interaction of this contact area with the surface causes rolling friction. The force of rolling friction can be calculated in terms of a coefficient of rolling friction ${\mu }_{\text{r}}:$

FIGURE 6.14 Rolling friction is also opposite the direction of motion

Rolling friction acts very much like kinetic friction, but values of ${\mu }_{\text{r}}$ (see Table 6.1) are much lower than values of ${\mu }_{\text{k}}$. This is why it is easier to roll an object on wheels than to slide it.