Types of friction

Depending on whether, and how, one surface is actually moving relative to another, friction is classified into three distinct types.

1. Static friction. Suppose a wooden block rests on a horizontal surface (Fig. 6.10), and a small horizontal force $F$ is applied to it. If the block does not move, it is because the frictional force between the block and the surface exactly balances $F$ — this balancing frictional force, present whenever the body remains at rest under an applied force, is called the force of static friction, $F_s$. As the applied force $F$ is gradually increased from a very small value, the force of static friction increases right along with it, always adjusting itself to exactly match $F$, which is why static friction is described as a self-adjusting force; if the direction of the applied force is reversed, the direction of the static friction reverses too. This self-adjustment continues only up to a certain maximum: at a critical applied force $F_{max}$, the object finally starts to move, and this maximum value of static friction, reached just before sliding begins, is called the limiting force of friction, $F_L$. For any $F < F_{max}$, the force of static friction simply equals $F$; once $F \geq F_{max}$, kinetic friction takes over instead. Because static friction opposes the impending motion — the motion that would occur if there were no friction — under the applied force, it is sometimes described as opposing impending relative motion rather than actual motion.

The laws governing static friction are:

1. The limiting force of static friction $F_L$ is directly proportional to the normal reaction $N$ between the two surfaces in contact: $F_L \propto N$, so $F_L = \mu_s N$, where the constant of proportionality $\mu_s$ is called the coefficient of static friction, defined as $\mu_s = F_L/N$ (Table 6.4 lists values for common surface pairs).
2. The limiting force of friction is independent of the apparent (visible) area of contact between the two surfaces, provided the normal reaction stays the same.
3. The limiting force of friction depends on the materials in contact and the nature (roughness, treatment) of their surfaces.

2. Kinetic friction. Once the block actually begins sliding, the frictional force it experiences typically drops — the force needed to keep a body sliding steadily is smaller than the force that was needed to first get it moving. Friction that comes into play once one surface is actually sliding steadily over another is called kinetic friction (or dynamic friction). Its laws parallel those of static friction: the force of kinetic friction $F_k$ is directly proportional to the normal reaction, $F_k = \mu_k N$, where the constant $\mu_k$ is the coefficient of kinetic friction, $\mu_k = F_k/N$ (Table 6.5); it is independent of the shape and apparent area of the surfaces in contact; it depends on the nature and material of the surfaces in contact; and its magnitude is independent of the relative sliding velocity between the two surfaces, provided that velocity is neither too large nor too small.

3. Rolling friction. A body's motion over a surface is called rolling motion if its point of contact with the surface keeps changing continuously as it moves (as with a wheel or ball). The friction between two bodies in contact when one is rolling over the other is called rolling friction. For the same pair of surfaces, the ordering is always: static friction $>$ kinetic friction $>$ rolling friction — rolling friction is the smallest of the three, which is exactly why ball bearings (which convert sliding/kinetic friction into rolling friction) are deliberately used inside machines to reduce friction and improve efficiency. …