Static Friction

Another type of frictional force is static friction, otherwise known as stiction. Like all friction, it acts to resist the motion of an object moving over a surface. Unlike kinetic friction, however, static friction acts to resist the start of motion.

Static friction is friction between two objects that are not moving relative to each other. This frictional force is what prevents a parked car from sliding down a hill, for example. Before an object at rest on a surface can move, it must overcome the force of static friction.

Static friction originates from multiple sources. For any given material on another material of the same composition, friction will be greater as the material surfaces become rougher (consider sandpaper) on the macroscopic level. Additionally, intermolecular forces can greatly influence friction when two materials are put into contact. When surface area is below the micrometer range, Van der Waals' forces, electrostatic interactions and hydrogen bonding can cause two materials to adhere to one another. A force is required to overcome these interactions and cause the surfaces to move across one another.

Like kinetic friction, the force of static friction is given by a coefficient multiplied by the normal force. The normal force is the force of the surface pushing up on the object, which is usually equal to the object's weight. The coefficient of static friction is usually greater than the coefficient of kinetic friction and is usually represented by $\mu_s$.

Putting these elements together gives the maximum force of static friction as:

$F_{s} = \mu_sF_n$

In general, the force of static friction can be represented as:

$F_s \leq \mu_s F_n$

As with all frictional forces, the force of friction can never exceed the force applied. Thus the force of static friction will vary between 0 and $_sF_n$ depending on the strength of the applied force. Any force smaller than $_sF_n$ attempting to slide one surface over the other is opposed by a frictional force of equal magnitude and opposite direction. Any force larger than that overcomes the force of static friction and causes sliding to occur. The instant sliding occurs, static friction is no longer applicable—the friction between the two surfaces is then called kinetic friction.

Static Friction

To move a block at rest on a surface, a force must be applied which is great enough to overcome the force of static friction.