Stiction

Stiction or static friction (also adhesion ( friction (ing ) ) force ) is a force that sliding is prevented from touching the body. The state of sufficient static friction is, sticking and might even include creep. Liability in this context is about as binding as used in chemistry. In the static friction is the connection of bodies by means of adhesion. In contrast, a material connection ( connection of multiple bodies to a body consisting of several substances ) is when, for example, when bonding produced. Furthermore, there is the positive connection, the ( eg passport springs in mechanical engineering ) is caused by the entanglement of surface irregularities.

Forces in liability

Is a prerequisite for the occurrence of static friction that two bodies touch and the contact surface is loaded by an external force to shear. There is an opposite and equal amount force builds up, which prevents relative movement of the two surfaces ( see Figure, Figure 1). Increases the shear force, initially grows also the force ( see Figure, Figure 2). However, this can happen only up to a limit. Exceeds the shear force these static friction limit, they will not be fully compensated by the adhesive force. It is a resultant force, which leads to an acceleration of the body ( as shown in Figure, Figure 3). The body is in motion, static friction acts no. The force that opposes the sliding of the movement, called sliding friction and is generally less than ( see Figure, Figure 4). The difference between static and dynamic friction can excite vibrations ( squeaky brakes) and is responsible for earthquakes; see stick-slip effect.

Calculation of static friction

Because the static friction is highly dependent on material properties and surface properties can only be described in a rough approximation, by simple physical principles. Thereafter, the maximum static friction is proportional to the normal force and independently of the size of the contact surface. The normal force is the force ( in the illustration, for example, the gravitational force) acting against perpendicular to the contact surface, the contact force. Thus, the following applies:

The constant of proportionality is called the coefficient of static friction or stiction number, see the article coefficient of friction for tabulated values ​​, and other details. With increasing pressure, the transferable shear stress increases only up to the yield point.

Unlike other forms of friction

Frictionally a dissipative process is usually understood, that a " power scavenging " process, in which the (kinetic ) energy is converted to heat, with an increase of entropy. In contrast to the rolling and sliding friction this is at the static friction is generally not the case, since a force that does not cause movement of the body, does no work. For this reason, the designation as friction and the classification as such by Physikdidaktikern is strongly criticized.