Static friction

Representation of the forces when pushing a box that initially sticks to the floor.

Static friction or static friction (also static (frictional) force ) is a force that prevents bodies from sliding . The condition of sufficient static friction is called sticking and may include creep , i.e. the gradual deformation of the joined parts. In this context, adhesion is used in much the same way as binding in chemistry. Static friction is the connection of bodies through a force fit . In contrast, when gluing , for example, a material bond (connection of several bodies to one body consisting of several substances) is established.

Forces in liability

The prerequisite for the occurrence of static friction is that two bodies are touching and the contact surface is loaded by an external force on shear . An opposing force of the same magnitude builds up , which prevents a relative movement of the two surfaces (see figure, Figure 1). If the shear force increases , the force also increases initially (see figure, Figure 2). However, this only happens up to a limit value . If the shear force exceeds this static friction limit, it is no longer fully compensated by the adhesive force. A resulting force remains, which leads to an acceleration of the body (see figure, figure 3). If the body is in motion, there is no longer any static friction. The force that counteracts the movement when sliding is called sliding friction and is generally less than (see figure, Figure 4). The rapid change between static and dynamic friction can stimulate vibrations (squeaking door or brakes) and is responsible for earthquakes ; see stick-slip effect . ${\displaystyle {\vec {F}}}$ ${\displaystyle {\vec {F}}_{\mathrm {H} }=-{\vec {F}}}$ ${\displaystyle {\vec {F}}}$ ${\displaystyle {\vec {F}}_{\mathrm {H} }}$ ${\displaystyle {\vec {F}}_{\mathrm {H} }^{\text{krit.}}}$ ${\displaystyle {\vec {F}}_{\mathrm {R} }}$ ${\displaystyle {\vec {F}}_{\mathrm {H} }^{\text{krit.}}}$

Calculation of static friction

Weight and normal force of a box on a flat surface

Since static friction depends heavily on material properties and surface properties, it can only be described as a rough approximation using simple physical laws. According to this, the maximum static friction is proportional to the normal force and regardless of how large the contact area is. The normal force is the force that counteracts the contact force perpendicular to the contact surface (e.g. the weight force in the figure opposite). Thus: ${\displaystyle F_{\mathrm {H} }^{\text{krit.}}}$ ${\displaystyle F_{\mathrm {N} }}$ ${\displaystyle A}$ ${\displaystyle F_{\mathrm {G} }}$

${\displaystyle \left|{\vec {F}}_{\mathrm {H} }^{\text{krit.}}\right|=\mu _{\mathrm {H} }\cdot \left|{\vec {F}}_{\mathrm {N} }\right|}$

The constant of proportionality is called the coefficient of static friction or coefficient of static friction , see the article coefficient of friction for tabulated values ​​and further details. With increasing contact pressure, the transferable shear stress only increases up to the yield point . ${\displaystyle \mu _{\mathrm {H} }}$

Difference from other forms of friction

Under frictional a commonly dissipative process understood, so an "energy-sapping" process, in which the ( kinetic ) energy with an increase of the entropy in heat is converted. In contrast to rolling and sliding friction, this is generally not the case with static friction, since a force that does not move the body does no work . For this reason, the designation as friction and the classification as such is criticized by some physics educators .

literature

• Wolfgang Stamm: Modeling and simulation of multi-body systems with surface friction contacts , Diss., KIT , 2009, limited preview in the Google book search.
• Valentin L. Popov: Contact Mechanics and Friction: A text and application book from nanotribology to numerical simulation , Springer-Verlag, 2009, ISBN 978-3-540-88836-9 , limited preview in Google book search.

Individual evidence

1. ↑ Demtröder: Experimentalphysik 1 . S. 130 ( google.de ).
2. ↑ Norbert Jost: Friction and wear - a brief introduction to materials . S. 6 ( google.de ).