Statics focuses on dry friction. Dry friction occurs between two contact surfaces and always acts in the opposite direction to the relative movement of the contact surfaces. We can represent this distributed force as a concentrated frictional force. Depending on the situation, dry friction can keep the object in balance, accelerate or slow down. In statics, we only deal with objects in balance; The acceleration and deceleration of objects are dealt with in a later course. The origin of kinetic friction at the nanoscale can be explained by thermodynamics. [46] When sliding, a new surface forms at the back of an actual sliding contact and the existing surface disappears at the front. Since all surfaces contain thermodynamic surface energy, work must be devoted to creating the new surface and the energy is released as heat during surface removal. Therefore, a force is required to move the back of the contact, and frictional heat is released at the front.

Depending on the situation, the calculation of the normal force N {displaystyle N} may involve forces other than gravity. If an object is on a flat surface and is exposed to an external force P {displaystyle P} that tends to slide, then the normal force between the object and the surface is only N = m g + P y {displaystyle N=mg+P_{y}} , where m g {displaystyle mg} is the weight of the block and P y {displaystyle P_{y}} is the descending component of the external force. Before slipping, this frictional force is F f = − P x {displaystyle F_{f}=-P_{x}} , where P x {displaystyle P_{x}} is the horizontal component of the external force. Thus, F f N {displaystyle F_{f}leq mu N} ≤ μ in general. Slippage does not begin until this frictional force reaches the value F f = μ N {displaystyle F_{f}=mu N}. Until then, friction is what it needs to be to establish balance so that it can simply be treated as a reaction. Elastic deformation in solids is a reversible change in the internal molecular structure of an object. Stress does not necessarily cause lasting changes. When deformation occurs, the internal forces are opposed to the applied force. If the applied tension is not too great, these opposing forces can completely resist the applied force, allowing the object to take on a new state of equilibrium and return to its original shape when the force is removed. This is called elastic deformation or elasticity. The use of these vertical vectors is most convenient for block and corner problems and can be applied equally in all three phases of friction (static but not threatening, imminent movement and kinetic friction).

A plastic box rests on a steel beam. One end of the steel beam is lifted slowly, increasing the angle of the surface until the box begins to slide. If the box starts to slide, if the beam is at an angle of 41 degrees, what is the static coefficient of friction between the steel beam and the plastic box? John Leslie (1766-1832) noted a weakness in the views of Amontons and Coulomb: if friction occurs, if there is a weight that is pulled up the inclined plane of successive bumps, why is it not compensated by descending the opposite slope? Leslie was equally skeptical of the membership role proposed by Desagulier, who, by and large, should have the same tendency to accelerate as to delay the movement. [12] According to Leslie, friction should be considered a time-dependent process of flattening and backflow of bumps that creates new obstacles in previous cavities. Coulomb friction F f {displaystyle F_{mathrm {f} },} can take any value from zero to μ F n {displaystyle mu F_{mathrm {n} },}, and the direction of frictional force against a surface is opposite to the movement that the surface would undergo without friction. In the static case, the frictional force is exactly what it should be to prevent movement between surfaces; It balances the net force that tends to cause such a movement. In this case, the Coulomb approximation does not provide an estimate of the actual frictional force, but a threshold value for that force above which motion would begin. This maximum force is called traction. The key to deciding what type of friction is appropriate for a particular problem depends on the specifics of the problem statement.

Most combined dry materials have coefficient of friction values between 0.3 and 0.6. Values outside this range are rarer, but Teflon, for example, can have a coefficient of only 0.04. A value of zero would mean no friction at all, an elusive property. Rubber that comes into contact with other surfaces can result in friction coefficients of 1 to 2. It is sometimes claimed that μ is always < 1, but this is not true. Whereas in most relevant applications 1 μ <, a value greater than 1 simply means that the force required to push an object along the surface is greater than the normal force of the surface on the object. For example, silicone rubber or surfaces coated with acrylic rubber have a coefficient of friction that can be significantly higher than 1. Mention of the body moving at a constant speed under the action of friction.

(You will learn how to solve friction problems with a non-constant speed in Dynamics.) Dry friction resists the relative lateral movement of two solid surfaces in contact. The two dry friction regimes are “stiction” between stationary surfaces and kinetic friction (sometimes called sliding friction or dynamic friction) between moving surfaces. The frictional forces acting on the motorcycle in Figure 9.1.2 are more complicated. Both wheels rotate clockwise, but the rear wheel is driven by the motor and chain, while the front wheel is turned by the friction of the road. The frictional force on the rear tire acts to the right, allowing the bike to maintain or accelerate speed. The dry friction on the front tire acts to the left and slows down the movement of the motorcycle. The object of research in the 20th century. In the nineteenth century, the goal was to understand the physical mechanisms behind friction. Frank Philip Bowden and David Tabor (1950) showed that at the microscopic level, the actual contact surface between surfaces is a very small fraction of the apparent surface. [14] This actual contact surface, caused by bumps, increases with pressure. The development of the atomic force microscope (circa 1986) allowed scientists to study friction at the atomic scale,[13] showing that at this scale, dry friction is the product of the shear stress between the surface and the contact surface.