 The coefficient of static friction is typically denoted as ?s. The initial force to get an object moving is often dominated by static friction. The coefficient of friction (also known as the frictional coefficient) is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. Coefficients of friction need not be less than 1—under good conditions, a tire on concrete may have a coefficient of friction of 1.7. Lubricants to overcome friction need not always be thin, turbulent fluids or powdery solids such as graphite and talc; acoustic lubrication actually uses sound as a lubricant. A value of 0.0 would mean there is no friction at all. Rubber in contact with other surfaces can yield friction coefficients from 1.0 to 2.0. Limiting friction is the maximum value of static friction, or the force of friction that acts when a body is just on the verge of motion on a surface. On the other hand, friction also causes wear and tear on the materials in contact. The static friction is in most cases higher than the kinetic friction. The coefficient of kinetic friction is typically denoted as ?k, and is usually less than the coefficient of static friction. Friction is the force that opposes the relative motion or tendency of such motion of two surfaces in contact. A sliding hockey puck comes to rest due to friction as its kinetic energy changes into heat. Devices such as ball bearings or rollers can change sliding friction into much smaller rolling friction by reducing the points of contact on the object. The science of friction and lubrication is called tribology. According to the law of conservation of energy, no energy is destroyed due to friction, though it may be lost to the system of concern. The force of friction is always exerted in a direction that opposes movement (for kinetic friction) or potential movement (for static friction) between the two surfaces. The coefficient of friction, when multiplied by the reaction force on the object by the contact surface, will give the maximum frictional force opposing sliding on the object. Friction between solid objects and fluids (gases or liquids) is called fluid friction. Rolling friction occurs when one object "rolls" on another (like a car's wheels on the ground). The work done by friction can translate into deformation and heat that in the long run may affect the surface's specification and the coefficient of friction itself.

image: i.ytimg.com When the surfaces are adhesive, Coulomb friction becomes a very poor approximation (for example, transparent tape resists sliding even when there is no normal force, or a negative normal force). The coefficient of rolling friction is typically denoted as ?r. To minimize work against kinetic friction, the movement is performed at the lowest velocity that's practical. Sliding (kinetic) friction and static friction are distinct concepts. where R is the normal reaction force acting between the interface and the object and is the coefficient of limiting friction. Most dry materials in combination give friction coefficient values from 0.3 to 0.6. Magnetically attractive surfaces can have very large friction coefficients, and, theoretically, glued or welded together surfaces have infinite friction coefficients. A common way to reduce friction is by using a lubricant, such as oil or water, that is placed between the two surfaces, often dramatically lessening the coefficient of friction. The coefficient of friction is an empirical measurement—it has to be measured experimentally, and cannot be found through calculations. You have to pull with a force greater than the maximum value of friction to move the object. Static friction occurs when the two objects are not moving relative to each other (like a book on a desk). Friction can, in some cases, cause solid materials to melt. Frictional force in mechanics is the force which opposes the relative motion of an object. The coefficient of friction (also known as the frictional coefficient) is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together.