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Sliding friction (also called kinetic friction) is a contact force that resists the sliding motion of two objects or an object and a surface. Sliding friction is almost always less than that of static friction; this is why it is easier to move an object once it starts moving rather than to get the object to begin moving from a rest position.
The friction coefficient is an empirical (experimentally measured) structural property that depends only on various aspects of the contacting materials, such as surface roughness. The coefficient of friction is not a function of mass or volume. For instance, a large aluminum block has the same coefficient of friction as a small aluminum block.
Table of static and dynamic friction coefficients most used Contact surfaces Static friction Dynamic friction Wood–wood 0.25–0.5 0.2 Wood–cardboard 0.32 0.23 Ice–ice 0.1 0.02 Scioled wood ski–snow 0.04 0.04 Glass–glass 0.9–1.0 0.4 Steel–steel (smooth) 0.6 0.6 Steel–steel (lubricated) 0.09 0.05 Steel–ice 0.1 0.05
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Kinetic friction on the other hand, occurs when two objects are undergoing relative motion, as they slide against each other. The force F k exerted between the moving objects is equal in magnitude to the product of the normal force N and the coefficient of kinetic friction μ k: | | =. Regardless of the mode, friction always acts to oppose the ...
heat transfer, fluid dynamics (change in internal energy versus kinetic energy) [12] Fanning friction factor: f: fluid mechanics (fraction of pressure losses due to friction in a pipe; 1/4th the Darcy friction factor) [13] Fourier number: Fo =
This theory is exact for the situation of an infinite friction coefficient in which case the slip area vanishes, and is approximative for non-vanishing creepages. It does assume Coulomb's friction law, which more or less requires (scrupulously) clean surfaces. This theory is for massive bodies such as the railway wheel-rail contact.
The former is concerned with static friction (also known as "stiction" [3]) or "limiting friction", whilst the latter is dynamic friction, also called "sliding friction". For steel on steel, the coefficient of friction can be as high as 0.78, under laboratory conditions, but typically on railways it is between 0.35 and 0.5, [4] whilst under ...