Kinetic friction defined

The friction coefficient can be measured in two ways the static friction coefficient Qus) and the dynamic or kinetic friction coefficient (fikX The static friction coefficient is defined as the ratio of the force required to initiate relative movement and the normal force between the surfaces the dynamic or kinetic friction coefficient is defined as the ratio of the friction force to the normal force when the two surfaces are moving relative to each other. For simplicity, much of the research has focused on the dynamic friction coefficients wherein the two surfaces move at a relative constant velocity. Most of the friction studies on skin have dealt with the dynamic friction coefficient and the subscript k is usually dropped. This overview references the dynamic coefficient of friction unless otherwise noted. 

Two types of friction are commonly measured and calculated. The static friction Fj is defined as the minimum lateral force needed to initiate sliding of one object over a second, while the kinetic friction Fk v) is the force needed to maintain sliding at a steady velocity v. Observation of static friction implies that the contacting solids have locked into a local free-energy minimum, and Fj represents the force needed to lift them out of this minimum. It is a threshold rather than an actual force acting on the system, and it limits lateral motion in any direction. No work is done by the static friction, since no motion occurs. The kinetic friction is intrinsically related to dissipation mechanisms, and it equals the work done on the system by external forces divided by the distance moved. 

N (7-6) where is the negative of the tangential force required to keep the slider moving at constant velocity. Equation 7-6 is a formal statement defining the coefficient of kinetic friction and also the expression of a phenomenological fact. Its counterpart for the coefficient of static friction is... 

It is noted that the kinetic viscosity, defined by (4.109) and the dilute regime viscosity (4.113), dominates in the dilute regime. The collisional viscosity, defined by the linear sum of the first (4.130), second (4.148) and the bulk viscosity (4.146) parts, usually dominate in the dense high shear flow regime. The frictional viscosity, defined by (4.156), normally dominates for the dense low shear flow regime. 

In other words, it is assumed here that the particles are surrounded by a isotropic viscous (not viscoelastic) liquid, and is a friction coefficient of the particle in viscous liquid. The second term represents the elastic force due to the nearest Brownian particles along the chain, and the third term is the direct short-ranged interaction (excluded volume effects, see Section 1.5) between all the Brownian particles. The last term represents the random thermal force defined through multiple interparticle interactions. The hydrodynamic interaction and intramolecular friction forces (internal viscosity or kinetic stiffness), which arise when the macromolecular coil is deformed (see Sections 2.2 and 2.4), are omitted here. 

RELATION BETWEEN SKIN FRICTION AND WALL SHEAR. Equation (4.29) can be written over a definite length AL of the complete stream. In Chap. 4, Ap was defined as p — p , but usually (though not always) > p and thus p — p is usually negative. The term Ap is commonly used for pressure drop, i.e., Pa Pb, and this terminology is employed in this and subsequent chapters. Here, then, Pa - P> Pb = P Zb = 0, and the two kinetic-energy terms cancel. Also, the only kind of friction is skin friction between the wall and the fluid stream. Denote this by hj-. Then Eq. (4.29) becomes... 

In turbulent flow the frictional pressure drop down the drill pipe must be calculated from equations that have been determined empirically. The commonest method in drilling fluid hydraulics (91-93) is to use a friction factor/, the so-called Fanning friction factor, defined by the ratio of the wall shear stress rw to the kinetic energy per unit volume of the flowing fluid 0.5 pvm2 (94)... 

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