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Kinetic friction, also known as dynamic friction or sliding friction, occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μ k , and is usually less than the coefficient of static friction for the same materials.
Static coefficient of friction (SCOF) testing has always been unreliable for assessing safety in the wet condition, so any reliable slip resistance test will be measuring the available slip resistance to someone who is moving (dynamic) across the floor, and therefore will be assessing dynamic coefficient of friction (DCOF).
μ (some authors use the symbol η) is the dynamic viscosity (Pascal-seconds, kg m −1 s −1); R is the radius of the spherical object (meters); is the flow velocity relative to the object (meters per second). Note the minus sign in the equation, the drag force points in the opposite direction to the relative velocity: drag opposes the motion.
The proportionality coefficient is the dimensionless "Darcy friction factor" or "flow coefficient". This dimensionless coefficient will be a combination of geometric factors such as π, the Reynolds number and (outside the laminar regime) the relative roughness of the pipe (the ratio of the roughness height to the hydraulic diameter).
In systems with significant nonuniform stress fields, the macroscopic static friction coefficient depends on the external pressure, system size, or shape because local slip occurs before the system slides. [18] The following table shows the values of the static and dynamic friction coefficients for common materials:
The load then starts sliding, and the friction coefficient decreases to the value corresponding to load times the dynamic friction. Since this frictional force will be lower than the static value, the load accelerates until the decompressing spring can no longer generate enough force to overcome dynamic friction, and the load stops moving.
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 ...
where η is the dynamic viscosity of the fluid, ... The graphs plotted by Martens show the coefficient of friction either as a function of pressure, speed or ...
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