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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.
A simple free-body diagram, shown above, of a block on a ramp, illustrates this. All external supports and structures have been replaced by the forces they generate. These include: mg: the product of the mass of the block and the constant of gravitation acceleration: its weight. N: the normal force of the ramp. F f: the friction force of the ramp.
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 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.
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.
Coulomb damping dissipates energy constantly because of sliding friction. The magnitude of sliding friction is a constant value; independent of surface area, displacement or position, and velocity. The system undergoing Coulomb damping is periodic or oscillating and restrained by the sliding friction.
Vibrational energy induced by either kinetic or breakaway friction can cause modal excitation of a subset of the contacting bodies or the vibratory coupling of the multiple bodies, depending on the strength of coupling. Friction noise can be the product of multiple distinct dynamic processes, sliding and stick-slip. Sliding generally leads to ...
The angle of friction, [7] also sometimes called the angle of repose, [8] is the maximum angle at which a load can rest motionless on an inclined plane due to friction without sliding down. This angle is equal to the arctangent of the coefficient of static friction μ s between the surfaces. [8]