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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.
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.
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 :
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.
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 static friction force will exactly oppose forces applied to an object parallel to a surface up to the limit specified by the coefficient of static friction multiplied by the normal force (). In other words, the magnitude of the static friction force satisfies the inequality: 0 ≤ F s f ≤ μ s f F N . {\displaystyle 0\leq \mathbf {F ...
Kinetic (or dynamic) friction occurs when the ski is moving over the snow. [8] The coefficient of kinetic friction, , is less than the coefficient of static friction for both ice and snow. [9] [10] The force required for sliding on snow is the product of the coefficient of kinetic friction and the normal force: =. [11]
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