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• # Kinetic friction

Posted by on March 17, 2023 at 1:49 pm

Kinetic friction, also known as dynamic friction, occurs when two objects are already in relative motion to each other and rub together. The coefficient of kinetic friction is typically denoted as $$\mu_k$$, and is usually less than the coefficient of static friction for the same materials. The formula of dynamic friction force is structurally identical to that of static friction force.

$F_k=\mu_k\cdot F_{\perp}$

With the same materials and surfaces in contact, the dynamic friction coefficient has lower values than the static one. In general, therefore:

$\mu_k<\mu_s$

If the external force exerted on the moving body is less than the kinetic friction, the body slows down in its motion; on the contrary, it accelerates. If, however, the two forces are perfectly equal in value, the resultant force will be null and therefore the body will move with uniform rectilinear motion.

The origin of kinetic friction at the nanoscale can be explained by thermodynamics. Upon sliding, new surface forms at the back of sliding true contact, and the existing surface disappears at the front of it. Since all surfaces involve the thermodynamic surface energy, work must be spent in creating the new surface, and energy is released as heat in removing the surface. Thus, a force is required to move the back of the contact, and frictional heat is released at the front.

replied 1 week, 6 days ago 1 Member · 1 Reply
• ### Encyclios

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March 17, 2023 at 1:49 pm

The dynamic manifestation of friction

Between the surfaces of two bodies in contact develops a tangential force able to oppose their relative motion. Its dynamic manifestation is, therefore, the deviation (of a certain friction angle) of the straight line of action of the mutual contact force from the perpendicular, on the side of which the tangential component opposes the relative motion of the body to which it is applied (with respect to the other).

Considering the contact between two solid bodies, and assuming a punctiform contact between the respective surfaces (for example between two very small spheres), the contact force exchanged between the two bodies passes through the common contact point and its straight line of action can assume an inclination of various sizes with respect to the contact normal (of the tangent in the common point). This inclination is not determined only by the geometry of the bond.

The mutual force can be distinguished from the perpendicular component, whose straight line of action is the contact perpendicular, and the tangential component, lying in the tangent contact plane. It is shown that for a given perpendicular component, there is a maximum for the tangential component of the contact force, i.e. a maximum of the inclination of the force itself on the perpendicular; this maximum value depends on the conditions that characterize the contact locally.

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