which is the kinetic friction is smaller
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Answer:
With most of the adhesion and abrasion being overcome to induce movement, the resistance to motion between the surfaces is reduced, and the surfaces are now moving under the influence of kinetic friction, which is much lower than static friction.
Kinetic friction that arises in the region of slip is essentially an irreversible process. It is not surprising, therefore, that shear traction in the contact during tangential unloading differs from that during loading. We start with the situation described in the previous section where solids under a constant normal load are subjected to a tangential load T = T1, such that T1 < μp. Subsequently, if the bodies are tangentially unloaded by reducing the tangential force to T2 where T2 < T1, the unloading step may be viewed as reversed loading by force ΔT = T2 - T1 < 0 (directed opposite to T1). The shear traction resulting from this additional loading may be superposed on the existing traction due to T1. For a finite coefficient of friction, the addition of ΔT results in slip in the opposite direction (counterslip), again starting from the contact boundary and extending to the concentric inner circle of radius b given by: b = a{1 - (T1 - T2)/(2μP)}1/3. The shift δ2 in the unloaded situation becomes
Kinetic friction is used to describe the changing resistance in this slip process. The friction coefficient μ, which has been determined experimentally for sliding contacts, provides an average of both static and kinetic resistance. Only when the difference between the static friction coefficient μs and the kinetic friction coefficient μk is small, is the expression μs used in this chapter useful in understanding the value of friction coefficient obtained by a sliding test. The friction coefficient μ obtained under the conditions of monolayer lubrication of oil in Fig. 3.9 illustrates this. For multiple junctions under a constant load, the local load on each one junction changes during relative motion caused by external tangential force. The observed maximum resistance in sliding of such multiple junctions between contact surfaces is a statistical value of all junctions and it is not for the static friction coefficient μs defined by Eq. [3.12].
(1) Static friction
It is the friction, experienced by a body, when at rest.
(2) Dynamic friction
It is the friction, experienced by a body, when in motion. The dynamic friction is also called kinetic friction and is less than the static friction. It is of the following three types:
(a) Sliding friction
It is the friction, experienced by a body, when it slides over another body.
(b) Rolling friction
It is the friction, experienced between the surfaces which has balls or rollers interposed between them.
(c) Pivot friction
It is the friction, experienced by a body, due to the motion of rotation as in case of foot step bearings.