Tangential frictional forces

When two bodies are in contact and there is a tendency for them to slide with respect to each other, a tangential friction force is developed that opposes the motion. For dry surfaces this is called dry friction or coulomb friction. For lubricated surfaces the friction force is called fluid friction, and it is treated in the study of fluid mechanics. Consider a block of weight W resting on a flat surface as shown in Figure 2-5. The weight of the block is balanced by a normal force N that is equal and opposite to the body force. Now, if some sufficiently small sidewise force P is applied (Figure 2-5b) it will be opposed by a friction force F that is equal and opposite to P and the block will remain fixed. If P is increased, F will simultaneously increase at the same rate until... 

The friction coefficient is defined as the ratio of the tangential friction force, resisting relative motion of the surfaces, to the normal load pressing the surfaces together. In the case of high adhesion of wet smooth surfaces, the normal load shall be considered as a sum of the externally applied normal force and the internal adhesion (stiction) between surfaces. For simplification, the normal load is usually considered equal to the externally applied normal force. 

A normal force P acting on an interface between two solids (Fig. 2.11) creates a tangential friction force pP, p being the (dynamic) coefficient of friction. Then, the friction power is... 

Let us consider the uniform deformation of a unit volume of a disperse system under the condition of a steady-state viscoplastic flow or the slow stage of the elastic aftereffect under the action of a shear stress t. The system consists of flbers of length / and diameter d occupying the volume fraction V. The contact between the crossed flbers can be characterized by the average tangential friction force (which in turn characterizes the mean force of resistance to the shear motion in the contact), Ptg- The number of particles, v, per unit volume of the disperse system is... 

The second theory considers the contact between surface asperities to be elastic and when the Gaussian distribution of surface heights of the asperites of the mating counterfaces is taken into account the real area of contact is once again linearly dependent on normal load. In the simplest view of Mclion it is assumed that the tangential friction force is proportional to the real area of contact, such that. 

Now we must consider how the frictional force experienced by this segment is related to the tangential velocity whose components are given by these expressions. 

Figure 5-22b shows this phenomenon. When contact is made between the surface and the rotating shaft, the coulomb friction will induce a tangential force on the rotor. This friction force is approximately proportional to the... 

In addition to gravity, atmospheric forces act on the ascending rocket, which unlike gravity are surface forces. The surrounding atmosphere flows along the surface of the rocket when in flight. A pressure force is exerted perpendicular to each surface element due to the adhesion of air molecules on the surface, while frictional forces arise in a tangential direction, in accordance with the internal friction of the air. 

Friction is defined as the resistance encountered when one body moves tangentially over another and they are in contact. Friction often embraces two classes of relative motion sliding and rolling. In industrial processes, frictional energy is usually dissipated as waste heat. The friction force is represented by F and the friction coefficient by p. Under many sliding conditions, the p for a given pair of materials and fixed conditions of lubrication is mostly constant. The three laws of friction are ... 

Figure 8-12 shows some typical results for platinum against platinum and for steel against steel. The tangential traction coefficient ((i = T/N, where T is the tangential tractive force and N is the normal loading force. The displacements at low values of T are almost entirely irreversible when T is relaxed and hence are due to the plastic shear of asperity junctions. The tangential traction coefficient approaches the coefficient of friction (as measured in the usual way) while the dis-... 

Before monitoring DD curves on PDMS, the actuator and the cantilever were thermally stabilized. The laser spot in contact with the tip was positioned in such a way that tangential forces, due to frictional forces, were minimized. We carried out these measurements on the basis of the above prerequisites so that force measurements could be reliable and comparable. Force-indentation (F-I)... 

To determine the macro-friction properties, the tangential or friction force was measured by a tribometer, and the contact area evolution was recorded by video camera. Friction stress was calculated by dividing each force value by the corresponding contact area. Results of the macro-friction experiments are presented in Table 5.2. 

On sliding hard bodies along other hard bodies, the frictional coefficient M FixjFw is given by the ratio of tangentially active forces to perpendicularly active forces. The frictional coefficient determined in this way lies between 0.15 and 0.5. It is independent of the chemical nature of the bodies, which suggests that it is influenced by surface roughness. 

---