# FRICTION.pdf - FRICTION MECHANICS OF MACHINES BDA 20303...

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Each surface is not free from ridges and depressions. We can see theridges visible on microscopical examination.When 2 surfaces are in contact with each other, these ridges anddepressions interlock. When one surfaces slides over the other, aforce is introduced. This force retards the motion. It is called force offriction.Friction force is tangential to the surfaces of two bodies at the pointof contact and acts in the opposite direction to that of the relativemotion.
Friction force is required for the successful operation of machinecomponent such as belt, rope drives, clutches, screws, nuts, rollingwheel etc. Thus to preserve the friction force, some laws applies;1) Friction force, F is independent on area and shape of the contactingsurfaces.2) Friction force,F depends on the nature of thesurfaces in contact.μ= Coefficient of friction3) Friction force, F directly proportional to thenormal reaction between surface of contactRNFAccelerationNRF
Consider that a body A of weight (W) is resting on a horizontal plane B, as shownin Figure below. If a horizontal force P is applied to the body, no relative motionwill take place until the applied force P is equal to the force of friction F, actingopposite to the direction of motion. The magnitude of this force of friction is F =μ.W = μ.RN, where RNis the normal reaction. In the limiting case, when themotion just begins, the body will be in equilibrium under the action of thefollowing three forces :1.Weight of the body (W),2.Applied horizontal force (P), and3.Reaction (R) between the body A and the plane B.Limiting angle of frictionFigure 1.0
NNNRRRFtanThe reaction R must, therefore, be equal and opposite to the resultant of W andP and will be inclined at an angle φ to the normal reaction RN.This angle φ isknown as the limiting angle of friction. It may be defined as the angle whichthe resultant reaction R makes with the normal reaction RN. From Fig. 1.0, then;

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