Friction

1. Friction

2. Introduction Friction is a force that resists the movement of two contacting surfaces that slides relative to one another This force always acts tangent to the surface at the points of contact and is directed so as to oppose the possible or existing motion between the surfaces

3. Types of friction • Dry friction: Friction between unlubricated surfaces of bodies in contact • Fluid friction: Occurs when adjacent layers in a fluid are moving in different velocities • Internal friction: Occurs in all solid materials whichare subjected to cyclic loading Click to view animation of types of friction

4. When P is given increasing values that are insufficient to cause the motion, the frictional resistance F increases correspondingly to maintain equilibrium • When the block is on the verge of moving and at this instant, F attainsits maximum available value • Further increase in P causes motion, but surprisingly, the value of F does not stay at its maximum but decreases rapidly to a kinetic value which remains fairly constant Click to view Image

5. Laws of friction If friction is neglected, the reactions are always normal to the surfaces in contact 2. Friction always acts to oppose the relative motion of the free body. It is tangential to the surfaces in contact. The total frictional force that can be developed (which is also the static friction as long as the body is still not moving) is proportional to the normal force transmitted across the surface of contact F is proportional to N the normal force or F = μs.N If motion occurs , the kinetic friction always acts at its constant value and then F = μk. N

6. Angle of friction The particular value of the angle when maximum frictional resistance is acting is defined as the angle of friction Click to view angle of friction Wedge friction A wedge may be defined as a simple machine which is commonly used to transform an applied force into much larger forces, directed approximately at right angles to the applied force Click to view wedge friction

7. Relation between the angle of friction and the angle of incline Angle of inclination is equal to angle of friction, when thebody is just about to slide For different combinations of materials Click to view Image 1 Click to view Image 2 The co efficient of friction will be different, because the angle of incline will change for the body to just slide, for each of the combinations of materials

8. Ladder friction The forces acting on the ladder: • Self weight of the ladder W acting at the centre of gravity of the ladder • Normal reaction offered by the floor Nfacting at A Click to view Image • Friction force offered by the floor Ff acting along the floor • Normal reaction offered by the wall Nwacting at B • Friction force offered by the wall Fwacting along the wall. This is considered negligible if the wall is smooth

9. Belt friction • Belts are extensively used to transmit power from one shaft to another shaft • Belts are extensively used to transmit power from one shaft to another shaft Click to view Image

10. Classification of belt drives

11. Click to view Open belt Drive Click to view Crossed belt Drive Click to view Compound belt Drive

12. Length of the open belt drive Length of the crossed belt drive Ratio of belt tensions Power transmitted by a belt drive