Friction

FRICTION

Friction

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

Friction

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 which are subjected to cyclic loading

Click to view animation of types of friction

Friction

• 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 attains its 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

FrictionLaws of friction

1. 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.

3. 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

4. If motion occurs , the kinetic friction always acts at its constant value and then F = μk. N

Friction

Angle of friction

The particular value of the angle when maximum frictional resistance is acting is defined as the angle of friction

FTan

R

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

Click to view angle of friction

Friction

Relation between the angle of friction and the angle of incline

Angle of inclination is equal to angle of friction, when the body is just about to slide

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

For different combinations of materials Click to view Image 1

Click to view Image 2

Friction

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 Nf acting at A

• Friction force offered by the floor Ff acting along the floor

• Normal reaction offered by the wall Nw acting at B

• Friction force offered by the wall Fw acting along the wall. This is considered negligible if the wall is smooth

Click to view Image

Friction

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

Friction

Classification of belt drives

Friction

Click to view Open belt Drive

Click to view Crossed belt Drive

Click to view Compound belt Drive

Friction

Length of the open belt drive

Length of the crossed belt drive

21 21 2 2o

r rL r r x

x

21 21 2 2c

r rL r r x

x

Ratio of belt tensions

1

2

Te

T

Power transmitted by a belt drive

1 2P T T v