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
Friction
Laws 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
T an 
Wedge friction
F
R
Click to view angle of 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
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
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
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
Click to view Image
• 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
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
L o    r1  r2  
 r1  r2 
2
 2x
x
Length of the crossed belt drive
L c    r1  r2  
 r1  r2 
2
 2x
x
Ratio of belt tensions
T1
e

T2
Power transmitted by a belt drive
P   T1  T 2  v