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1. 1
On completion of this period, you
would be able to know about:
•The importance of friction
•Desirability of friction
•Types of friction
Objectives

2. 2
Known to unknown
1.How can you retard the moving vehicle
2.What factor is effecting the motion of
vehicle ?
3.Can you run on smooth glass or glazed
tiles?
4.Whether the friction is desirable or not for
a machine component

3. 3
Friction
•Whentwosmoothsurfacesareincontact
thenthereisnoresistancetomovement
•Whentworealsurfacesareincontact
thereisaresistancebetweenpartsdueto
surfacesirregularities

4. 4
Friction (contd.)
•Limiting amount of resistance to sliding is called friction

5. 5
Significance of friction
•Friction is undesirable in bearings and moving
parts
•Reduces the efficiency of machines
but
•Desirable in clamping and friction devices
•Friction plays major role in braking systems

6. 6
Friction (contd.)
Types of friction
1.According to nature of surfaces in contact
a. Dry or solid friction
b. Fluid friction
2. According to state of rest or type of motion
a. Static friction
b. Dynamic friction

7. 7
Friction (contd.)
Dynamic friction is again divided into
sliding friction
Dynamic friction
rolling friction

8. 8
Dry friction
Friction that exits between perfectly clean
and dry solid surfaces
•No interference of fluids in between the
surfaces

9. 9
Fluid friction
•No direct contact between the surfaces
•Lubricant is present between contacting
surfaces
•Friction between two surfaces separated
by a film of lubricant is called viscous or
fluid friction

10. 10
Static friction
•Friction acting on a body which is at rest is called static
friction
•i.e, P is unable to move the body

11. 11
Dynamic friction
•Friction acting on a body which is actually in motion.

12. 12
Dynamic friction contd
•Sliding friction
Friction between surfaces when the relative
motion is sliding
•Rolling friction
Friction between surfaces separated by ball or
rollers

13. 13
Summary
•Friction developed due to surfaces
irregularities
•Limiting amount of resistance to sliding is
called friction
•Frictional force always acts parallel to two
contacting surfaces and resist motion

14. Quiz
1.Friction is desirable in
a. Machines
b. Bearings
c. Clamping devices
d. All of the above
14

15. 15
Objectives
On completion of this period the
student will be able to know
•Limiting friction
•Laws of solid friction
•Laws of solid friction

16. 16
16
Introduction To Friction
•Forces acting on a body under friction which is at rest
W is weight of the body
N is normal reaction of the surface
P is effort applied
F is friction encountered against force P
P
F
W
N

17. 17
Friction(F)
Effort (p)
static
Kinetic
A
B
Limiting friction
P=F P>F
Friction contd

18. 18
Limiting Friction
•If force P is increased
gradually up to certain value
it is insufficient to cause
motion ( i.e. to overcome
friction)
•At a certain value the body is
on verge of sliding ( i.e. At
point A)
•If P is increased beyond point
A the value of f decreases
slightly and then remains
constant Friction(F)
Effort (p)
static
Kinetic
A
B
Limiting friction
P=F P>F
Friction(F)
Effort (p)
static
Kinetic
A
B
Limiting friction
P=F P>F
Friction(F)
Effort (p)
static
Kinetic
A
B
Limiting friction
P=F P>F

19. 19
Limiting Friction ( Contd)
•The max frictional force
exerted at the state of
impending motion
•In the graph friction force
(F) value at point A Friction(F)
Effort (p)
static
Kinetic
A
B
Limiting friction
P=F P>F
Friction(F)
Effort (p)
static
Kinetic
A
B
Limiting friction
P=F P>F
Friction(F)
Effort (p)
static
Kinetic
A
B
Limiting friction
P=F P>F

20. 20
Laws Of Solid Friction
1.Thefrictionalforceisdirectlyproportionalto
normalreactionbetweenthetwosurfaces
2.Frictionalforcedependsonmaterialofthetwo
contactingsurfacesandroughness
3.Frictionalforceisindependentofareaofcontact
betweenthesurfaces
4.Forlowvelocitiesofsliding,Kineticfriction
developedisindependentofvelocity

21. 21
Laws Of Fluid Friction
1.Frictionalforcedependsonthe
propertiesoflubricants
2.Intensityoffrictionisindependentof
loadacting
3.Frictionalforceisindependentof
materialofthecontactingsurfaces
4.Withincreaseintemperatureofthe
lubricant,frictionalforcereduced

22. 22
Summary
In this period we discussed about
•Limiting friction
•Laws of solid friction
•Laws of fluid friction

23. 23
Quiz
1.Static friction is
a. Constant force
b. Variable force
c. Less than kinetic friction
d. more than kinetic friction

24. 24
Recap
In the previous class, we have learnt
•limiting friction
•Laws of solid friction
•Laws of fluid friction

25. 25
Objectives
Oncompletionofthisperiod
youwouldbeabletoknow
• Coefficient of friction
• Angle of friction
• Angle of repose

26. 26
Coefficient Of Friction
•Frictional force (F) is directly
proportional to normal reaction (N)
Fα N
F = µ N
•where ‘µ’ is a
proportionality constant
known as coefficient of
friction

27. 27
Coefficient Of Friction
•Coefficient of friction is defined as the
ratio of limiting
force of friction to Normal reaction
•It is dimensionless quantity
•Co efficient of friction (μ)
=
Limiting frictional force(F)
Normal reaction(N)

28. 28
Angle Of Friction
•Angleoffrictionisthe
maximumanglewhich
theresultant(R)of
limitingfrictionalforce
andnormalreaction
makeswiththedirection
ofnormalreaction
Ø
W
F
R
N
P

29. 29
Angle Of Friction
P
Ø
W
F
R
N
ResultantRoflimiting
frictionalforceand
normalreactionis
calledtotalreaction

30. 30
Angle Of Friction
•Total reaction ‘R’ is
calculated as
μ = tan Ø
Ø = tan
-1
(μ )
where’ μ ‘ is coefficient of
friction22
= R N F =
F
Tan
N


31. 31
Angle Of Repose
•Angle of repose can be
defined as the maximum
angle of the inclined
plane at which a body
can remain in equilibrium
on the plane entirely by
the assistance of friction
•Frictional force on the
body at this angle of
repose is equal to limiting
frictionW
W Cosα
W Sin α
N
F
α
α
W
W Cosα
W Sin α
N
F
α
α

32. 9M303.23 32
Angle Of Repose
•Thevalueofαis
increasedtosuchan
extentthatthebodyison
thevergeofmovingdown
theplane,
atthatpoint
F = W Sinα
N = W Cosα
µ = tanΦ
tan Φ = tan α
Φ = αW
W Cosα
W Sin α
N
F
α
α
W
W Cosα
W Sin α
N
F
α
α
32

33. 33
Summary
•Coefficient of friction is
defined as the ratio of
limiting friction to normal
reaction
•Angle of friction is the
angle which the total
reaction makes with
normal reactionF
N
 1
tan
F
N






34. 34
Recap
In the previous lesson we have learnt:
•Coefficient of friction
•Angle of friction
•Angle of repose

35. 35
objectives
•On completion of this period the
student will be able to
–Compute the effort required to
move a body along a rough
horizontal plane when
•The effort is parallel to the plane
•The effort is inclined to the plane

36. 36
Equilibrium Of A Body
•Effort P parallel to the
surface:
•W is weight of the body
•N is normal reaction of
the surface
•P is effort applied
•F is friction encountered
against force P F
P
W
N
F
P
F
P
W
N
W
N

37. 37
Equilibrium Of A Body
•Effort P parallel to the
surface:
•ΣH = 0
•P-F = 0
•P = F
•ΣV = 0
•N-W= 0
•N=WF
P
W
N
F
P
F
P
W
N
W
N

38. 38
Equilibrium Of A Body
•Effort P parallel to the
surface:
•frictional force
•F = µN
•P = µN
•P = µW
•Horizontal effort required
to move the body on
rough horizontal plane
•P = µWF
P
W
N
F
P
F
P
W
N
W
N

39. 39
Equilibrium Of A Body
•Effort P inclined to the
surface:
•W is weight of the body
•N is normal reaction of
the surface
•P is effort applied
•F is friction encountered
against force P F
P cosθ
W
N
P
P sinθ

F
P cosθ
W
N
P
P sinθ


40. 40
Equilibrium Of A Body
•Effort P inclined to the
surface:
•Resolving the forces
horizontally
•F = P cosθ
•Resolving the forces
vertically
•N + P sinθ=WF
P cosθ
W
N
P
P sinθ

F
P cosθ
W
N
P
P sinθ


41. 41
Equilibrium Of A Body
•Effort P inclined to the
surface:
•Frictional force
•F = µN
•P cosθ= µN
•P cos θ= µ [W-P sinθ]
•P cos θ+ µ P sinθ= µWF
P cosθ
W
N
P
P sinθ

F
P cosθ
W
N
P
P sinθ


42. 42
Equilibrium Of A Body
•Effort P inclined to the
surface:
•µ = tan Ø
•substituting µ = tan Ø
•P is minimum when θ= Ø
•the least force required to
move the body
•P = W sin Ø =
cos + sin
W
P

   sin
=
cos( )
W
P

