Friction

Introduction to Friction
S.D.Vetrivel
AP/MECH
Ref: S.S.Rattan Theory of machines

 Introduction - When a body slides over another, the
motion is resisted by a force called the force of friction.
 The force arises from the fact that the surfaces, though
planed and made smooth, have ridges and depressions that
interlock and relative movement is resisted.
 Thus, the force of friction on a body is parallel to the
sliding surfaces and acts in a direction opposite to that of
the sliding body.
Friction

 Kinds of Friction
There are three kinds of friction depending upon the
conditions of surfaces are considered.
1. Dry Friction
 Dry friction is said to occur when there is relative motion
between two completely unlubricated surfaces. It is further
classified into two types:
a)Solid friction – when the two surfaces have a sliding motion relative
to each other, it is called a solid friction.
b)Rolling friction- friction due to rolling of one surfaces over another
(e.g., ball and roller bearings) is called rolling friction.
Friction

1.Skin or Greasy or Boundary Friction
 When the two surfaces in contact have a thin layer or lubricant
between them, it is known as skin or greasy friction. Higher spots on
the surface break through the lubricant and come in contact with other
surface.
2.Film friction
 When the two surfaces in contact are completely separated by a
lubricant, friction will occur due to the shearing of different layers of
the lubricant. This is known as film friction or viscous friction.
Friction

Laws of friction
Experiments have shown that the force of solid friction
 Is directly proportional to normal reaction between the
two forces
 Opposes the motion between the surfaces
 Depends upon the materials of the two surfaces
 Is independent of the area of contact
 Is independent of the velocity of sliding
The last of these is not true in the strict sense as it has been
found that the friction force decreases slightly with the
increase in velocity.
Friction

Coefficient of Friction
 Let a body of weight W rest on a smooth and dry
plane surface. Under the circumstances, the plane
surface also exerts a reaction force Rn on the body which
is normal to the plane surface. If the plane surface
considered is horizontal, Rn would be equal and opposite
to W [Fig (a)]
Friction

 Let a small horizontal force F be applied to the body to
move it on the surface [Fig (b)]. So long the body is unable
to move, the equilibrium of the body provides.
Rn = W and F = F’
 Where F’ is the horizontal force resisting the motion of
the body. As the force F is increased, the relative force F’
also increases accordingly. F’ and Rn, the friction and the
normal reaction forces can also be combined into a single
reaction force R inclined at an angle θ to the normal. Thus
R cos θ = W and R sin θ= F
Friction -Coefficient of Friction

 At a moment when the force F would just move the body, the value of F’
or R sin θ (equal to F) is called the static force of friction. Angle θ attains
the value φ and the body is in equilibrium under the action of three forces
[Fig (c)]:
F, in the horizontal direction
W, in the vertical downward direction, and
R, at an angle φ with the normal (inclined towards the forces of friction)
F’ α Rn
= μ Rn
Where μ is known as the coefficient of friction.
Or μ=
𝐹′
Rn
Also, in Fig(c), tan φ =
𝐹′
Rn

Or tan φ =
μ𝐹′
𝑅𝑛
= μ, The angle φ is known as the
limiting angle of friction or simply the angle of friction.
 Now, if the body moves over the plane surface, it is
observed that the friction will be slightly less than the
static friction force. As long as the body moves with a
uniform velocity, the force F required for the motion of the
body will be equal to the force of friction on the body.

 However, if the velocity is to increase, additional force
will be needed to accelerate the body. Thus, while the body
is in motion, it can be written that
tan φ = μ
where φ is approximately the limiting of friction.
 Also, no movement is possible until the angle of reaction
R with the normal becomes equal to the limiting angle of
friction or until φ = μ

Friction

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Friction