This document discusses friction, which is the force that opposes the relative motion between two objects in contact. There are two types of friction: dry friction, which occurs between non-lubricated surfaces, and fluid friction, which occurs when a lubricant is present between surfaces. Dry friction includes static friction, which acts on stationary surfaces, and dynamic or kinetic friction, which acts on moving surfaces. The laws of friction state that the frictional force is proportional to the normal force and depends on the roughness and material of the surfaces. Friction is quantified using the coefficient of friction.

1. UNIT V FRICTION
E.M - B.K.P
ENGINEERING MECHANICS
Mr.B.K.Parrthipan, M.E., M.B.A., (Ph.D).,
Assistant Professor / Mechanical Engineering,
Kamaraj College of Engineering and Technology.

2. Friction
When a body moves over a another body, a
force opposing the motion is developed at the
contact surfaces.
This force which opposes the movement is
called frictional force or friction.
It is also called as frictional resistance or
force of friction.
E.M - B.K.P

3. Types of Friction
There are two types of friction.
(i) Dry friction or Solid friction or Coulomb
friction
(ii) Fluid Friction
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4. Dry Friction
If the two contact surfaces (which are in
contact) are non- lubricated and have direct
contact between the surfaces is called dry
friction. Since the characteristics of dry
friction was studied by C.A.Coulomb in 1781,
it is often called as coulomb friction.
Example:
(i) Friction between shoe brake and drum
(ii) Friction between the wheel and the road
E.M - B.K.P

5. Fluid Friction
The friction between the layers of a fluid
that acts as a lubricant between the two
contacting surfaces is called fluid friction. This
type of friction exists in problems involving
flow of fluids through pipes and orifices.
Example:
(i) Friction developed in machine parts having
the lubricants.
E.M - B.K.P

6. Types of Dry Friction
The following are the two types of dry friction.
(i) Static Friction
(ii) Dynamic Friction or Kinematic Friction
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7. Types of Dry Friction - Static Friction
It is the frictional force experienced by the
surface of a body that is in contact with a fixed
surface while it is in static condition under the
action of external forces.
If two surfaces which are in contact are at
rest then the force experienced by one surface
is called static friction.
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8. Types of Dry Friction - Dynamic
Friction
It is the frictional force experienced by the
surface of a body that is in contact with a fixed
surface moving over it or with in.
If one surface starts moving and the other
is at rest, then the force experienced by the
moving surface during the moving condtion is
called as dynamic friction.
It is also called as kinetic friction.
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9. 1. Sliding Friction
The friction experienced by a body when it
slides over another body.
2. Rolling Friction
The friction experienced by a body when it
rolls over the another
Types of Dynamic Friction
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10. Laws of Dry Friction
1. The force of friction is directly proportional
to the normal load between the surfaces.
2. The force of friction depends upon the
material of which the contact surfaces are
made.
3. The force of friction is independent of the
velocity of sliding of one body relative to the
other body.
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11. Laws of Static Friction
1. The force of friction always acts in a direction,
opposite to that in which the body tends to move.
2. The magnitude of the force of friction is exactly
equal to the force, which tends the body to move.
3. The magnitude of the limiting friction (F ) bears a
constant ratio to the normal reaction (RN ) between
the two surfaces. Mathematically, F/RN= constant
4. The force of friction depends upon the roughness of
the surfaces.
E.M - B.K.P

12. Laws of Kinetic or Dynamic Friction
1. The force of friction always acts in a direction,
opposite to that in which the body is moving.
2. The magnitude of the kinetic friction bears a
constant ratio to the normal reaction between the
two surfaces. But this ratio is slightly less than
that in case of limiting friction.
3. For moderate speeds, the force of friction
remains constant. But it decreases slightly with
the increase of speed.
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13. Laws of Fluid Friction
1. The force of friction is almost independent
of the load.
2. The force of friction reduces with the
increase of the temperature of the lubricant.
3. The force of friction is independent of the
substances of the bearing surfaces.
4. The force of friction is different for different
lubricants.
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14. Coefficient of Friction
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15. Friction Angle
• Angle made by the resultant of normal reaction
and limiting friction with the normal reaction
is called the angle of friction.
• The coefficient of static friction is equal to the
tangent of the angle of friction.
E.M - B.K.P

16. Angle of Repose
It is the maximum angle of inclination of
an inclined plane on which a body begins to
slide down due to the action of the component
of its own weight acting alone the plane. The
angle of repose (α)is equal to the friction
angle(θ).
E.M - B.K.P