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Understanding Friction Types

This document discusses different types of friction: 1. Static friction occurs when a body is at rest, while dynamic or kinetic friction is less than static friction and occurs when a body is in motion. Dynamic friction can be sliding, rolling, or pivot friction. 2. Friction is also classified as occurring between unlubricated surfaces, which is known as dry or solid friction, or between lubricated surfaces. 3. When surfaces are lubricated, the friction is either boundary friction with a very thin lubricant film, or fluid friction with a thick lubricant layer where the surfaces do not touch. Boundary friction follows solid friction laws while fluid friction depends on lubricant viscosity.

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212 views2 pages

Understanding Friction Types

This document discusses different types of friction: 1. Static friction occurs when a body is at rest, while dynamic or kinetic friction is less than static friction and occurs when a body is in motion. Dynamic friction can be sliding, rolling, or pivot friction. 2. Friction is also classified as occurring between unlubricated surfaces, which is known as dry or solid friction, or between lubricated surfaces. 3. When surfaces are lubricated, the friction is either boundary friction with a very thin lubricant film, or fluid friction with a thick lubricant layer where the surfaces do not touch. Boundary friction follows solid friction laws while fluid friction depends on lubricant viscosity.

Uploaded by

rajmeha
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10.

2 Types of Friction

In general, the friction is of the following two types:

1. Static friction. It is the friction, experienced by a body, when at test.


2. Dynamic friction. It is the friction, experienced by a body, when in motion. The dynamic
fricytion is also called kinetic friction and is less than the static friction. It is of the
following three types:
a) Sliding friction. It is the friction, experienced by a body, when it slides over
another body,
b) Rolling friction. It is the friction, experienced between the surface which has balls
or rollers interposed between them.
c) Pivot friction. It is the friction, experienced by a body, due to the motion of
rotation as in case of foot step bearings.
The friction may further be classified as:

1. Friction between unlubricated surface, and 2. Friction between lubricated surfaces.

These are discussed in the following articles.

10.3 Friction Between Unlubricated Surfaces

The friction experienced between two dry and unlubricated surfaces in contact is
known as dry or solid friction. It is due to the surface roughness. The dry or solid friction
includes the sliding friction and rolling friction as discussed above.

10.4 Friction Between Lubricated Surfaces

When lubricant (i.e. oil or grease) is applied between two surface in contact, then
the friction may be classified into the following two types depending upon the thickness
of layer of a lubricant.

1. Boundary friction (or greasy friction or non-viscous friction). It is the friction,


expereienced between the rubbing surfaces, when the surfaces have a very thin
layer of lubricant. The thickness of this very thin layer is of the molecualr
dimension. In this type of friction, a thin layer of lubricant forms a bond between
the two rubbing surfaces. The lubricant is absorbed on the surfaces and forms a
thin film. This thin film of the lubricnat results in less friction between them. The
boundary friction follows the laws of solid friction.
2. Fluid friction (or film friction or viscous friction). It is the friction, experienced
between the rubbing surfaces, whrn the surfaces have a thick layer of the
lubricant. In this case, the actual surfaces do not come in contact and thus do not
rub against each other. It is thus obvious that fluid friction is not due to the
surfaces in contact but it is due to the viscosity and oiliness of the lubricant.
Note: The viscosity is a measure of the resistance offered to the sliding one layer
of the lubricant over an adjacent layer. The absolute viscosity of a lubricant may
be defined as the force required to cause a plate of unit area to slide

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