FRICTION AND ELEMENTS OF RIGID BODY

DYNAMICS

1 Friction
1.1 Explanation
2 Types of Friction
2.1 Static Friction
2.2 Dynamic Friction
3 Limiting Friction
4 Normal Reaction
5 Co Efficient of Friction

1 Friction
A force which prevents the motion or movement of the body is called friction or
force of friction and its direction is opposite to the applied external force or motion
of the body. Friction is a force of resistance acting on a body which prevents or
retards motion of the body. Or When a body slides upon another body, the property
due to which the motion of one relative to the other is retarded is called friction.
This force always acts tangent to the surface at points of contact with other body
and is directed opposite to the motion of the body.
1.1Explanation
The other end of the string is connected to the spring balance. Apply an external
force on the balance. Gradually increase the magnitude of the external force.
Initially the body will not move and the effect of the applied force is nullified. This
is because there acts a force on the block which opposes the motion or movement
of the block. The nature of this opposing force is called friction. It depends upon
many factors. The major cause of friction is the microscopic roughness of the
contact surfaces. No surface is perfectly smooth. Every surface is composed of
crests and falls as shown in fig b. It is the interlockingof the crests of one surface
into the falls of the other surface which produces the resistance against the
movement of one body over the other body. When the force exerted is sufficient to
overcome the friction, the movement ensures and the crests are being sheared off.
This gives rise to heat and raises the local temperature. This is also the reason of
the wear of the contact surfaces. This phenomenon of friction necessitates the
presence o fluid film between the two surfaces to avoid wear of surfaces. The
process of creating the fluid film is called lubrication

2 Types of Friction

Friction is of the following two types.

It is the friction acting on the body when the body is at the state of rest or
the friction called into play before the body tends to move on the surface is called
static friction. The magnitude of the static friction is equal to the applied force. It
varies from zero to maximum until the movement ensures.

2.2 Dynamic Friction

It is the friction acting on the body when body is in motion is called

i. Sliding friction

The sliding friction acts on those bodies, which slide over each other for
example the friction between piston, and cylinder will slide friction because the
motion of the motion of the piston in cylinder is sliding and there is surface contact
between piston and cylinder.

ii. Rolling Friction

The rolling friction acts on those bodies which have point contact with each
other for example the motion of the wheel on the railway track is the example of
rolling motion and the friction between the wheel and railway track is rolling
friction. It is experimentally found that the magnitude of the sliding friction is
more than the rolling friction because

in the rolling friction there is a point contact rather than surface contact.

3 Limiting Friction

The maximum friction (before the movement of body) which can be produced by
the surfaces in contact is known as limiting friction .It is experimentally found that
friction directly varies as the applied force until the movement produces in the
body.
 Let us try to slide a body of weight w over another body by a force P as
shown in fig

A little consideration will show that the body will not move because the friction F
which prevents the motion. It shows that the applied force P is exactly balanced by
the force of friction acting in the opposite direction of applied force P. if we
increase the force P by increasing the weight in the pan, the friction F will adjust
itself according to applied force P and the body will not move. Thus the force of
friction has a property of adjusting its magnitude to become exactly equal and
opposite to the applied force which tends to produce the motion.There is however a
limit beyond which the friction cannot increase. If the applied force increases this
limit the force of friction cannot balance applied force and body begins to move in
the direction of applied force. This maximum value of friction, which acts on body
just begin to move, is known as limiting friction. It may be noted that when the
applied force is less than the limiting friction the body remains at rest, and the
friction is called static friction, which may have any values zero to limiting
friction.

4 Normal Reaction

Let us consider a body A of weight “W” rest over another surface B and a force P
acting on the body to slide the body on the surface B as shown in fig
 A little concentration will show that the body A presses the surface B
downward equal to weight of the body and in reaction surface B lift the body in
upward direction of the same magnitude but in opposite direction therefore the
body in equilibrium this upward reaction is termed as normal reaction and it is
denoted by R or N.

Note

It is noted the weight W is not always perpendicular to the surface of

5 Co Efficient of Friction

The ratio of limiting friction and normal reaction is called coefficient of