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Friction
About
We all know that what would happen if we slide a book kept along a horizontal table .Yes it would first sliding and then finally would come to rest
We all know that what would happen if we slide a book kept along a horizontal table .Yes it would first sliding and then finally would come to rest
- This force opposing
continous motion of book on the table is called force of friction
- Whenever surface of
one object slides over another ,both the bodies exerts force of friction
on each other
- Here in this
chapter,we will consider frictional forces acting between a pair of
surfaces
- Frictional force
comes into action whenever two surfaces of two bodies comes in contact
with each other(Both the bodies could be moving or rest).Both the bodies
exert a force on each other which is basically E.M in nature and this
force is a contact force
- Magnitude of contact
forces on two bodies are equal but they are opposite in direction
- Contact forces also
depend on the nature of the surfaces of the two bodies kept in contact,how
both the bodies are moving and all the forces acting on them
- Consider a block
resting on the horizontal,Now two bodies exert equal and opposite contact
force on each other . Let Fc is the contact force both the
bodies exert on each other
·
Direction of the contact force
acting on a body need not necessarily be perpendicular to the surface of
contact
·
Contact force Fc can
be resolved into two components.The component of Fc along the normal
to the surface is called Normal
contact force and component parallel to the surface is called friction (f) Friction is subdivided into two catagories namely
1. Static Friction
2. Kinetic Friction
Static friction is experienced betweem the non moving surfaces i.e the bodies at rest and kinetic friction or dynamic friction is experienced between the moving surfaces or between the bodies in motion
contact force and component parallel to the surface is called friction (f) Friction is subdivided into two catagories namely
1. Static Friction
2. Kinetic Friction
Static friction is experienced betweem the non moving surfaces i.e the bodies at rest and kinetic friction or dynamic friction is experienced between the moving surfaces or between the bodies in motion
Static Friction
·
We already know that frictional
forces can also act between the bodies in contact with each other even if they
are not moving and such type frictional force is known as static friction
·
Consider a heavy metal block kept
on the floor and you are trying very hard to push it to another location and
you are not able to slide it even by a centimeter
·
Since the block is at rest
resultant force on it should be zero.To counter balance the force applied by
you ,floor exerts a frictional force on the block s
·
Now if you begin to increase the
magnitude of force gradually then bloack does not start moving until force
applied is greater then a minimum value of force
·
This force of static friction
must be overcome by the applied force before the body at rest begin to move
·
Force of friction is always equal
and opposite to the external applied force as long as the body is at rest
·
This means that static friction
force is a self adjusting force.It adjust its value accordingly with the
increase in magnitude of applied force
·
This frictional force cannot be
unlimited and its value cannot go beyond a maximum value fms
·
This maximum value of static
friction between the two surfaces in contact is known as limiting friction
·
Thus magnitude of static friction
can not go beyond the magnitude of the limiting friction i.e, fs
<= fms
·
This limiting friction is
proportional to the normal contact force (N) between the bodies i.e,
fms is proportional to N
fms=μsN
fms is proportional to N
fms=μsN
·
Where N =Normal contact force
μs is the proportionality constant known as coefficent of static friction
μs is the proportionality constant known as coefficent of static friction
·
Value of coefficent of static
friction depends on the material and roughness of the surfaces of bodies in
contact
·
fms is the maximum
value that force of static friction acting between two bodies can reach
·
The actual force of static
friction can be equal to zero or less then fms and its value depends
on the force applied on the body thus
fs <= fms=μsN
fs <= fms=μsN
Kinetic or dynamic friction
·
We already know that static
frictional force exists between two surfaces in contact before there is
relative motion between two surfaces
·
When applied force becomes
greater then limiting friction force,then motion of the body starts and kinetic
friction comes into existence
·
Thus when bodies in contact moves
relative to each other then friction devloped between them is called kinetic or
dynamic friction
·
Initially the block is at
rest,now we push the block on the surface such that it begins to move or start
sliding on the surface
·
When the block is sliding over
the surface each body exerts a frictional force on the other side parallel to
the surface in contact
·
The force of friction acting on
the block B due to surface S is along the direction opposite to the motion of B
with respect to S.Thus force of kinectic friction opposes the relative motion
between the bodies in motion
·
The frictional force acting on
the block is along the direction towards the left and an equal force acts on
the surface S directed towards the rights
·
Thus we can say that sliding
friction on a body B against surface S is opposite to the velocity of the body
B with respect to S
·
Force of kinetic friction is
denoted by fk and its magnitude is always less then the magnitude of
limting friction i.e maximum static friction
i.e fk < fms
i.e fk < fms
·
kinetic friction force is
proportional to the normal contact force between the surfaces i.e
fk is proportional to N
=> fk =μkN
Where N =Normal contact force
μk is the proportionality constant known as coefficent of kinetic friction and its value depends on the nature of the two surfaces in contact
fk is proportional to N
=> fk =μkN
Where N =Normal contact force
μk is the proportionality constant known as coefficent of kinetic friction and its value depends on the nature of the two surfaces in contact
·
fk < fms,
this inequality shows that force required to start the motion is greater then
the force required to maintail the further motion of the body
Rolling Friction
- Consider
a situation of the ring or a sphere rolling without slipping over a
horizontal plane.In this case there is only one point of contact between
the body and the plane
- The
frictional forces developed between two surfaces in case described above
is called rolling friction
- Rolling
friction developes between two surfaces when one body rolls over the
surface of another body
- We
know that it is very difficult to pull a heavy metal box on a rough surface
and if we attah four metal wheel to the box it becomes easiar to move the
- Thus
resistance offered by the surface during rolling is relatively less than
offered during sliding friction
- This
is because while rolling surfaces in contact do not rub each other
- Rolling
friction is negligible in comparision to the kinetic and static friction
which are present simlutanously
- In
many parts of the machine where this type of friction is undesirable ball
bearings(small steel balls) are generally kept between the rotating parts
of
Angle of friction
- It
is the angle which the normal force N makes with the contact force when
the equlibrium is limiting i.e when the condition of maximum static
friction
- Consider
a block of mass m resting on a horizontal surface .Weight W=mg of this
block is balanced by the normal force (N) of reaction
- Mathematically
W=mg=-N - if F is the contact
force which each body exerts on the another body then this contact force
can be resolved into two components
- Perpendicular
Components which is normal force N
- Components parallel
to the contact surface known as friction which is usually denoted by the fs,fk
or fms
- Angle BCD
respresented by λ is angle of friction
Since N and fms are components of the contact force F,so we can write
Fcosλ=N
and Fsinλ=fms
or we can say
tanλ=fms/N
Also from the defination of static friction we know that
fms=μsN So tan λ=μs
Thus we can conclude that coefficient of static friction is equal to the tangent of the angle of friction
Methods to
Reduce Friction;
Friction can be reduced by the numbers of methods.Some of them
are listed below:
(a)Lubrication: when a lubricant is applied between the two
surfaces in contact then a thin layer of lubricant is formed between the two
surfaces resulting in reduction in friction.Example of Lubricant Grease oil, graphite
, compressed air
(b)Polishing : Irregularities of a surface can be reduced by
polishing the surface smooth which results in reduction in friction
(c) Friction between the two surfaces can be reduced by using ball bearing between the two surfaces.Also sliding friction can be converted into rolling friction which is much less then sliding friction
(c) Friction between the two surfaces can be reduced by using ball bearing between the two surfaces.Also sliding friction can be converted into rolling friction which is much less then sliding friction
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