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MECHANICS OF SOLID
FRICTION
FRICTION
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When a body moves over another body, its motion is
opposed by a resisting force along the surfaces of contact.
This resisting force is called Friction.
Magnitude of this frictional (resisting) force depends mainly
upon the materials of the two bodies, upon the quality of
the surface roughness and upon the pressure between the
two surfaces of contact.
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Force of the friction will be zero for 100% smooth or
perfectly smooth surfaces.
Practically it is impossible to make ideal or 100% smooth
surfaces.
Best possible smooth surfaces do have minute
irregularities on the surfaces and hence force of friction will
be negligible but will not be zero.
Definition Of Friction Force
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When a motion occurs or about to occur between the two
surfaces in contact, at that time because of interlocking of
irregularities these surfaces oppose the motion.
Resisting force, generated due to interlocking, opposite to
the motion is called the Force of Friction.
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If contact surface is smooth. friction force (F) will be less.
If contact surface is rough, friction force (F) will be more.
In machine parts like piston, bearings, liner etc. attempts
are made to reduce friction to increase life of parts and
efficiency of machine.
Outer surface of vehicle tyres is made rough to increase
friction between tyre and road surface for safe driving.
High friction (lots of friction) – will slow something down
Low friction (not much friction) – will keep things moving
Limiting Friction
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Consider a body resting on the surface. If a little force is
applied on the body, it will not move, because friction force
(F) will resist the motion.
With further increase in external force, body will not
move, But there is a limit of developing friction force.
If external force becomes greater than that friction force
body will move.
The maximum friction force that can be developed at the
contact surface, when body is just on the point of moving
is called limiting force of friction.
Types of Friction
There are two types of friction:I.
Static Friction
II.
Dynamic Friction:-Sliding friction & Rolling friction
 Static friction: Friction experienced by a body when it is at rest is
called static friction. if (Applied force < Friction force) then there is
no Motion.
 Dynamic friction: Friction experienced by a body, when it is in
motion is called dynamic friction. Its always less than static friction.
 Sliding friction: Friction experienced by a body when it slides over
another body, is called sliding friction. e.g. to push the table on
floor.
 Rolling friction: Friction experienced by a body when it rolls over
another body is called rolling friction. e.g. - to move the drum by
rolling, Ball bearing used in machines.
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A Flower pot on top of a
table
striking a match against a rough
surface
Rolling friction slows down the motion of a tire
rolling
along the pavement
Angle Of Friction
5 - 11
Coefficient Of Friction
5 - 12
Angle Of Repose
Ladder Friction
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The ladder is a device used to climb
on the wall. The ladder is made of
wood or iron.
There are two long pieces in the
ladder to which many small cross
pieces called rungs are connected at
equal distances.
If the ladder is uniform throughout
its length then the C.G. of the ladder
will be at the mid-point of it.
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1. WL ( ) Weight of the ladder through C.G. of ladder [G]
2. WM ( ) Weight of the man through [M], where man is standing.
3. P ( ) Applied force P through [C] at a height h from the floor.
4. RA ( ) Reaction by the floor on the ladder.
5. FA ( ) Friction force on the ladder at point A. Ladder slips rightward on
the floor and
hence FA is leftward.
6. RB ( ) Reaction by the wall on the ladder.
7. FB ( ) Friction force on the ladder at point B. Ladder slips downward on
the wall and
hence FB is upward.
5 - 20
Ladder Friction Sum
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A 5 meter long ladder,
weighing 750 N is
resting as shown in
the fig. Check
whether ladder will be
stable or not. Suggest
options to make it
stable, if it is
unstable. The vertical
wall is smooth and
floor has coefficient of
friction 0.25.
Wedge Friction
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Wedges are simple machines used to raise
large stone blocks and other heavy loads.
These loads can be raised by applying to
the wedge a force usually considerably
smaller than the weight of the load.
In addition, because of the friction
between the surfaces in contact, a
properly shaped wedge will remain in
place after being forced under the load.
Wedges can thus be used advantageously
to make small adjustments in the position
of heavy pieces of machinery.
Wedge Friction
5 - 24
Wedge Friction
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Problems of wedges are the problems of equilibrium of
forces.
When wedge is inserted frictional forces are generated on
the rubbing surfaces of the wedge.
Problems of equilibrium of forces acting on the wedge are
solved by anyone of the following three method.
1. Lami'sTheorem
2. By taking (i) BH = 0 (ii) BV = 0 i.e., by resolution of
forces.
3.Graphical Method
Made By
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Parth Gajjar
Roll No.
02