2. Friction
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.
5-2
3. 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
5-3
4. Definition Of Friction Force
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.
5-4
5. 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
5-5
6. High friction or Low friction?
Ski’s on the snow
Car tyre
Brakes on a bike Water on a slide
Pencil and rubber
5-6
7. Limiting Friction
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.
5-7
8. Types of Friction
Friction
Static Dynamic
Friction Friction
Sliding Rolling
Friction Friction
5-8
9. Static friction :
Friction experienced by a body when it is at rest is called static friction.
In case of 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.
Dynamic friction is 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.
5-9
10. striking a match against a rough surface
A Flower pot on top of a table
Rolling friction slows down the motion of a tire rolling
along the pavement
5 - 10
14. Variation Of Friction (F) With Applied Force (P)
If the force P is increased, the friction force F also
increases, continuing to oppose P, until its magnitude
reaches a certain maximum value Fm.
If P is further increased, the friction force cannot balance it
any more and the block starts sliding.
As soon as the block has been set in motion, the
magnitude of F drops from Fm to a lower value Fk.
This is because there is less interpenetration between the
irregularities of the surfaces in contact when these
surfaces move with respect to each other.
From then on, the block keeps sliding with increasing
velocity while the friction force, denoted by Fk called
the kinetic-friction force, remains approximately constant.
5 - 14
15. Laws of static friction
Following are the laws of static friction.
The friction force always act in a direction, opposite to that in which the body
tends to move.
The magnitude of friction force is equal to the external force.
F=P
The ratio of limiting friction (F) and normal reaction (N) is constant.
F/N =
The friction force does not depends upon the area of contact between the two
surfaces.
The friction force depends upon the roughness of the surfaces.
5 - 15
16. Laws of dynamic friction
Following are the laws of dynamic friction :
The friction force always act in a direction, opposite to that in which the body
is moving.
The ratio of limiting friction (F) and normal reaction (N) is constant & it is
known as co-efficient of friction ( ).
F/N = =Constant
For moderate speeds, the friction force remains constant. But, it decreases
slightly with the increase of speed.
5 - 16
17. Ladder Friction
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.
5 - 17
19. 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 - 19
21. A 5 metre 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.
5 - 21
22. Wedge Friction
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.
5 - 22
23. Wedges are used as shown to split tree trunks because the
normal forces exerted by the wedges on the wood are much
larger than the forces required to insert the wedges.
5 - 23
25. 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) ∑H = 0 (ii) ∑V = 0 i.e., by resolution of forces.
3. Graphical Method
5 - 25
