Physics

1.  Force of Friction
 Laws of Static and Dynamic Friction
 Limits of Friction
 Friction – a necessity & evil
FRICTION

2.  INTRODUCTION
 When an object (e.g., wooden block) moves over the
surface of another object (e.g., floor) , its motion is always
opposed by a retarding force that resists motion.
 This retarding force is called friction and always acts in a
direction opposite to motion.
 For example, an object sliding down an inclined plane
experiences a frictional force directed up the inclined
plane.
 Similarly, an object that moves to the right on a level
floor experiences the frictional force to the left.

3.  Friction can operate between two solids, between a
solid and a fluid or between two layers of a fluid.
However, in this chapter, we shall discuss only the
friction between solid surface.

4.  FRICTION
 When a solid object moves over the surface of
another solid object, its motion is always opposed
by the retarding force, called friction.
 Hence friction is an opposing force that comes
into play when one body moves or attempts to
move over the surface of another body. The
frictional force is always parallel to the surfaces in
contact.
 Let us discuss how frictional force behaves.

5.  Consider a wooden block of mass m placed on a
horizontal surface as shown in Figure 1.
 The block pushes down on the horizontal surface
with a force equal to block’s weight (w = mg).
 The supporting horizontal surface pushes back with
an equal and opposite force R (= N) so that R = mg.
Fig.1

6.  The force R is perpendicular to the surface and is
called normal reaction ( normal means
perpendicular).
 Suppose a small horizontal force F is applied on the
block to the right but the block does not move. It
means that a force has come into play to oppose the
motion of the block.
 This is the force of friction and is called static
friction because the blocks remains at rest or static
even though we have applied force.
 As we increase the applied force, the static friction
also increases till a stage is reached when the block is
about to move.
s
F
s
F

7.  The value of static friction is now maximum and is
called limiting friction or critical friction .
 Once the limiting friction (i.e., maximum static
friction ) is overcome, the block begins to move and
frictional force decreases. The friction is now called
kinetic friction .
 Note that the word kinetic means that the block is
moving. Thus the behaviour of friction can be
summed up as under :
1) Before the body begins to move, the static friction
plays the role ; the static friction increases as the
applied force increases upto the value of limiting
friction (i.e., maximum static friction).
s
F
c
F
k
F

8. 2) Once limiting friction is overcome by the applied
force, the role of static friction is over. Now smaller
frictional force (=kinetic force) opposes the
motion.
3) Both limiting friction ( ) and kinetic friction( )
are important. The limiting friction must be
overcome to start the body moving and then smaller
frictional force to oppose the motion.
k
F
k
F
k
F
c
F

9.  TYPES OF FRICTION
 There are two common types of friction viz.
1. Static friction 2. Kinetic friction.
1. Static friction :
The opposing force that comes into play when an
object does not move over the surface of another even
though force is applied to make it move is called static
friction.
Consider a wooden block lying on a horizontal surface.
If we apply a small horizontal force (say 1N) on the
block and the block does not move, it means that a
force (=1N) has come into play to oppose the motion.

10.  This is the force of friction and is called static friction
 As the applied force is increased, the static friction
also increases (being always equal and opposite to the
applied force) till a stage is reached when the block is
about to move.
 The value of static friction is now maximum and is
called limiting friction or critical friction .
 Note that static friction is always adjusts itself equal
to the applied force upto the value of limiting friction.
 Thus limiting friction is the maximum static friction
that comes into play when one body is just at the verge
of moving over the surface of another body.
s
F
c
F

11.  Once the limiting friction is overcome by the applied
force, the block begins to move and limiting friction
is converted into kinetic friction .
 The subsequent motion of the block is determined by
the applied force (F) and the kinetic friction .
 It is reminded that <
Discussion : The following point may be noted :
1. Static friction comes into play only when the
applied force tends to move a body on the surface of
another body. If the applied force is zero, static friction
is also zero.
2. Static friction is a self –adjusting force. As the
applied force increases, static friction also increases
upto limiting friction.
c
F
c
F
k
F
k
F
k
F

12. 2. Kinetic friction :
 Kinetic friction is the opposing force that comes into
play when one body moves over the surface of
another body. It is sometimes called dynamic friction.
 Once the limiting friction is overcome by the applied
force, the body begins to move and the limiting
friction is converted into kinetic friction.
 Fig. 2 shows the graph between the applied force
and force of friction.
 The part OA of the graph is a straight line showing
that static friction increases as the applied force
increases upto limiting friction .
c
F

14.  The body will remain at rest till the applied force does
not exceed OC. Once the applied force exceed OC, the
body starts moving and friction decreases to (kinetic
friction).
 Thus the limiting friction must be overcome to start
the body moving and then a smaller kinetic frictional
force acts on the body to oppose the motion of the
body.
k
F
k
F

15. Types of kinetic friction :
 Kinetic friction is of two types viz.
(a) Sliding friction (b) Rolling friction.
 When the body slides over the surface of another
body, then opposing force that comes into play is
called the sliding friction.
 For example, when a rectangular wooden block is
moved on a level floor, the opposing force is the
sliding friction.
 When one body rolls over the surface of another body,
then opposing force that comes into play is called the
rolling friction. Thus when boll rolls over a surface,
the opposing force is the rolling friction.

16.  LAWS OF FRICTION
 Static friction as well as kinetic friction is a
complicated phenomenon. Many aspects of friction
even today are not completely understood.
 However, experiments show that to a good
approximation, the force of friction obeys the
following laws :
1) The frictional force (static as well as kinetic)
depends upon the nature of the two surfaces in
contact and their state of roughness.
2) The frictional force always acts parallel to the
surfaces in contact and its direction is opposite to
motion or attempted motion.

17. 3) The frictional force is independent of the area of
contact of the two surfaces.
4) The magnitude of frictional force (kinetic and
static) is directly proportional to the perpendicular
force (normal reaction R) between the two surfaces
in contact.
5) The frictional force is independent of the speed
(applicable to kinetic friction only) of one surface
relative to the other surface.

18. MAGNITUDE OF LIMITING
FRICTION
 Consider a wooden block of
m lying on a horizontal surface
as shown in figure.
 The normal reaction R is the
perpendicular force exerted
by the surface on the block.
 Clearly, R = mg. If is the limiting friction (i.e.,
maximum static friction), then according to laws of
friction, is directly proportional to the normal
reaction R i.e.,
Applied
force
←Fc
c
F
c
F

19. Fc α R
or Fc = R
 Where is a constant of proportionality and is
called coefficient of static friction. Its value depends
upon the nature and condition of the surfaces in
contact.
 Therefore, co-efficient of static friction between two
surfaces in contact is equal to the ratio of limiting
friction and the normal reaction.
 Since is a ratio of two forces, it has no units i.e.,
it is a pure number.
s

s

c
s
F Limiting friction
R Normal reaction
  
s


20.  ADVANTAGES AND
DISADVANTAGES OF FRICTION
 Friction always opposes motion. It has advantages as
well as disadvantages.
Advantages : Although we often view friction as an
undesirable evil and go to great pains to reduce it, we
would really be in deep trouble without it. The following
examples illustrate this point.
1) We would not able to walk if there had been no friction
between the soles of our shoes and the ground. While
walking, we push on the ground and the force of friction
acts in the opposite direction. In the absence of friction,
for example, on icy surface, the person’s shoes would
slip when placed on the ground. The frictional force
always opposes the motion of the shoes.

21. 2) The frictional forces are very beneficial for cars and
other moving vehicles. Without friction between tyres
and the road, the car could not stop, start or turn
corners.
3) The break of car or any moving devices depend upon
friction. When we apply breaks, the car is stopped due
to the force of friction between the break lining and
the drum on the wheel.
4) Friction helps (a) hold screws in wood (b) drive nails
in wood etc.

22. Disadvantages : Friction is associated with many
disadvantages and much of our efforts are directed to
reduce it. The following examples illustrate this point.
1) Much of energy is wasted in overcoming frictional
forces. For example, as much as 20% of the gasoline
burnt in car is used to overcome friction in the
engine.
2) The energy used to overcome the frictional forces is
converted into heat. This raises the temperature. In
almost every machinery, the generation of heat
causes several problems.
3) Since energy due to friction is converted into heat, it
lowers the efficiency of very machinery.