4.Friction (Biomedical Physics).pdf

About friction and types , laws, advantages and disadvantages of friction

1. FRICTION

2. INTRODUCTION
When object moves over the surface of another
object, 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
Friction can operate between two slides,
between a solid and fluid or between two layers
of a fluid.
In this chapter , we shall discuss only the friction
between solid surface

4. FRICTION
KINETIC
(DYNAMIC)
SLIDING ROLLING
STATIC

6. TERMINOLOGY
The frictional force between two stationary surfaces
is called the static friction,f
s.
The frictional force between two sliding surfaces is
called the kinetic friction,f
k.
static frictionf
sranges from zero to a maximum
value.
0 <f
s<μ
sN
kinetic frictionf
kis constant and independent of
speed.
f
k=μ
kN
maximum static friction is larger than kinetic friction.

7. The component perpendicular to the surfaces
is called the normal contact forceN. This force
arises because the two surfaces are pressing
into each other.
The component parallel to the surfaces is
called frictionf. This force arises when two
surfaces are sliding, or are tending to slide,
relative to each other.

8. 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
Consider a wooden block of mass m placed on a horizontal
surface as shown in figure
The block pushes down on the horizontal surface with a force
equal to block’s weight (w=m.g)
The supporting horizontal surface pushes back with an equal
and opposite force R (=N) so that R=mg
F push
N
F static
W

9. 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 plat to
oppose the motion of the block
This is the force of friction and is called static
friction F, because the blocks remains at rest or
static even though we have applied force
As we increase the applied force, the static friction
F , also increase till a stage is reached when the
block is about to move

10. The value of static friction Fsis now maximum and is
called limiting friction or critical friction Fc
Once the limiting friction is overcome, the block
begins to move and frictional force decreases. The
friction is now called kinetic friction Fk.
Note that the work kinetic means that the block is
moving.
Thus the behaviour of friction can be summendup
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.emaximum static friction)

11. 2) once limiting friction is overcome by the
applied force, the role of static friction is over.
Now smaller frictional force Fk(= kinetic force)
opposes the motion
3) both limiting friction (Fc) and kinetic friction
(Fk) are important. The limiting friction must be
overcome to start the body moving and then
smaller frictional force Fkto oppose the motion

12. TYPES OF FRICTION
There are two common types of friction
1.1. Static friction
2.2. Kinetic friction

13. 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 on the
block and the block does not move, it means that a
force has come into play to oppose the motion
This is the force of friction and is called static friction Fs
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

14. The value of static friction is now maximum and is called
limiting friction or critical friction Fc
Note that static friction is always adjusts itself equal to
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
Once the limiting friction is overcome by applied force ,
the block begins to move and limiting friction Fc is
converted into kinetic friction Fk
The subsequent motion of the block is determined by the
applied force (F) and the kinetic friction Fk
It is reminded that Fk<Fc

15. DISCUSSION:
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

16. 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. shows the graph between the applied force
and force of friction
The part OA of the graph is straight line showing
that static friction increases as the applied force
increases upto limiting friction Fc

17. O

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

19. Types of kinetic friction
1) Sliding friction
2) Rolling friction
When the body slides over the surface of another
body, then opposing force that comes into play is
called the sliding friction
Exa:-when a rectangular wooden block is moved
on a level floor, the opposing force is sliding
friction
When body rolls over the surface of another body,
then opposing force that comes into play is called
the rolling friction. Thus when ball rolls over a
surface, the opposing force is the rolling friction

20. 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 surface 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

21. 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

22. MAGNITUDE OF LIMITING
FRICTION
Consider a wooden block of “m” lying
on a horizontal surface as shown in fig
The normal reaction R is the
perpendicular force exerted by the
surface on the block
Clearly, R=mg. if Fc is the limiting friction
(i.e, maximum static friction), then
according to laws of friction, Fc is
directly proportional to the normal
reaction R, Fc ∞ R => Fc = µs R (µs is
Constant proportional, its value
depends upon the nature and condition
of the surfaces in contact

23. µs = Fc/R = Limiting friction/Normal
reaction
Therefore, co-efficient of static friction
between two surface in contact is equal
to the ratio of limiting friction and the
normal reaction
Since µs is a ratio of two force, it has no
units because it is pure number

24. ADVANTAGES AND
DISADVANTAGES OF FRICTION
ADVANTAGES:
1) we would not able to walk if there had been no friction
between the soles of our shoes and the ground. The
frictional force always opposes the motion of the shoes
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 stooped due to
the force of frictional between the break lining and drum on
the wheel

25. DISADVANTAGE:
1) Much of energy is wasted in overcoming
frictional force. For exa, as much as 20% of thr
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