what is friction and its types friction explained full in this ppt

1. Friction

2. Friction
• is that effect which prevents or tends to prevent
the relative motion of two surfaces in contact.
• Like most things in nature, friction has it's
advantages and disadvantages.
– Without friction, it would be very difficult to
walk, the wheels of a car would spin round
endlessly without moving forward, etc.
– On the other hand, a lot of money is spent to
lubricate moving parts of machines to prevent
their wearing out from friction.

3. • Whenever two surfaces are in contact and one tries to
move relative to the other, frictional forces are
developed between the two surfaces.
• It is generally assumed that these forces are due to the
irregularities of the surfaces in contact and also to a
certain extent, to molecular attraction between the two
surfaces.

4. There are two types of friction:
• Dry friction (coulomb friction) - Friction along
dry (non lubricated) surfaces (wedges, belt
friction)
• Fluid friction - Between layers of fluids moving
at different velocity e.g. flow of fluids through
pipes and orifices or around bodies (a boat in
water, plane in the air)

5. • In this course, we will be considering only dry
friction and will analyze the equilibrium of
various rigid bodies and structures, assuming dry
friction at their surfaces of contact.
• Engineering applications of dry friction include
wedges, belt friction, square thread screws, journal
bearings, thrust bearings, rolling resistance, etc.

6. Coefficients of Friction
• Consider a piece of block of weight W which is placed on
a horizontal plane. The block will be in equilibrium under
its own weight, W, which acts vertically downwards and
the reaction, N normal to the horizontal plane and acting
vertically upwards

7. • A light cord is attached to the block and passed over a
small smooth pulley A to a scale pan B whose weight is
known.
• A small weight is placed in the scale pan. This will set up
a tension T in the string which is transmitted to block.
• If the weight is sufficiently small, i.e. T is small, the block
will still remain in equilibrium and will not move.
• For this state to exist, it implies that a certain force, F,
must have developed which exactly balances T since the
block is still stationary.
• This force F is the friction force. We note that when T is
zero, F is zero. Hence, the friction force begins to act
only when there is a pull, T in the string (which attempts
to move the block) and the friction force ceases when the
pull ceases

8. • If the weight in the scale pan is gradually increased, i.e. T
is gradually increased, so long as the block remains
stationary.
• The force of friction F is also increased and must be equal
T.
• A point is reached however, at which movement of the
block is imminent at the point, the block had been in
equilibrium under it's slight increase in T beyond the
value for the limiting friction will cause the block to move
and the block will no longer be in equilibrium.
• There is therefore a limiting value of the force of friction
Fm beyond which it cannot increase and the body will not
move until the force applied to it is greater than Fm, the
limiting static friction

9. • As soon as the body is in motion, the friction force drops
Fm to a lower value, Fk called the kinetic friction force
which remains approximately constant. A plot of the
friction force F vs the force causing motion, T will be as
shown below:

10. Experimental evidence has shown that
• Experimental evidence has shown that the ratio of
the maximum (limiting) static friction Fm to the
normal component N of the reaction from the
surface of contact is a constant, called the
coefficient of static friction s i.e.

11. • Similarly, the kinetic friction Fk is proportioned
to the normal compliment of reactions from the
surface of constant and the constant of proportions
is called the coefficient of kinetic friction, Mk;
i.e. Fi = MiN
• Both s and Mk are strongly dependent on the
nature of the surfaces in contact (the type of
materials) and on the condition of these surfaces
(smooth, polished, rough). Typically,  is about
25% less than s.

12. Summary
• Friction why exists if the forces applied to a body is such
as to try to move it along its surface of contact with another
body.
• The friction force acts in an opposite direction to that in
which a body tends to move.
• While there is equilibrium, i.e. no motion friction force
varies with the magnitude of the applied force acting in the
direction of the potential motion but he's a waiting value
equal to sN.
• Once motion begins, friction force reduces to Fk= kN. If
motion is not impending, then

13. Summary (2)
Four different situations may occur when a rigid body is in
contact with a horizontal surface.
• The frictional force F and normal force N are the
components of the resultant force R which the force exerts
on the body.
• The forces applied to the do not tend to move it along the
surface of contact; there is no friction force.
• The applied forces tend to move the body along the surface
of contact but are not large enough to set it in motion. The
friction force F which has developed may be found by
solving the equations of equilibrium for the body. Since
there is no evidence that F has reached its maximum value,
the equation cannot be used to determine friction force.

14. Summary (3)
Four different situations may occur when a rigid body is in
contact with a horizontal surface.
• The frictional force F and normal force N are the
components of the resultant force R which the force exerts
on the body.
• The forces applied to the do not tend to move it along the
surface of contact; there is no friction force.
• The applied forces tend to move the body along the surface
of contact but are not large enough to set it in motion. The
friction force F which has developed may be found by
solving the equations of equilibrium for the body. Since
there is no evidence that F has reached its maximum value,
the equation cannot be used to determine friction
force.

15. Summary (4)
• The applied forces are such that the body is just about to
slide. We say that motion is impending. The friction force
F has reached its maximum value Fm and, together with the
normal force N, balances the applied forces. Both the
equations of equilibrium and the equation
may be used. We also note that the friction force has a
sense opposite to the sense of impending motion.
• The body is sliding under the action of the applied forces,
and the equations of equilibrium do not apply any more.
However, F is now equal to Fk and the equation Fi = iN
may be used. The sense of Fk is opposite to the sense of
motion

16. Angle of Friction
• If a body rests on a surface, the resultant reaction of the
surface on the body is inclined to the normal to the
surface at an angle of  , such that
• The angle the resultant reaction of a surface on a body
resting in it makes with the normal to that surface is
defined as the angle of friction. Where the angle  is the
angle of friction and m is the coefficient of friction. Note
that with both  and m increase as frictional force F
increase and reach their maximum value when Fm is of
weight W. Consider a piece of block of weight W placed
on a horizontal surface. A force P is applied as shown
N
F
M 


tan

17. Angle of Friction
• The reaction R acts in a vertical direction, hence  = 0
and tan  =  = 0. Since friction force F =  N and  = 0,
it follows that F = 0 (which is the case since the force P
does not try to cause motion in the direction of surface of
contact).
• If P is applied as shown below
• The horizontal component of the applied force will give
rise to the friction force F while the vertical forces W and
R, will be balanced by N. The resultant reaction of the
surface R is inclined at  to the normal to the surface,
such that where N = W + P, and N = W + P
F

 

tan

18. Angle of Friction
• As P increases, Px will increase and hence F will increase.
Consider the case where P is applied in a horizontal
direction, it is easily seen that F and hence  and  will
increase until F attains the maximum value, F. At that
point  will have also reached it's maximum value s
such that where s is the angle of static
friction.
• If P is increased further, Fm drops down to Fk and
subsequently s reduces to k and we have that
where k is the angle of kinetic friction.
N
Fm
s
s 
 

tan
N
Fk
k
k 
 

tan

19. 5 Points to Note
• Note that for static equilibrium, the angle of friction 
must be such that 0 <  < s
• At the instant of motion,  = s
• Once motion is in place  = k
• At every stage ; hence if the angle of
friction is known, the friction force can be determined.
• For a body resting on an inclined plane with no other
force but its weight acting, the angle of inclination of the
plane  will be equal to the angle of friction , provided
that the body is at rest e.g.
N
F

 

tan

20. Classification of Friction Problems
• Free body diagrams must be drawn and
equilibrium equations written with respect to three
key points.
• * Note:
– In problems involving only three forces, it may
be more convenient to solve the problem by
drawing the relevant force triangle

21. 1. Given the forces acting on a body and the coefficients of
friction for the surfaces of contact and required to determine
if the body will slide or remain stationary, i.e. obtain the
friction force F needed to maintain equilibrium
A. Determine the resultant of all the forces acting on the
body and find the components of this resultant force in a
direction parallel to the surface of contact F (the
component trying to move the body) and in a direction
normal to the surface of contact, N.
B. Compute Fm = sN
C. If F < Fm, required force is F and the body is stationary.
If F = Fm, required friction force is Fm and the body is
just about moving
If F > Fm, then required friction force is equal to Fk = kN
and the body is in motion.

22. 2. Given all the applied forces and told that motion is
impending and required to determine the coefficient of static
friction
A. Repeat as for 1(a)
B. Since motion is known to be impending F = Fm = sN
C. Compute ms from s = F/N

23. 3. Given the coefficients of friction and that motion is
impending and required to determine magnitude or direction
of one of the applied forces
A. This is the same as case(2) except that this contains
an unknown, which is the magnitude or direction of
one of the applied forces.
B. F = Fm = sN because motion is impending
C. Determine the required magnitude or direction from
F = sN

24. Example
• Missing info on WEBPAGE.

25. WEDGES & Square Threaded Screws
• Wedge: simple device that uses and inclined plane to
move heavy objects by applying relatively little load
• Square threaded screws in jacks presses and a host of
other mechanisms operate using essentially the same
principles
• Hence, their analyses are very similar
• Consider a heavy piece of block of weight W which needs
to be raised over a height h. In ancient times, this
problem was solved by building a ramp (inclined plane)
and moving the weight along the inclined plane

26. Case 1:
• Total work done by effort, F to lift load = F x h
• Total work done by block = W x h
• We have that F = W

27. Case 2:
• Total work done by effort, (input) F = F x s
• Total work done by block, (output force) = W x h
• Neglecting friction (i.e. assuming a perfectly
efficient system) F x s = W x h ==>
• Hence if s is sufficient larger than h, then F will be
much smaller than W.
s
h
W
F



28. • In analyzing wedges, frictional forces are always
present and need to be considered. A free body
diagram of the object being lifted and the wedge
should be drawn and used for analysis. Wedges
are used to make small adjustments in the
positions of heavy objects.

29. Analysis of Square - Threaded Screws
• Square Threaded screws are often employed in devices that
are used to raise a load or apply compressive force on an
object.
• The load carried by the screw is subsequently transferred to
the threads. Considering the jack shown in figure 8.8. By
turning the jack handle through one complete revolution,
the screw will advance vertically through a distance known
as the lead L of the screw.
• If r is the mean radius of the threads in the base of the
jack, then the path around the threads can be stretched out
(i.e. unwrapped) and considered as a straight inclined line,
where the angle of inclination  (lead angle) is obtained as
shown on next slide….

30. SHOULD HAVE DIAGRAM,
NOT ON WEBPAGE

31. Free body diagrams of
forces on the Thread
• Note that the forces acting on thread include the load being
lifted W, a horizontal force having the same effect (i.e. same
moment about the axis of the screw) as the force P exerted
on the handle ==> Q x r =Pa and finally the reaction R of
the base thread to the applied loads
• Once the lead angle  is computed using the lead L and the
mean radius of the threads r as shown above and Q is
determined from the expression Q= Pa/r, then the analysis
reduces to that of a block of weight W sliding on a plane
inclined at 

32. Free body diagrams of
forces on the Thread
• Typically,  is taken equal to s to commence motion and 
= k to maintain motion that is already occurring.
• If the angle of friction s is larger than the lead angle , i.e.
s >  , the screw is said to be self-locking and will not
unwind under load. A force Q3 will have to be employed to
lower the load. On the other hand, if s <  , the screw will
unwind under load unless a force Q4 is applied to maintain
equilibrium