Transcript Friction

Chapter 2: Friction
If we examine the surface of any object, we
observe that it is irregular.
It has protrusions and valleys.
When two surfaces are in contact, their
irregularities intermesh, and as a result there
is a resistance to the sliding or moving of
one surface on the other.
This resistance is called friction.
Metal Surface under Microscopic Inspection
Intergranular fracture in a nickel-chromium alloy, viewed
under the scanning electron microscope.
The surface of any object when closely examined is seen irregular.
It has protrusions and valleys.
When two surfaces are in contact, their irregularities intermesh, as a
result there is a resistance to the sliding or moving of one surface on
the other.
This resistance is called friction.
If one surface is to be moved with respect to
each other, a force has to be applied to
overcome friction.
The direction of a frictional reaction force always opposes the motion.


Ff //  v ,

where v is the velocity of the moving object
F f  μFn
1. The magnitude of the frictional force depends on the nature of the surfaces; clearly,
the rougher the surfaces, the greater is the frictional force. The frictional property of
the surfaces is represented by the coefficient of friction μ.
2. The magnitude of the frictional force depends also on the force Fn perpendicular to
the surfaces that presses the surfaces together. This force is always referred as the
normal force.
1. The kinetic frictional force is the frictional force that acts on a moving object.
F f  μk Fn
2. The static frictional force is the frictional force that acts on a stationary object.
Ff , max  μs Fn
μs  μk
Other types of frictions
A rigid sphere rolling on a plane
Viscous friction in fluid flows
This type of friction is strongly velocity dependent.
Fiction is everywhere around us and is an indispensable factor in the ability of
animals to move.
It is the frictional force that dissipates kinetic energy into heat and eventually stops a
moving object.
Without friction we could not walk; nor could we balance on an inclined plane.
Sometimes for some mechanical purposes it is
desirable to reduce the effect of friction. By
introducing lubrication oils the friction can be
greatly reduced. This is because the fluid oil
fills the irregularities and thereby smoothes out
the surfaces.
A natural example of such lubrication occurs in the
joints of animals, which are lubricated by a fluid called
the synovial fluid.
This lubricant reduces the coefficient of friction by
about a factor of 100.
Nature indeed provides very efficient joint lubrication.
The coefficient of friction is significantly lower than
for steel on ice.
Standing at an Incline
Calculate the angle of incline θ of an oak board on which
a person of weight W can stand without sliding down.
The force Fn normal to the inclined surface is
Fn  W cos θ
The static frictional force Ff is
F f  μFn  μsW cos θ  0.6W cos θ
The force parallel to the surface Fp, which
tends to cause the sliding, is
Fp  W sin θ
The person will fall if
To keep the balance on the oak board
sin θ
 tan θ  0.6
cos θ
Fp  F f
F f  Fp
Therefore
0.6 W cos θ  W sin θ
θ  31
When the joints are in motion, forces acting
on the joints are very large. These forces
tend to damage the joints unless they are
well lubricated.
Frictional wear at the joints is greatly
reduced by a smooth cartilage coating at the
contact ends of the bone and by synovial
fluid which lubricates the contact area.
When a man walks, the full weight of the body rests
on one leg through most of each step.
Because the center of gravity is not directly above
the joint, the force of the joint is greater than the
weight and is about 2.4 times the weight.
The frictional force on the joint is then
F f  μFn  μ( 2.4W )
If the joint is not lubricated, the coefficient of friction (μ) would be about 0.3 and so the
joint would have experienced a frictional force that is almost 72% of the total body weight.
However when the joint is well lubricated the fractional coefficient is only 0.003 and this
will greatly reduce the frictional force down to 0.72% of the total body weight.
A Catfish
Catfish can be rather huge!!
Although in most cases good lubrication
of bone-contact surfaces is essential,
there are a few cases in nature where
bone contacts are purposely unlubricated
to increase friction.
Normally the fin is folded flat against
the body, but when the fish is attacked,
the appropriate muscles pull the bone of
the fin into a space provided in the
underlying skeleton.
The coefficient of frictional on points B
and C must be high in order to lock the
fin in the position.
The erect sharp fin discourages predators
from eating the catfish.
An example of applying
frictional force in nature
Assume that a dislodging force of
0.1 N is applied at θ= 20° and the
angle between the fine bone and
the spine is 45°.
After the calculation we may show
that the minimum value for the
coefficient of friction between the
bones to prevent dislodging of the
bone is μ = 1.95 which is a large
value when compare to a more
usual value of ~ 0.5.
An example of applying
frictional force in nature