Chapter 2Friction

By: Mashael Saud Alharbi

*If we examine the surface of any object, we observe that it is irregular.

*Such surfaces that appear smooth to the eye, show irregularities.

*Under microscopic examination ,it has protrusions and valley.

*When two surfaces are in contact, their irregularities intermesh. This results in 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 another, a force has to be applied to overcome friction.

*Consider a block resting on a surface as shown in Fig. 2.1 .

*If we apply a force “F” to the block, it will tend to move. But the intermeshing of surfaces produces a frictional reaction force Ff that opposes motion.

*In order to move the object along the surface, the applied force must overcome the frictional force.( i.e: F> Ff)

*The magnitude of the frictional force depends on the nature of the surfaces;

•Note that :*Clearly, the rougherthe surfaces, the greater is the

frictional force *The frictional property of the surfaces is represented

by the coefficient of friction “μ.”*The magnitude of the frictional force depends also

on the force Fn perpendicular to the surfaces that presses the surfaces together.

*The magnitude of the force that presses the surfaces together determines to what extent the irregularities are intermeshed.

The frictional force Ff is given by

Static & Kinetic Friction Coefficients

Types of Friction*Rolling motion is not encountered in living

systems, but viscous friction play an important role in the flow of blood and other biological fluids.

*Without friction an object that is pushed into motion would continue to move forever (Newton’s first law, a body remains at rest or in a state of uniform motion in a straight line unless it is acted on by an applied force)

*The slightest force would send us into eternal motion.

*It is the frictional force that dissipates kinetic energy into heat and eventually stops the object.

*Without friction we could not walk; nor could we balance on an inclined plane.

*In both cases, friction provides the necessary reaction force.

*Friction also produces undesirable wear and tear and destructive heating of contact surfaces.

*Both nature and engineers attempt to maximize friction where it is necessary and minimize it where it is destructive.

*Friction is greatly reduced by introducing a fluid such as oil at the interface of two surfaces.

*The fluid fills the irregularities and therefore smooths 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 .

*As is evident from Table 2.1, nature provides very efficient joint lubrication.Note that:

*The coefficient of friction here is significantly lower than for steel on ice.

*We will illustrate the effects of friction with a few examples.

2.1 Standing at an InclineReferring to Fig. 2.2, let us calculate the angle of incline (θ) of an oak board on which a person of weight (W) can stand without sliding

down .•Assume that she is wearing leather-soled

shoes and that she is standing in a vertical position as shown in the figure

We will now examine the effect of lubrication on the hip joint in a person.

When a person 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 on the joint is greater than the weight.

Depending on the speed of walking, this force is about FR=2.37≈2.4) times the weight (see Chapter 1).

In each step, the joint rotates through about 60◦. Since the radius of the joint is about 3 cm, the joint slides about 3 cm inside the socket during each step.

The frictional force on the joint is:

Therefore, the work expended in counteracting friction and the resultant heating of the joint are negligible .

•However, as we age, the joint cartilage begins to wear, efficiency of lubrication decreases, and the joints may become seriously damaged .

•Studies indicate that by the age of 70about two-thirds of peoplehave knee joint problems and about one-third have hip problems.