Mechanical Engineering Principles

15

Friction

At the end of this chapter you should be

able to:

• understand dynamic or sliding friction


• appreciate factors which affect the size and
  direction of frictional forces


• define coefficient of friction,μ


• perform calculations involvingF=μN


• state practical applications of friction


• state advantages and disadvantages of fric-
  tional forces


• understand friction on an inclined plane


• perform calculations on friction on an
  inclined plane


• calculate the efficiency of a screw jack

15.1 Introduction to friction

When an object, such as a block of wood, is placed
on a floor and sufficient force is applied to the
block, the force being parallel to the floor, the block
slides across the floor. When the force is removed,
motion of the block stops; thus there is a force
which resists sliding. This force is calleddynamic
orsliding friction. A force may be applied to the
block, which is insufficient to move it. In this case,
the force resisting motion is called thestatic friction
orstiction. Thus there are two categories into which
a frictional force may be split:

(i) dynamic or sliding friction force which occurs

when motion is taking place, and

(ii) static friction force which occurs before
motion takes place.

There are three factors that affect the size and
direction of frictional forces.

(i) The size of the frictional force depends on the

type of surface (a block of wood slides more

easily on a polished metal surface than on a

rough concrete surface).

(ii) The size of the frictional force depends on

the size of the force acting at right angles

to the surfaces in contact, called thenormal

force; thus, if the weight of a block of wood is

doubled, the frictional force is doubled when

it is sliding on the same surface

(iii) The direction of the frictional force is always

opposite to the direction of motion. Thus the

frictional force opposes motion, as shown in

Figure 15.1.

Frictional

force

Motion

Pulling

Block force

Surface

Frictional

force

Motion

Pushing

force

Block

Surface

Figure 15.1

15.2 Coefficient of friction

Thecoefficient of friction,μ, is a measure of the

amount of friction existing between two surfaces.

A low value of coefficient of friction indicates that

the force required for sliding to occur is less than

the force required when the coefficient of friction

is high. The value of the coefficient of friction is

given by:

μ =

frictional force(F)

normal force(N)