. . . g o i n g o n e s t e p f u r t h e r
Mechanic s / Force s
Static and Dynamic Friction
UE102050
BAS IC PRINC IPLES
In order to move an object from rest along a level surface, a
force of inertia needs to be overcome. This results from static
friction between the body and the surface on which it rests. If,
once moving the body is to continue sliding along the surface,
a force of
F
Dyn
needs to be applied to overcome the dynamic
friction. This force is smaller than the initial force needed to
overcome the inertia caused by static friction
F
Stat
, as the degree
of contact between the sliding body and the surface beneath is
less.
Neither of these forces are dependent on the area in contact,
instead being determined primarily by the types of materials and
the roughness of the surfaces in contact. They are also proportional
to the force that is pushing the surfaces together in a plane perpen-
dicular to that of the surfaces themselves. This is called the normal
force
F
N
(it acts normally, i.e. perpendicular to the surface). The
coefficients of static friction μ
Stat
and dynamic friction μ
Dyn
are the-
reby defined as in the following two equations:
(1)
In order to measure dynamic friction, an apparatus for measuring
such friction is used, in which rough strips are pulled out at cons-
tant speed from under a body that remains stationary and is also
connected to a dynamometer. Measurements are made for various
combinations of materials and contact areas. To alter the normal
force the track can be tipped up so that the component of the stati-
onary body’s weight that acts normally to the plane of the surface
changes.
EVALUATION
If the track is tilted by an angle
α
, the normal force exerted by
a body of mass
m
in the direction perpen-dicular to the inclined
plane is as follows:
Fig. 1: Dynamic friction
F
Dyn
for four different materials on a smooth sur-
face (1) and a rough surface (2)
Fig. 2: Dynamic friction
F
Dyn
depending on normal force between the two
surfaces
F
N
and
N Stat
Stat
F
F
⋅
μ=
N Dyn
Dyn
F
F
⋅
μ=
cos
α ⋅ ⋅ =
N
gm F
