FRICTION
Friction : is a force that resists motion. It involves objects that
Are in contact with each other.
The cause of friction is not simple. As the surfaces of two objects
Rub against each other, they tend to interlock and thus offer
Resistance to being moved over each other.
5 PRINCIPLES OF FRICTION:
1. Friction acts parallel to the surfaces that are in contact and
in the direction opposite to the motion of the object or to
the net force tending to produce such motion.
2. Friction depends on the nature of the materials in contact
and the smoothness of their surfaces.
3. Sliding friction is usually less than starting friction
4. Friction is practically independent of the area of contact.
5. Starting or sliding friction is directly proportional to
the force pressing the two surfaces together.
Friction
Frictional resistance to the relative motion of two solid objects is
usually proportional to the force which presses the surfaces together
as well as the roughness of the surfaces. Since it is the force
perpendicular or "normal" to the surfaces which affects the frictional
resistance, this force is typically called the "normal force" and
designated by N. The frictional resistance force may then be written:
Ffriction = N
= coefficient of friction
k = coefficient of kinetic friction
s = coefficient of static friction
N = normal force
Ff = Frictional force
The frictional force is also presumed to be proportional to
the coefficient of friction. However, the amount of force
required to move an object starting from rest is usually
greater than the force required to keep it moving at
constant velocity once it is started. Therefore two
coefficients of friction are sometimes quoted for a given
pair of surfaces - a coefficient of static friction and a
coefficent of kinetic friction.
Friction and Surface Roughness
In general, the coefficients of friction for
static and kinetic friction are different.
Frictional force is equal and opposite the
Applied force.
Normal Force
Frictional resistance forces are typically proportional to the force which
presses the surfaces together. This force which will affect frictional
resistance is the component of applied force which acts perpendicular or
"normal" to the surfaces which are in contact and is typically referred to
as the normal force. In many common situations, the normal force is just
the weight of the object which is sitting on some surface, but if an object
is on an incline or has components of applied force perpendicular to the
surface, then it is not equal to the weight.
Coefficients of Friction
Friction is typically characterized by a coefficient of friction which is the
ratio of the frictional resistance force to the normal force which presses
the surfaces together. In this case the normal force is the weight of the
block. Typically there is a significant difference between the coefficients
of static friction and kinetic friction.
Static Friction
Static frictional forces from the interlocking of the irregularities
of two surfaces will increase to prevent any relative motion up
until some limit where motion occurs. It is that threshold of
motion which is characterized by the coefficient of static
friction. The coefficient of static friction is typically larger than
the coefficient of kinetic friction.
The difference between static and kinetic
coefficients obtained in simple experiments like
wooden blocks sliding on wooden inclines roughly
follows the model depicted in the friction plot
from which the illustration above is taken
This difference may arise from irregularities,
surface contaminants, etc. which defy precise
description
Kinetic Friction
When two surfaces are moving with respect to one another,
the frictional resistance is almost constant over a wide range
of low speeds, and in the standard model of friction the
frictional force is described by the relationship below. The
coefficient is typically less than the coefficient of static
friction, reflecting the common experience that it is easier to
keep something in motion across a horizontal surface than to
start it in motion from rest.
Friction Plot
Static friction resistance will match the
applied force up until the threshold of
motion. Then the kinetic frictional
resistance stays about constant. This
plot illustrates the standard model of
friction.
The experimental procedure described below
equates the vector component of the weight
down the incline to the coefficient of friction
times the normal force produced by the weight
on the incline.