Friction
Friction is a force between two surfaces that are
sliding, or trying to slide, across each other. For example, when you try to
push a book
Along the floor, friction makes
this difficult. Friction always works in the direction
opposite to the direction in which the object is moving, or trying to move. TYPES OF FRICTIONS
Static Friction
Static
friction acts on
objects when they are resting on a surface. For example, if you are hiking in
the woods, there is static friction between your shoes and the trail each time
you put down your foot (see figure below).. Without
this static friction, your feet would slip out from under you, making it
difficult to walk. In fact, that’s exactly what happens if you try to walk on
ice. That’s because ice is very slippery and offers very little friction.
LIMITING
FRICTION
The maximum friction that can be
generated between two static surfaces in contact with each other. Once a force
applied to the two surfaces exceeds the limiting friction, motion will occur. For two dry surfaces, the limiting friction is a product
of the normal reaction force and the coefficient of limiting friction.
DYNAMIC FRICTION
Kinetic friction, also known as dynamic friction or sliding friction, occurs when two
objects are moving relative to each other and rub together (like a sled on the
ground).
ROLLING
FRICTION
Rolling friction is the frictional force that occurs when
one object rolls on another, like a car's wheels on the ground. This is
classified under static friction because the patch of the tire in contact with the ground, at any point while the tire spins, is stationary relative to the ground.
SOME BASIC DEFINATIONS
Angle of friction
For
the maximum angle of static friction between granular materials
For certain applications, it is
more useful to define static friction in terms of the maximum angle before
which one of the items will begin sliding. This is called the angle of
friction or friction angle.
Angle of repose
The angle of repose, or critical angle of repose, of a granular material
is the steepest angle of descent or dip relative to the horizontal plane to
which a material can be piled without slumping. At this angle, the material on
the slope face is on the verge of sliding. The angle of repose can range from 0° to
90°
Coefficient of
friction
A coefficient
of friction is a value that shows the relationship between two objects
and the normal reaction between the objects that are involved. The coefficient
of friction depends on the objects that are causing friction. The value is
usually between 0 and 1 but can be greater than 1. A value of 0 means there is
no friction at all between the objects; such is possible with Superfluidity.
NORMAL
REACTION
The
component of contact force normal to the surfaces in contact
is called normal reaction. It is the force acting perpendicular to
two surfaces in contact with each other. It is a measure of the force holding
the two surfaces together.
FRICTION
IN EQUBILLIUM
When
the frictional force is at its maximum, the body in question
will either be moving or will be on the verge of moving. The maximum frictional force
(when a body is sliding or is in limiting equilibrium) is equal to
the coefficient of friction × the normal reaction force.
Everyday Examples of
Sliding Friction
· Rubbing both hands together to create heat.
·
A sled sliding across snow or ice.
·
Skis sliding against snow.
·
A person sliding down a slide is an example of
sliding friction.
·
A coaster sliding against a table.
·
A washing machine pushed along a floor.
·
An iron being pushed across material.