now we will study about the types of friction
between two surfaces in contact . we can say
when, two surfaces , are in contact. the friction
between. these surfaces. can be classified.
in three categories . these are , the first
one is static friction , second one is limiting
friction. and the third one is. kinetic friction
.
as the name implies static friction acts.
when body is at rest . kinetic friction when.
body is in . motion . limiting friction is
basically. it 
is the maximum limit of . static friction
. that is up to which the body remain at rest
, beyond which the body will start its motion
. let's discuss these three, in detail independently
.
when we talk about static friction we can
say it acts, when body is at rest, but the
other condition is if the body is at rest
it will start moving only when there is any
external force acting on it , so we can say
static friction, will act when body is at
because external force , will provide a tendency
of body to move, and the static friction will
not allow body to move , so it is called static
friction. you can say when on a rough horizontal
surface. a body of mass m a block of mass
m is placed at rest , we can say, it is not
having any tendency of motion so in this situation
we can say that friction is equal to zero
. or, no friction is acting on the body .
but in the similar situation say. if on this
rough surface. a block is placed and an external
force f is applied on it . still it is at
rest . due to force it has , a tendency to
move towards right , but due to roughness
and interlocks , the ground applies a horizontal
friction opposite to this force, and here
the friction is balancing the external force,
that is why the body is not moving it is at
rest or we can say the body is still at equilibrium
.
because the friction is balancing the external
force acting on it , so this is the phenomena
, where we call this friction as static friction
. in this situation one more thing we can
say, that if we increase the value of this
force the friction will also increase , because
if we increase the force the body will apply
more force on the interlocks on the ground
value of f ,
but there is always a limit up to which the
interlocks can tolerate the force applied
by the body on the ground , so the maximum
limit. up to which. the static friction. balances
the external force . acting on body. is termed
as. limiting friction . we can say this limiting
friction is the maximum limit of static friction
, up to which the friction balances the external
force ,
if the value of external force crosses the
value of limiting friction, the body will
start , moving. or we can say that the force
approaches the limiting friction because it
is the maximum limit the body starts moving.
so we can say, if this limiting friction.
is f l and we can say here , body will start
moving . when this external force is more
than or equal to this limiting friction, but
for force less than this limiting friction
this friction balances this f , we can say,
it happens for capital f less than limiting
friction , this friction balances this external
force, and the body is kept at rest, this
is called static friction , so the maximum
limit of this static friction is limiting
friction as soon as this external force approaches
limiting friction , the body will start moving.
