Students we have only learned about the
laws of static friction and kinetic
friction now I just move quickly through
the concepts of the friction then we'll
go to the numerical so others concepts
that is required for concepts for new
miracles of friction first is that force
of static friction is directly
proportional to knock on reaction that
is thus therefore force of static
friction T is equal to because this is
the proportionality sign this will go
and so far shot is introduced this is mu
s into M so this is the formula that
force of static friction is mu s into M
where this mu s is called coefficient of
static friction
where n is the normal reaction FS is the
limiting force of friction FS is the
limiting force of friction limiting
force of static friction
limiting is maximum value of static
friction this is the maximum value of
saturation so this is the maximum value
of static friction that means now we'll
discuss in detail so first thing is this
one the proficient of static friction
has no unit because it is a ratio of
this force by displace okay this is the
ratio of frictional force by a normal
reaction normal reaction itself is a
force so that is so it is very nature
now the maximum the range of new is from
0 to 1
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this is the anemias between 0 to 1 it
can be zero any Max and one it cannot be
more than 1 or it cannot be negative the
surface is smooth then is zero it is
very rough then it is 1 ok now so the
other thing is that this is now in hit
and no dimension
perfectly smooth surface this is zero
and very rough surface this is very very
rough sticky surface here is yes is one
this is a constant of static friction
now let us apply this equation F s is
myris into n2 numerical so our one
equation is Ephesus or F Ely's or FS
maximum is nice again you cannot always
write SS maximum usage where sometime
this is a less than this if it is less
than that that force is not maximum
force what is the meaning of that if I
write
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FS when I write
like this
this can be any value the Equality sign
is only there if this is maximum okay
that we'll discuss now with help of
example so I will take one numerical
here and see how we can solve this first
of all we will to start with we will
take a simple proportion then gradually
the level of difficulty will increase
okay to start let us start with a simple
fashion caption is let us consider an
object of mass flight program is resting
on a horizontal surface with sensation
say some friction coefficient mu oh I'll
take by 0.3 then I apply a for certain
force then I am to see if the object is
moving if it is moving then what is the
acceleration produced and what is the
maximum value of friction possible there
but all these chapters will find so
first let me write the question then
we'll proceed
let a block of mass 5 kg is placed on a
rough horizontal surface
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the first part is to find
the force of maximum static friction
okay
second the floats of maximum static
friction is independent of the applied
force that you must remember but the
force of maximum static friction is
independent of the applied external
force so the second part of the question
is
if an external force
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50 Newton
it's applied to the body
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then find the oxidation of the body so I
apply a force of 50 Newton and see if
the body is moving then then I have to
find the acceleration and that money so
the applied force is 15 Newton so let us
first find these two then we'll extend
the question is a very simple equation
and what is given is given the
coefficient of friction static friction
is superb 0.4 this is given to you now
how to solve this solution so let us
understand the question before we
proceed to solve this the first is today
there is a block whose mass is 5 kg so
that
this is block smart is a right here sigh
thank you okay
it's placed on a horizontal surface so
there is a surface I will say this is
the surface on which it is finished this
is rough
rough surface now and the real value for
this surface is 0.4 so this is
nuez is 0.4 now first thing is to find
our force of maximum static friction the
force of maximum static friction the
maximum static friction is called
limiting friction so you have to find FL
that is or FS maximum so this is what
this is FL or you write F is maximum
stratification okay then here we know
from lot of static friction that F is
equals to us into n if you write equal
to this is maximum so then what is the
answer
F is close to maximum okay yes jefes
maximum occurs to us because how much 0
into n what again and is the normal
reaction the surface is horizontal the
object is placed like this one force
will act through the object that is
known as gravitational force wait the
block will experience the weight mg
acting downward this is a mg which is
five kilogram into 10 G I will consider
as a 10 meter per second square so this
is equal to 5 into 10 this is 50 50
Newton is acting downward okay if 50
Newton is acting downward then normal
reaction will be
non-action only and which is equal to
this mg okay so 0.4 is yes which is
given to us the normal Direction normal
tension is equal to the weight of the
body because when the body is subject on
the surface what force is acting down
what that is mg we weight and equate it
to that the normalization left because
this point of contact is not moving up
and down
so the normal lecture and a way to
balance so n is equal to 50 so this
comes out to be 20 Newton that means
this means that to this block if I apply
a force of maximum force of 30 Newton it
can prevent because it can prevent up to
20 that means if I apply force of one
Newton the friction will also be 1
Newton in that direction if you apply
force of to Newton description will be 2
Newton because its maximum capacity is
20 Newton so maximum is 20 so if I apply
force of one Newton friction only that I
will just make a table over here okay to
this cushion I'll continue there so this
is a block this is mg this is 50 the
projection is again coming out to 50 the
coefficient of friction is 0.4 now the
maximum the force of static friction
limiting friction comes out to be 20
Newton this is what we have got so far
so this 20 Newton that means if I apply
here I'm writing apply force
a breakfast here and Sentinel force here
in this side and see what you get
to start with first if I applied my
applied force is zero if I do not apply
any force then what would happen if I
don't apply any force then there will be
no friction friction will not to be
little
right so because section will act only
when that is applied force when you
apply force there is a tendency of
motion then always a channel act if
there is no applied force there is no
tendency of motion there is not
impending motion there is no section
this is very very straightforward so if
applied force is zero friction force
will be zero second suppose I increase
up to five Newton that means to this
apply force of five Newton then section
will not be 22 turn that is interesting
because section does not believe in
wasting the time it will not apply 20
Newton it has the ability to apply it
has the capacity to apply 20 Newton but
if you not apply it will just apply the
external force when you that is right
Newton because the main purpose of the
section is to keep the two particles or
two surfaces in contact with each other
if so the basic idea is to keep it to
surfaces are in contact his basic idea
is to this point and the other point who
that issue can take any two pair of
point the basic aim of friction is not
to separate those two points that is the
basic aim of friction what I told you if
this is a block this is another surface
I am just separating those two sources
to show the my point that is a point
here suppose name is a that is another
point say here be a point is on this
first block this block B point 8 point
is on this second surface now initially
these two points are together together
the basic end of section is to keep
these two points together at least as
far as possible
so far as possible it always tries to
prevent these two from separation it
doesn't want is to have relative motion
don't this want the friction always
wants these two points together that is
the basic idea of fiction so now what
happens is that if I apply a force or
suppose five Newton force or find due to
this direction this point will tend to
move in this direction if this point
tends to move then they will not be
remaining women together they will get
separated if they get separated then to
prevent the point L from moving towards
right the surface develops a force which
applies on this block a to this point a
force which is towards split so if I
apply force on this block any force I
apply the same amount of force is
applied by this body on this block or
along the surface so that these two
forces will cancel out and these two
points still remain together that is the
basic idea of fishin so if my force is 5
Newton force this is 5 Newton force the
friction will be definitely 5 Newton
because its maximum value as you can see
here is 20 it can prevent up to 20 okay
now I increase the force to 15 Newton
then also fiction will be 15 Newton no
problem because you can spend up to 20
Newton now I apply 20 Newton here let's
it shall be continued it is thought this
is the maximum value the
friction that's it is the maximum value
the friction s it cannot be more than 20
Newton applied force can be anything
because a crackphones you can put
increasing that depends on us how to
apply the force that force can be more
but then friction has certain
limitations it cannot be more than 20
for this situation this is Cochin under
consideration so here is 20 Newton so
when I apply 20 here also will be 20 but
when it is more than 20 suppose I apply
21 Newton force then the question is how
much force will be the section providing
to the object now in this case when the
applied force is greater than the
limiting value of the study section then
also it will be 20 because it is a
maximum it can apply so if it my applied
force is more than 20 Newton the maximum
limiting value take the friction who not
like maximum the friction will try its
best it will give it this if you give
the maximum section that it has that is
a it will oppose with 20 Newton force
because it has become that and this is
21 Newton is the force that I have
applied this is the external force and
this is the frictional force so now in
this case it is 20 so if mine force is
greater than its limiting all the time
you won't keep on increasing the applied
force the static friction would always
remain 20 feet the body starts moving
because after the body starts moving
then kinetic friction will be proper to
play that we'll discuss some other day
now so as you can see here basic is we
have to understand up to this
if there is no applying force friction
is zero that you understand it is 0 this
is 0 if this is maximum 20 which also is
next or 20 if it is less than the
limiting that means if I applied force
is less than D then friction that will
apply 20 here that would remember
because if you apply 5 in applies offers
5 and switch and apply 20 because it has
the ability to operate 20 supposing boot
applies 20 then delta T motion the
relative motion is this force will
determine the source and the body will
start moving in that direction
so what was the basic form for
sufficient basic process of film the
basic idea of the friction is to keep
these two points to
keep these two bodies together so if you
apply 5 Newton is of this objective
should be to apply 5 Newton to the
opposite direction so that the forces
cancel out and these two bodies are
still together if you apply 5 Newton the
Fisher can provide a maximum of 20
Newton but it cannot apply so this is
not possible if you do so then body
start moving in that direction the
purpose is not sorry
so therefore when you apply 5 Newton
this also applies if I do 10 okay thank
you
