Contact Force, Friction (Part - 12) - Laws of Motion, Physics Video Lecture - Class 11

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motion part 12 is brought to you by
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please make sure that you have watched
all the videos from part 1 to 11 before
going ahead with part 12 we will now
study the introduction to contact forces
so what are contact forces as the name
suggests the forces which arise due to
contact of one object with another the
forces that result when two bodies are
in physical contact with each other any
two bodies when they come in physical
contact with each other there is a force
which acts along the surface of contact
so that is called contact force let us
take some examples of contact force for
example the very famous frictional force
normal force tension force spring force
etcetera these are some of the contact
forces so let us look at a scenario
where we have this box which is lying on
this table so that means this box is in
contact with the table so which is the
surface where it is in contact with the
table we can say this is along this
surface here we can see that the box is
physically in contact with the table so
the forces which arise due to the
contact of the box with the table are
termed as the contact forces so here we
have one contact force that is the
frictional force the frictional force
acts along the horizontal direction and
as we all know it opposes the motion of
the object similarly the other force
that is the normal force it acts
perpendicular to the surface of contact
here we see this is the surface of
contact so a force which acts
perpendicular to the surface of contact
that is the normal force so we will see
that the component of contact force
normal to the surfaces in contact is
called normal reaction it is generally
denoted by Kappa
n so this is the surface in contact for
the two bodies so the force which is
perpendicular to the surface that is
normal reaction force similarly what is
friction it is the component parallel to
the surfaces in contact so these are the
surfaces in contact frictional force
will act parallel to the surfaces on
contact so this is the frictional force
so I hope you got a basic understanding
of what are contact forces now in this
chapter we will go ahead and study in
detail the frictional force we will now
start with what is friction so friction
is a contact force that opposes relative
motion that is we already discussed what
is a contact force a force which comes
into picture when two bodies are in
physical contact with each other so
friction is a contact force which
opposes a relative motion please note
that it opposes not motion but relative
motion because when we talk of friction
we need at least two bodies to be in
contact with each other so it opposes
the motion of one body with respect to
the other body another important factor
is no friction else'll exists till an
external force is applied to the body as
long as the body is at rest and there is
no force applied on it there will be no
friction coming into play now let us
take an example here let us suppose that
this box was lying on the table we just
saw that before it started moving it was
at rest when the box was at rest there
was no external force no force acting
over it as a result the friction at that
time was equal to zero so when there is
no force acting on the body the friction
is zero now as we started applying some
force in order to move the box in this
direction so we started applying some
force on the box as we started applying
of force in this direction the force of
friction comes into play and it acts in
the opposite direction to the motion of
the box so if I am applying a force and
direction to make the box move along
right the frictional force will operate
in the left direction now initially but
now if you see this graph it will become
even more clearer you see that when
force is zero when it is this is the
force and the graph which is plotted
here in blue that indicates friction so
when the applied force is zero there is
no friction the friction also is zero
now after that as the applied force
increases the value of friction also
increases it reaches a maximum point
that there is a that is there is a
maximum value of friction which it can
deliver which it can reach after that it
will fall down and then attain a
constant value so in this example we saw
that the box was initially at rest it
started moving only when the applied
force became greater than the frictional
force like this at first we applied
force the but still the box didn't move
after that the Box started moving only
when the applied force became greater
than the frictional force the Box moved
again if we remove the applied force if
we take off the applied force that is we
do not apply any external force here the
box will again stop because in that case
the friction will dominate again and it
will oppose the motion of the box and it
will come to rest so I hope now you are
very clear with the basic idea of
friction what is friction it is a
contact force that is it comes into play
only when two bodies are physically in
contact with each other
also friction opposes relative motion
that is motion of one body with respect
to the other body that is in this case
let us suppose the box was first body
and the table was the second body the so
friction opposes motion of the box with
respect to the table now we will see the
various types of friction there are
three categories in which fiction is
classified static friction kinetic
friction and rolling friction we will
discuss each of the types in detail one
by one so first we'll start with static
friction thank you visit exam Viacom to
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