Rectilinear Motion - Motion in a Straight Line Video Lecture - Class 11

hello friends this video on motion in a
straight line
part 1 is brought to you by exam Viacom
no move here from exams the topics which
we'll cover in this video are as
introduction position path length and
displacement average velocity and
average speed instantaneous velocity and
speed acceleration kinematic equations
for uniformly accelerated motion and
relative velocity so let's start with
what is motion motion is nothing but
change of position with time let us
suppose we consider any object say any
object which is lying at this particular
place so the position of the object is
this place at this time now with the
passage of time when the object moves to
some position say this so that means
with time there was a change in position
so that is what is known as motion let
us look at some examples we all have
seen the blades of a fan right now they
are not in motion they are at rest so we
can see these are the position of the
blades this is the position of blade 1
this is the position of the blade 2 and
this is the position of the blade
3 now when the blades start moving the
position keeps changing as you can see
very clearly so we say that the position
of the blades is changing with time
hence the moving blades are in motion
let us look at another example
a moving car right now we can see this
pink car is at rest this is the position
of the car where it is standing now the
driver starts driving and we see that
with time the car changes its position
initially the car was at this position
then it moved to this there's there's
this and so on so the position of the
Chari so the position of the car kept
changing as time passed so motion is
nothing but change of position with time
well a very important thing to discuss
when we talk of motion is motion is a
relative term what do we mean when we
say relative the term itself suggests
relative that means in relation to
something or related to something
correct
that means motion of an object is always
with respect to some frame of reference
this is a very important term to
remember here with respect to some frame
of reference when I say some frame of
reference what do I mean I mean to say
whenever I talk of motion of any object
suppose I talked of the moving blades of
the fan so the blades of the fan were
moving with respect to something so
whenever we see some object as in motion
we say that the object is in motion with
respect to some other object now that
some other object with respect to which
we define the motion of another object
is known as the frame of reference
for example let us suppose we have these
three people waiting at a bus stop what
do we get word to be see from this
picture we see that three of them are at
rest when I say they are at rest they
are at rest with respect to with respect
to the ground they are at rest now for
let us suppose we name these persons as
a b c now with respect to a when a is
looking at b for him
b is at rest similarly with respect to b
a is at rest
similarly c is at rest with respect to
both B and a and so on so all of them
are at rest with respect to each other
correct okay now let us see what happens
we see a buses in motion now just
consider the peep the bus is in motion
that means the people who are inside the
bus the people who are inside the bus
they are in motion with respect to a B
and C correct that means people inside
bus they are in motion with respect to a
b c right but for two people inside the
bus let us suppose the two people who
are inside the bus are say f
and gee now for two people who are
inside the bus they are at rest with
respect to each other that means if at
rest with respect to G similarly G is at
rest with respect to F that means for
people who are inside the bus with
respect to each other they are at rest
because the people inside the bus are
not moving similarly for people outside
the bus that is a B and C they are at
rest with respect to each other but for
a b and c the people inside the bus are
in motion
similarly for when we see the motion of
a B and C they are at motion with
respect to people inside the bus that
means a B C are in motion with respect
to people inside the bus that means you
just try to visualize the situation when
we sit inside a bus and we look at
people who are standing outside it seems
to us as if the people are moving in the
opposite direction that means for
somebody like f or g who are sitting
inside the bus they will get a feeling
as if these three people are moving in
this direction so this is what is known
as the relative term of a motion that
means what we always define motion of an
object with respect to some frame of
reference
I hope this is clear to you now we'll
have a quick review see in this case
them these three people are at rest with
respect to each other the people inside
the bus are at rest with respect to each
other for these three people a B and C
people inside the bus are in motion
similarly for people inside the bus ABC
are in motion and in the opposite
direction now let us proceed further and
study about rectilinear motion
rectilinear motion is what we will study
in detail in this chapter so what do you
think rectilinear motion would mean the
term rectilinear let us try to break
this down rectum linear all of you must
be familiar with the term linear linear
means line so when we say rectilinear we
mean to say straight line the term
rectilinear means straight line so
motion along a straight line is
rectilinear motion very simple and very
easy to remember rectilinear that's a
straight line so motion in a straight
line when an object is moving in a
straight line
then it is known as rectilinear motion
let us suppose we take this guy the
movement of the car is along a straight
line like this so this is rectilinear
motion now any straight-line motion is
rectilinear motion just that it should
be a straight line anywhere like this
there's this this whatever so they are
all they all come under rectilinear
motion
rectilinear motion need not be
one-dimensional necessarily some of us
have this misconception that when we see
that rectilinear motion means motion
along a straight line it should always
be along this direction or it should be
along this direction and nothing else
because it has to be one-dimensional if
it is along this direction it is only
along x axis this direction only along y
axis but this is a misconception
rectilinear motion can be any straight
line in any direction but it is always
possible that a rectilinear motion be
treated as one dimensional motion it
does not matter in which direction the
straight line goes but we can always
represent that straight line as a
one-dimensional motion so try to
understand the difference between the
two statements the first statement
states that it is not necessarily that
the rectilinear motion should have a
specific direction along x axis or Y
axis the second statement states that it
can be along any direction but it is
always possible for us to represent it
along x axis or y axis let us look at an
example which will make it very clear
say this is our x axis this is our Y
axis now let us suppose there is a car
which is moving in this direction so now
I told you any motion in a straight line
is rectilinear motion in this case the
car is moving somewhat like this so this
is a straight line undoubtedly that
means it is rectilinear motion so our
first point is correct by then we say
that we can treat any rectilinear motion
as one dimensional motion how just look
at this even now the car would be moving
in the same way as it was
moving in the previous case but in this
case what we will do is we will just
rotate the entire coordinate system in
this direction and try to overlap the x
axis with the direction of the car just
observe it it is something like this so
what we did is we just rotated the
entire coordinate system that is the
coordinate axis in such a way that the
direction of the part of the car
overlapped with the x axis now if you
see this figure will see it once again
just see the motion of the sky so
definitely this is along x axis so you
will treat it as one dimensional motion
so what did we observe we observed that
any rectilinear motion can be treated as
a one dimensional motion by just
rotating or adjusting the coordinate
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