L3 - Kinetic Energy and Potential Energy - Work and Energy, Class 9, Science Video Lecture

so you finally come to that part which
is at the same time both interesting and
challenging to discuss if I were to tell
you right that batsman has amazing
potential or you have amazing potential
you will become a great person what are
we saying in one sense we are saying
that and you say the word potential what
we're saying is not done anything now
but we expect you to do something in the
future it's a it's a statement about
your capacity in the future right so
when I even I used toward potential
energy it's going to mean something very
very similar so if I take a spring I
expand that spring right and if I call
you after that so that you see it see me
holding it at this moment then to you it
might seem like something much should it
nothing very interesting but the moment
I leave the spring slams into position
and then you might ask the question
where did this energy come from all of a
sudden I just saw this bring your boom
back and started started hitting it
started moving it had some kinetic
energy where did it come from because it
would be a good question if you ask that
because where does energy come from
energy can neither be created nor be
destroyed we always say so where this
has come from and to answer that
question is where we will bring in
what's called potential energy whereby
when I stretch the spring the fact that
it has been stretched the fact that its
shape has changed even though it does
not have any kinetic energy right now
makes it so that it can have kinetic
energy in the future and that energy is
what we call potential energy we say
that the spring has it even though it
doesn't look it looks very calm so if I
were to carry a block it right with me
up to some fourth floor and then I keep
it there and in my hand anybody looks at
that block is going to go like okay
mundane boring block but we know that
the moment I leave the block is going to
start falling down it's going to have
kinetic energy nobody's doing any what's
happening but at least I'm not pushing
it of course the earth is pulling it
that's what's happening but to somebody
in the outside it looks like energy was
created out of nowhere and that's not
possible so what do we see now a body
that's kept at a height has some
potential energy just like water that's
stored in the damn water that's stored
for a large height you break a hole what
who's is out right
starts rushing outside with large
velocity kinetic energy comes out of the
dams the water in the dams potential
so if you are into mythology or into
these games where is a lot of these book
I know what are they call our chips yeah
or crossbowmen so you have an archer
pulling the bow the string of the book
and at that moment when he's stopping
there there's nothing really that's
happening but the moment he lets go is a
huge amount of kinetic energy and in all
these cases we say that that kind of
thick energy comes from the potential
energy that that object had now we've
got a question a lot of things that we
are saying in a couple of minutes but as
of now what is the understanding that
you have you have to think about it an
object can have kinetic energy or it can
be in a configuration where the moment
it's left to itself it will have a netic
energy so whenever that is true whenever
there is a possibility of kinetic energy
in the future usually its kinetic energy
it could be any other energy as well
it's called potential energy so let's go
back to our definition what is energy
the capacity to do work so kinetic
energy is the capacity to do work so
kinetic energy is the capacity to do
work and potential energy is the
capacity to have kinetic energy in the
future so it's an interesting way of
looking at it so potential energy might
lead to kinetic energy which might lead
to some work being done so now point
number one if somebody were to tell you
right whenever you do some work and if
there is no change in kinetic energy
there is a change in potential energy
strong it's not true I might take a
block on the floor push it all the way
around the house and take it everywhere
I've done a lot of work but the moment I
leave the block nothing happens what is
just staying there hey whatever work I
did I finished the work and move the
block somewhere it's at rest now so the
block was at rest to begin with I moved
the block the block is at rest now in
the change in kinetic energies theorem
does that mean the potential energy of
the block is increased well naturally it
I did is all I work on the block and all
of that was lost as heat so that could
happen as well it's not necessary that
whenever there is no change in kinetic
energy there will be a change in
potential energy
that's why number one so we've defined
what is potential energy for you
we're also telling you it's not the only
other option from a netic energy now
there are other things that are pretty
important to understand one of them is
that potential energy is only defined
for very very special kinds of forces
not for every force
so if you go back in time and if you go
back for a while you'll begin to realize
that there was some kind of force is
called conservative forces is not going
to go too deeply into them but
conservative forces are those forces
where this quantity called work done
does not depend upon the path that you
take it respect to what path I take I
could go from here to here taking
whichever path I want to go like this I
go like that no matter what path I take
if I calculate that quantity called work
done force into displacement in every
single point and I add them up they
would add up to the same quantity in
other words work done does not depend
upon the path taken then such a force is
called a conservative force and
potential energy has meaning only the
conservative forces because I can push
an object against friction friction is
not a conservative force I can keep a
body on the floor push it all around
what work am i doing I'm doing work
against friction but none of that will
be converted into potential energy it
all goes away as heat so with this in
mind what we're trying to do here is
begin to take potential energy define it
in a way that is useful so what is
potential energy it is that energy that
might give rise to kinetic energy in the
future and our next model will try and
understand how to calculate this
potential energy in a way that's
measurable