L8 : Pressure - Force & Pressure, Science, Class 8 Video Lecture

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pressure part 8 is brought to you by
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with this we have discussed about force
now our discussion will remain
incomplete if we do not talk about
pressure so by now you have understood
what is force and what are the different
types of forces which exists so let us
talk about pressure so what is pressure
now pressure is nothing but force per
unit area now you might wonder that
while suddenly are we talking about
pressure
we already discussed about force and we
talked about the various types of force
why do we want to talk about pressure
that is because the amount of force
which we applied and the amount of
impact which we get many times that
differ like let us suppose you are
pushing a wall with the same amount of
force and you are pushing a small book
with the same amount of force but the
impact of the result of both the efforts
are extremely different the wall doesn't
move a bit but the book might move a
very large distance so the impact of in
order to understand the impact that
applied force will have we should talk
about pressure because it also depends
on the amount of area on which that
force has been applied so I will take
some examples here so as per this
definition pressure is equal to force
divided by area so let us take the
example of a needle and a thick blunt
cylindrical rod now what happens if I
pinch a little on your skin so in the
first example what do you see it is a
sharp pointed needle now if I try to
prick you through this needle what will
happen you will feel the pain because it
actually penetrates inside you on
through your skin now if I repeat the
same activity I just replace the needle
with a thick blank cell in gentle rod
maybe the back side of your pencil and
now I try to pinch your skin with that
thick blunt rod in that case what
happens
you do not get that much hurt in the
first is you might start to plead you
might get pain but in the second key is
nothing like that but the amount of
force which you have applied in both the
cases is the same step light forces the
same so what has changed only that area
has changed so here if you look at the
cross-sectional area of the needle that
is very less when compared to the
cross-sectional area of this blank
cylindrical rod so that means in this
case the area is less in this case a lis
is more now force is same in both case
so here also it is a eros it is M so
what would be the pressure in this case
this will be forced by area so here also
pressure will be equal to force by area
if you compare both of these force is
same in both of them but area is less
here and area is more here so now here
what do you see pressure is inversely
proportional to area so if area is less
here so that means pressure will be more
and in this case since area is more than
foot pressure will be less now since
pressure is less that is the pressure
exerted by the pencil or by this thick
cylindrical rods and the pressure is
less therefore the pain that infant is
also less but in this case the pressure
is more therefore the pain felt on the
impact is also more so this is why we
are talking about pressure because if we
want to know the impact of force we will
have to talk about area because we need
to know that the force was applied on
how much area now once we know that we
will get an idea about pressure and
pressure is helps us to identify the
impact of the applied force so by now we
understood that pressure is something
which tries to find out the impact of
force by considering the coverage area
that means the area which is being
covered by that application of that
force so we understood that great since
pressure is equal to force per unit area
their progression is the amount of force
greater would be the pressure
and greater would be the area lesser
would be the pressure now keeping this
in mind let us try to look at some of
the important applications where we will
see that how this concept helps us to
understand why these different things
happen have you ever observed people
showing shows on lying on a bed of nails
so they very comfortably lie on a bed of
nails and nothing happens to them which
sounds so surprising because if you
consider one single nail and if you try
to put it into your leg what happens a
single nail putting into your leg or
feet or arm or anywhere so one nail can
actually cause a lot of bleeding it can
hurt you it can cause you a lot of pain
and now here you have a bed of nails
so not only one nail but so many nails
put together but the person is able to
sleep over it and he is able to lay down
lie down over it so comfortably so how
does that happening is there any magic
behind it not really
this is a simple concept of pressure now
what happens is when you just try to put
one single nail in that case therefore
if you consider the scenario of a single
name so for a single nail what happens
is you two are just considering one
pointed object so the area here the
coverage area here is very less now a
Lea is very less therefore the pressure
is very high and when the pressure is
very high then the impact is very high
impact in terms of the pain the hurt so
everything is very high and that is why
the nail gets inside through your skin
and as it enters inside your body it
goes it's going to cause you troubles so
that is the scenario of a single nail
now you consider the scenario of a bed
of nails
now have you ever observed that in a bed
of nails all the nails are of the
exactly the same height now when all the
nails are of exactly the same height now
you have small small surface areas we
are which are very close to each other
in this
is you just have this much area with a
small area but in this case you have
small small surface areas which are very
close to each other so overall it makes
a bigger surface area so when you have a
bit of nails where all the nails are of
the same height and they are located
quite close to each other
so the overall surface area increases so
the overall surface area in case of a
bed of nails increases now when the
surface area increases the pressure
decreases when the pressure decreases
they impact also decreases now whenever
somebody shows you a magic of this kind
just try to ask him that can I replace
one nail with another nail which is of a
bigger height or a smaller height the
person would not agree that's because if
you have nails of different different
heights in that case your effective
surface area will not increase because
not all of them are at the same level so
only few of them will be at one level so
wherever you try to lie down on it so
because of lesser surface area the
pressure will be more and the impact
will also be more so this is why line
when a bed of nails also do not cause
any harm because all the needs are of
the same height and they are kept very
close to each other so that the overall
surface area increases and the pressure
decreases similar you think of an
example of cutting vegetables with a
knife or cutting a loaf of bread with a
knife which kind of a knife will you
prefer a blunt one or a sharp one
obviously a sharp one that's because a
sharp knife will have a lesser surface
area so sharp knife we have a lesser
surface area therefore the pressure will
be more and therefore the impact will be
more pressure is more the cutting the
blade or cutting vegetables' will become
easier think of the school bag have you
ever observed that the strap of the
school bag is always quite thick and why
do you have this thick strap that's
because the school that is quite heavy
right so and that weight is going to be
carried by your shoulders
now if the strap is quite
that means the area is going to be more
if area is more the pressure is going to
be less the pressure on your shoulders
is going to be less so your shoulders
are going to feel less pain by carrying
this heavy bag so that is why the
backpacks or the school bags they always
have thick straps so these are some
examples where you see that this very
simple concept of pressure helps you to
understand so many things which keep on
happening around you let us look at few
more examples have you ever seen the
porters and the railway stations who
just keep running behind you in order to
carry your luggages now if you have
observed them you would have also
observed that they put a cloth over
their head before they put any other
luggage like suitcase or any other box
so first they put a piece of clothes
over their head and on that top of that
clothes they keep them luggage now what
happens due to the presence of this
clothes so now this cloth increases the
area of contact so the area of contact
between the box and the head of the
person so this area of contact increases
now when area increases what happens to
pressure so the pressure tends to
decrease and when the pressure decreases
the pressure of this heavy box on the
person reduces and therefore it becomes
easy to carry let us look at one last
example of pressure so think of this
example which might be a very common
example in your day-to-day life let us
suppose you want to put a nail I mean
you want to construct something using a
different wooden parts or different
parts of Steel so how do you fix them
all together with the help of meals and
why do we choose nail because they have
a pointed end like this and it is not an
easy stuff to put in blunt things so we
want some tiny things with pointed ends
now what do you do with these kind of
small pins small and sharp pins now when
you apply
force so you are basically applying it
over a very small area because the area
of contact is very less
now when the area is very less so the
pressure becomes quite more because you
are applying a force at that force per
unit that area of the tiny nail that is
pressure so the pressure is very high
and when the pressure is very high then
it becomes easy to put that pin inside
so that is how it helps in kind of jobs
also so that is the concept of pressure
so pressure tells us that the impact of
force also depends on the area on which
the force is acting thank you
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