Let us perform a very simple experiment.
As you can see in the animation,
hold up a pencil in between your index finger
and thumb as has been placed. Now in this
way apply a force with your index
finger as well as your thumb on both ends
of the pencil and in this manner
hold the pencil 
for at least a minute.
With the pencil in between your fingers
in such a way, hold the pencil for at
least one minute. What will you find?
If you hold it in that way for at least
one minute, you will see that the depression
that is taking place in your index
finger is much more than the depression
taking place or occurring in your thumb.
In other words the depression is deeper
in the index finger than the depression
in the thumb. Now why do think it is so?
I had said that you have to apply the
same amount of force in between these
two ends, that is, the amount of force
applied with the your thumb has to be the
same as the amount of force applied with
your index finger. So let us find out the
explanation for why the depression
varies.
Now, let us consider the index finger. When you
are applying a force with your index
finger on the pencil nib, according to
Newton's third law of motion, an equal
and opposite force will be applied by
the pencil nib on your finger. So we consider
the pencil nib also applies an equal force
on the index finger. So as a result what
happens? The direction of force applied
by the pencil is perpendicular to the
index finger as given by Newton's third
law because the force that your finger
is applying on the pencil nib is also
perpendicular and these two forces are equal
and opposite.
So this force that is perpendicular to
your finger has a particular name to it.
As a matter of fact, any force that acts
perpendicular to any surface has name to it
and that name is thrust. So the force
applied by the pencil nib that is
perpendicular to the index finger is
also known as thrust and thrust is a
special name for the force which acts
perpendicular to a surface. So if we
have a surface like this and we have a
force
that is acting perpendicular to that surface, we
will say that force F is the thrust
acting on the surface.
Now we know that the unit of force is
Newton and since thrust is also a kind of
force, what can we say?
We can say that the unit of thrust, that
is, the SI unit for thrust is also Newton
and just like the C.G.S unit for force is
Dyne, since thrust is also force its C.G.S
unit is also Dyne. So the SI unit for
thrust is Newton and the C.G.S unit for thrust is
Dyne, Now consider this situation. The force that is
being applied by the pencil is the same on both
the index finger
and the thumb. Why, because initially when we are applying a
force on the pencil, it is being
applied equally in between the index
finger and the thumb. So if I am applying
an equal force on the pencil from both
ends, the pencil at both ends will also
apply an equal force on two of my fingers. So we
can say that this force that is acting
perpendicular to both the index finger
as well as the thumb will be the same or
in other words the thrust is the same on
the index finger and the thumb
So now consider this scenario that we are 
considering separately for the nib as
well as the base. Now we have just seen
that the force being applied by the
pencil nib and a pencil base on the index
finger and the thumb respectively
is the same.
Now when we are considering the
index finger it is located on top of the
pencil nib, that is, the index finger is
pressing down on the pencil nib. As you can
see, the area of the nib is very very
small. So, we can say that the area of
contact of the pencil nib
with the index finger is less. So since 
this is less, what can we say? we can say
that thrust is acting on a lesser area or
the thrust per unit area is more and
due to this, the depression
is more because
the thrust
per unit area is more because the area is
less.
In the case of the thumb which is
applying a force at the base of the
pencil, the thrust remains the same.
However, as you can clearly see the area
of contact is more in the case of the
base of the pencil that is directly attached to thumb.
So since thrust is acting on a larger area, 
the thrust per unit area 
will be less. Why, because
the thrust remains constant. However the
area is more. So in this case the thrust
per unit area is less and this is why
the depression is less.
Now this has a particularly name to it,
that is, thrust per unit area.
Thrust per unit area is commonly known in
scientific terms as pressure.
So how is pressure mathematically
defined? Pressure is given by
thrust divided by area. So, thrust per unit
area is also known as pressure.
So now let us find out what the unit of
pressure is in the SI system.
Now we have seen that the unit of thrust
in SI system is Newton and the unit
of area is metre square. Pressure, we
found, is given by this particular
expression, that is equal to thrust by
area.
So in other words, we can say unit wise
it is Newton by metre square. So, the unit-
SI unit of pressure is Newton per meter
Square. Newton per meter square is also
known by another name, that is, Pascal.
This unit Pascal has been named after
the famous scientist Blaise Pascal
who had conducted extensive studies in this
particular area of science.
Similarly, we can find out the 
C.G.S unit of pressure.
The C.G.S unit for thrust, which is a
kind of force is Dyne and the C.G.S unit
for area is centimetre square. So
if we place these units in the
expression, we find that the C.G.S unit
for pressure is Dyne by centimetre square
or in other words, it is Dyne 
per centimetre square.
Now let me ask you a very interesting
question.
Over here you'll find that two kids are at
the sea beach.
This kid is standing on the sea beach
whereas this kid is lying down flat
on the sea beach as you can see. Now if
you observe closely, you will find that for
the first kid, her feet are tug into the 
sand, that is, she has sunk into the sand
at the beach. However for the other kid,
he has not been dug into the sand or in
other words he is lying flat as he was
initially. So why do think that this is
happening? Now let me tell you that the
mass of both these kids or in other
words there weights are the same, that is,
the weight of this kid is the same as
the weight of the other kid.
So since their weights are same, we can say
that the thrust is the same in both cases
Because weight is a force which acts
perpendicularly downwards because it is
influenced by gravity acting from the
centre of the earth. So we can say the
thrust is the same in both cases.
Now since thrust is same, consider the
two cases separately. In the first case
since the kid is standing on her feet,
the area of contact is less.
So when the area of contact is less,
we can say that the pressure is more
because the thrust remains the same, whereas in
the other case, for the kid who is lying
flat, in his case the area of contact is
much more because he is lying flat on his belly and
the area is more, but the thrust remains the same.
So the pressure is much less.
Due to this, for the first kid, since the
pressure is more, she kind of sinks into
the sand whereas for the second kid
whose pressure applied is less, he does not sink into
the sand despite the fact that both of them
have the same weight or thrust on the
sand.
Now, I ask you another interesting question.
If you have ever done push-ups, you will know
that while doing push-ups on your palm, it
is comparatively easier than to do
push-ups
on fingers. Now why is it so that it is
difficult to do push-ups on fingers
rather than with your palms?
Take a close look at this video. When this
man is doing push-ups with his palm, as
you can see, he has a good contact area.
Now when the fingers are applied, as you
can clearly see, the area of contact is
significantly reduced. So keeping the
thrusts same, if the area is reduced, the
pressure increases and it is due to this
reason that pressure is equal to thrust
divided by area. The pressure increases
and it becomes more difficult to do
push-ups on fingers rather than on palm.
