Can
you stand on a single leg easily that you can do by using your two legs? The
shoes
pierce
into the mud when one walks on soft mud wearing a high hill. But if one does so
wearing
a flat pair of shoes, then it does not get into the mud. We will see that it
happens
due
to the change of pressure.
Fig-5.1
The
force exerted perpendicularly to unit are of a body is called pressure. Let a
force F
act
perpendicularly on area A of a surface, then pressure, P =
FA
i.e.
Pressure =
Force
Area
It
is noted that less the area A, more is the pressure P and more the force F,
more is the
pressure
P.
Example:
(i)The
area of the sharp edge of a nail is very small. So when the sharp end of the
nail is
placed
on a wood like surface and hammered on the flat head of the nail, comparatively
more
pressure is exerted on the sharp end as a result the nail easily enters into
the
substance.
(ii)The
area of the sharp edge of the knife is very small. So, by placing the sharp
edge of
the
knife on a substance if force is applied, then more pressure acts on the
substance
along
edge. Therefore the substance can easily be cut.
Do it yourself: Hole a paper by a very sharp pin and by a blunt pin. Which
one is easier to hole?
Explain.
More
pressure is felt on the sharper edge of the pin when force is applied on its
flat end.
When
force is applied on the flat end of the blunt pin, less pressure is exerted on
the
blunt
end .So, it is easy to hole a paper by a sharper pin.
Physics
89
Verify: Which one is more painful to walk bare footed on plain
bricks soling road or a brick
chucked
road? Explain
Unit of pressure:
If
we divide the unit of force by the unit of area, we get the unit of pressure. Therefore
the
unit of pressure is Nm-2.
It is called pascal (Pa).
∴ 1Pa = 1Nm-2
If
a force of 1N force is applied perpendicularly to an area of 1m2, then the pressure
generated
is known as 1Pa.
Mathematical Example 5.1: The mass of a woman wearing a shoe is 50kg. The area of
the
bottom of the shoe is 200cm2. Find the pressure.
We
know.
Pressure
P
= F/A = W/A
=
=
2.45×104Pa
Given,
mass,
m=50kg
force,
F = w = mg = 50kg×9.8ms-2
=
490N
Area
of the bottom of the shoe, A = 200cm2
=
200×10-4m2
5.2: Density
The
space occupied by a body is called its volume. If we drop a piece of cork and a
piece
of
iron of the same volume into water, we see that the piece of cork floats on
water but
the
piece of iron gets submerged into water. We may say in general that the density
of
iron
is more than the density of cork. So, iron gets sunk into water. Though the
volume
of
the substances are the same, actually the more the density, the heavier the
object is,
and
the less the density, the lighter the object is. The mass per unit volume of a
substance
is called the density of its material. Density is a general property of matter.
Density
of a body depends on its material and temperature.
Density
is denoted by ρ. If the mass of a body is m and its volume is V,
then
density ρ =
mV
=
mass
of the body
volume
of the body
The unit of
density is kgm-3.
