Page 118 - Understanding NCERT Science 09

You have learnt that weight is the force by the wooden block on the table top if
acting vertically downwards. Here the force is it is made to lie on the table top with its
acting perpendicular to the surface of the sand. sides of dimensions (a) 20 cm × 10 cm
The force acting on an object perpendicular to and (b) 40 cm × 20 cm.
the surface is called thrust.
When you stand on loose sand, the force, Solution:
that is, the weight of your body is acting on
The mass of the wooden block = 5 kg
an area equal to area of your feet. When you The dimensions
lie down, the same force acts on an area equal
to the contact area of your whole body, which = 40 cm × 20 cm × 10 cm
Here, the weight of the wooden block
is larger than the area of your feet. Thus, the
effects of forces of the same magnitude on applies a thrust on the table top.
different areas are different. In the above That is,
cases, thrust is the same. But effects are Thrust = F = m × g
= 5 kg × 9.8 m s –2
different. Therefore the effect of thrust
= 49 N
depends on the area on which it acts.
Area of a side = length × breadth
The effect of thrust on sand is larger while
= 20 cm × 10 cm
standing than while lying. The thrust on unit
= 200 cm = 0.02 m 2
2
area is called pressure. Thus,
From Eq. (9.20),
thrust
Pressure= (9.20) 49N
area Pressure = 2
0.02m
Substituting the SI unit of thrust and area in
-2
Eq. (9.20), we get the SI unit of pressure as N/ = 2450 N m .
–2
2
m or N m . When the block lies on its side of
In honour of scientist Blaise Pascal, the dimensions 40 cm × 20 cm, it exerts
SI unit of pressure is called pascal, denoted the same thrust.
as Pa. Area= length × breadth
Let us consider a numerical example to = 40 cm × 20 cm
2
understand the effects of thrust acting on = 800 cm = 0.08 m 2
different areas. From Eq. (9.20),
49 N
Example 9.6 A block of wood is kept on a Pressure = 2
tabletop. The mass of wooden block is 5 0.08 m
kg and its dimensions are 40 cm × 20 = 612.5 N m –2
cm × 10 cm. Find the pressure exerted The pressure exerted by the side 20 cm
–2
× 10 cm is 2450 N m and by the side
–2
40 cm × 20 cm is 612.5 N m .

Thus, the same force acting on a smaller
area exerts a larger pressure, and a smaller
pressure on a larger area. This is the reason
why a nail has a pointed tip, knives have sharp
edges and buildings have wide foundations.

9.5.1 PRESSURE IN FLUIDS
All liquids and gases are fluids. A solid exerts
pressure on a surface due to its weight.
Fig. 9.4 Similarly, fluids have weight, and they also

GRAVITATION 107

Rationalised 2023-24