Page 147 - Physics

Pressure
4. The density of a Pine wood block
is 700 kg/m . If it measures
3
1⋅5 m×1⋅2 m×1⋅0 m, calculate
the maximum and the minimum
pressure exerted by the block.
Pressure in liquids
A liquid will exert pressure on an
immersed object as well as on the wall
of its container. Note that, the pressure
exerted by the liquid at any point in the
liquid is due to the weight of the liquid
column above the point. As the depth
of the liquid increases, the weight of
the liquid also increases and hence the
pressure increases. This concept can be
explained using the marble pile model.
It is important to note that, since the
particles in liquid are loosely packed, the
pressure acts in all directions. Assume
that the molecules of liquid are packed
together but are not fi rmly tight to their
neighbours, as shown in Figure 7.10.
Figure 7.10: Marble pile model
If a downward force is applied shown
in Figure 7.10, the marbles are pushed
sideways as well as downwards. Similarly,
the gravitational force pulls the liquid.
This causes the liquid to be pushed out
sideways as well, and so the liquid exerts
a sideways pressure too.
Physics Form 1 Final.indd 141
Calculating the pressure in a liquid
Figure 7.11 shows a tank with base area A
2
(m ) fi lled with a liquid to a height h (m)
and the gravity, g (N/kg) pushes liquid
molecules down producing the force
of gravity or weight (N) in the liquid.
Because the area of the tank is fi xed,
increasing the height of the liquid in the
tank will also increase the weight of the
liquid and the pressure, as previously
stated. Therefore, the pressure in the
liquid depends on the weight of the liquid
and the base area of a tank.
Water
Height (h) Base area (A)
Figure 7.11: Pressure exerted by water in
a tank
To calculate the pressure in liquids,
consider the liquid column of cross-
sectional area, A (m ) at a depth, h (m)
2
below the surface of the liquid. If the
density of the liquid is ρ (kg/m ), then,
3
Volume of the vertical column of liquid
= Base area A ( ) × Height h ( ) = Ah
The mass due to the liquid column
= Volume of liquid × Density of liquid.
= Ahρ
But Force, F = mg = Ahρg
141
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