1. Teacher N TBS2011
PRESSURE
Definition : Force per unit area on the surface.
Formula : F P :………………, F :…………, A :……....….
P=
A
Unit S.I of Pressure : ……………… or ………………….
Example 1 : Which shoe will exert a greater pressure on the when it is worn by the same
women?
Answer :…….
A
B
Applications of Pressure :
(1) Tools like knives, scissors, axes and saws have sharp cutting edges. The
surface area of contact is …………… When a force is applied on the tools, the
small area of contact will produce a …………… pressure to cut the material.
(2) Walking over deep snow is difficult and tiring because your feet sink at every
steps. Skies or snowshoes make it much easier because they stop you from
sinking in. The skies causes the pressure becomes …………….
Exercise 1: A table of mass 50kg has 4 legs is placed on a floor. Each legs has a
cross sectional area of 25cm2. Find the pressure exerting on the floor (g=10ms-2).
Solution :
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2. Teacher N TBS2011
PRESSURE IN LIQUID
Density ( ρ ) is defined as: mass per unit volume. S.I Unit is kg m -3 . Oil
Density ( ρ ) =
mass
, ρ = m
volume V Water
Liquids have pressure because of their ……………... Honey
Pressure in liquids acts in ……….directions.
.
The pressure in a liquid is the product of depth (h), density ( ρ ) and gravitational acceleration
(g).
The above formula can be derived from the following steps:
Mass of a cylinder of water, m = ρV = ………………
Weight of the cylinder of water, W = ………..= ……………..
A cylinder of water
A
Volume h The pressure of water at the base of the cylinder of water is,
V = Ah
F W m g ρ Ah g
Water pressure, P = = = = = ρ g h
A A A A
From the final expression , we can say that the pressure at any point in a liquid at rest is
………………………… to the density of liquid and the depth.
The pressure in the liquid is not dependent on the surface area and the…………. of a
container.
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3. Teacher N TBS2011
Example 1 : A balloon is situated at 10 m below sea level, what is the total pressure experience
by the balloon ? [ The density of sea water is 1100 kg m-3 ]
Total Pressure,
P = Atmospheric pressure + Liquid pressureAtmospheric pressure at sea
level :
= 100,000 N m-2 + ρ g h
= 100,000 + …………………….. Patm = 1.0 x105 Pa
= 100,000 + ……………………..
= ………………N m-2
= ……………. Pa
Exercise 1: Given that the density of mercury is 13600kgm-3. Calculate the pressure of mercury
at a point 25 cm from the mercury surface (g =10ms-2).
Solution:
Applications of Pressure in Liquids.
(1) Water Tank : Normally a water tank is placed at higher level so as to supply water at greater
pressure.
(2) Dam : The wall of a dam is much thicker at the bottom than at the top because it must
withstand the increased lateral pressure in depths of the water.
(3) Submarine : Submarine is built with thick wall so as to withstand enormous pressure at
greater depth.
(4) Hydraulic System
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