the chapter is intended mostly for use students of class 9. based on the Selina publishers

1. PHYSICS
CHAPTER : 4
CL ASS 9
PRESSURE IN FLUIDS AND
ATMOSPHERIC PRESSURE

2. CONCEPTS IN MODULE
 Thrust and pressure
 Pressure in fluids
 Pressure exerted by a liquid column
 Laws of liquid pressure
 Some consequences of liquid pressure
 Transmission of pressure in liquids
 Applications of Pascal's law
 Example of hydraulic machine
 Atmospheric pressure
 Consequences of atmospheric pressure
 Measurement of atmospheric pressure
 Variations of atmospheric pressure with altitude
 Weather forecast by the use of barometer
 Altimeter

3. THRUST AND
PRESSURE

4. THRUST AND PRESSURE
THRUST:
When you are pushing any
object, your weight which is
a force acts downwards
Hammering a big nail into the
ground , acts perpendicular to
the force applied.
Thrust is the force acting normally ( or perpendicular) on a surface

5. Thrust exerted by a body on a surface = weight of the body
 Thrust is a vector quantity.
Unit of thrust:
 It is measured in the unit of force.
SI UNIT : newton (N)
C.G.S SYSTEM: Dyne
{1 N = 105 dyne }
M.K.S SYSTEM : kgf
{1 kgf = 9.8 N}

6. PRESSURE:
The effect of thrust depends on the area of surface on which it
acts.
 the effect of thrust is less on a large area.
 The effect of thrust is more a small area.
In both the cases, the thrust exerted is same but the effects are
different.

7. Pressure is the thrust per unit area of surface
If the thrust F acts on an area A, then
Pressure = thrust
Area
Pressure is a scalar
quantity .
UNITS OF PRESSURE:
S.I UNIT : Pascal N/m2
C.G.S SYSTEM: dyne/cm²
{1 dyne/cm² = 0.1N/m2 }
Others: bars and millibars
Constan
t thrust
Constant
thrust
More
area
Less area
Less pressure More
pressure
Pressure
=forceare
a

8. Factors on which the pressure exerted by a body
depends
The pressure exerted by one body on another depends on
two factors:
1. The magnitude of force applied:
To increase the pressure applied to any object increases
the amount of force applied.
So, greater the force greater would be the pressure
applied.
The reason this effect happens because pressure varies
directly with force as per our definition.
2. The area over which force is applied:
This area is the area of contact between two objects.
You can increase pressure due to the same force by
reducing the amount of area over which the force is
acting.

9. ACTIVITY : 1

10. ACTIVITY : 2
Case 1: Case 2:
 The brick placed vertical exerts
more pressure with its longest
side.
Area of the base: 5 x 10 = 50
cm2
Pressure on the ground = 4kg/
50cm2
= 0.08 kgf/cm2
20cm
10
cm
5cm
Shortest side vertical Longest side vertical
4kg
s
Note: thrust is same in both the cases

11. WAYS OF
INCREASING
PRESSURE
For the given
thrust , the
pressure on a
surface Is
increased by
reducing the
area of surface
on which it is
acting.
End of the nail or pin are made pointed so
that large pressure is exerted with less efforts.
The cutting tools also have either sharp or
pointed edges so that large pressure is exerted
with less efforts.

12. WAYS OF
DECREASING
PRESSURE
For the given
thrust, the
pressure on
a surface is
reduced by
increasing
the area of
surface.
Wide wooden sleepers are placed below the
railway tracks so that the pressure exerted
by the iron rails on the ground become less.
The foundation of buildings are made
wider than the walls so that the pressure
exerted by the building on the ground

13. PRESSURE IN
FLUIDS

14.  A Substance which can flow is called a fluid.
 All liquids and gases are, thus, fluids.
DIFFERENCE IN PRESSURE
• Pressure of solids
 a solid exerts pressure on a
surface due to its weight.
A solid exerts pressure only
on the surface on which it is
placed(bottom).
• Pressure of fluids
fluids also exerts pressure
due to its weight.
A fluid exerts pressure on
the bottom as well as on
the walls of the container
due to its tendency to flow
Solid liquid gas

15. LIQUID PRESSURE
 Liquid exert pressure on the surface of the body immersed in it and the container in
which it is placed.
DERIVATION:
Let liquid mass be m
Density = ρ (rho)
Be taken in a cylindrical container
Area of the base of the container = A
Height of the liquid level in the
container from bottom= h
Volume occupied by the liquid = v

16. The liquid exerts a force equal to its weight towards the bottom of the container.
The weight of the liquid in the container is
W = m x g
Where g is acceleration due to gravity
Mass = density x volume
So,
W = ρ x A x h x g
Hence, the force exerted by liquid is
F = W = ρ x A x h x g
We know that , pressure
p = F/ A
So ,
p = ρ x A x h x g = h ρg = h ρ
Since acceleration due to gravity is constant at a given place , the pressure exerted by a liquid
depends on the height of the liquid and density of a liquid
A
Total pressure in a liquid at a depth h
= atmospheric pressure + pressure due to liquid column
= Po + h ρ g

17. FACTORS AFFECTING THE PRESSURE AT
A POINT IN A LIQUID
From the given equation
h= Po + h ρ g
The liquid depends on following three factors:
i. Depth of the point below the free surface (h).
ii. Density of liquid (ρ).
iii. Acceleration due to gravity (g).
The pressure inside a liquid does not depends on :
i. The shape and size of the vessel in which liquid is contained.
ii. The area of the surface on which it acts.

18. PROPERTIES/LAWS OF
LIQUID PRESSURE
Pressure exerted by a liquid increases with increase in depth from
the liquid surface
Pressure exerted by a liquid at the same depth will be the same.
Liquid exerts pressure in all directions.
Pressure at the same depth is different in different liquids.
A liquid seeks its own level
i. ii iii iv v

19. Do you know…….
Why are the dams constructed
on river thicker at the bottom?
Why do deep sea divers wear
diving suit?
Why does the water flows at
the faster rate from the tap
located on the ground floor
than the taps located on the
first floor?
First
floor
Groun
d floor
Thus ,
 The divers wear the special dress to protect themselves from getting crushed.
 The dams are thicker at the bottom as the pressure increases with depth and
withstand the pressure .
 Since the pressure decreases with the increase in height therefore water from the
taps located on the on the first floor flows at a slower rate.
SOME CONSEQUENCES OF LIQUID
PRESSURE

20. Continue…..
The pressure at a certain
depth in sea water is more
than that at the same depth in
river water
Size of gas bubble inside water
rises as it grows in size.
Greater the height of the tank , more will be the pressure of water in
the taps

21. TRANSMISSION
OF PRESSURE
IN LIQUID
PASCAL’S L AW

22. TRANSMISSION
OF PRESSURE IN
LIQUID
French scientist
Blaise Pascal put
forward the law
While studying the
behavior of water as
an incompressible
fluid discovered that
when pressure is
applied at a point
in a confined fluid ,
it is transmitted
equally in all
directions.

23. Demonstration of Pascal's law
Initial level of
water
OBSERVATION:
The level of water is the same in all
side tubes.
Reason:
This is because the liquid seeks its
own level .
When compressed , the
level of water
Observation:
The level of water rises to the
same height.
Reason:
This happens because the
pressure applied at one point
exerts pressure in all

24. APPLICATIONS OF PASCAL'S LAW
https://www.youtube.com/watch?v=hV5IEooHqIw
https://www.youtube.com/watch?v=AtQ1oh09Uyo
https://www.youtube.com/watch?v=VxLTDtaRCZk
Hydraulic jack

25. CONCLUSION
In all the hydraulic machines:
 Effort is less than load.
 Distance move by effort is more than the distance move by load.
 Effort x distance =load x distance
(effort) (load)
(hence, there is no loss of energy)
 Work done by effort = work done by load
 Thus, mechanical advantage (M.A) = load > 1
effort
And velocity ratio (V.R) =distance moved by effort > 1
distance moved by load
Hence a hydraulic machine acts like a force multiplier

26. ATMOSPHERIC
PRESSURE
AT M O S P H E R I C P R E S S U R E A N D I T S
M E A S U R E M E N T

27. INTRODUCTION
The earth is surrounded by air up to a height of about 300km from its
surface. The envelope of air around the earth is called atmosphere
The thrust exerted per unit area on the earth surface due to column of
air, is called atmospheric pressure on the surface of earth

28. The graph of pressure in various layer of atmosphere

29. EXAMPLES
• Pressure on a table • Pressure on human body
Why doesn’t these collapse under so much pressure?
The blood
pressure is slightly
greater than tha
a.p

30. DEMONSTRATIO
N OF
ATMOSPHERIC
PRESSURE
A C T I V I T I E S

31. COLL APSING
TIN CAN
EXPERIMENT
Observation:
The can collapses inwards.
Reason: initially the pressure
due to steam inside the
heated can is same as the
air pressure outside the can
.
Reason:
But on pouring cold water
The steam inside the can
condenses, produces water
and water vapors at a very
low pressure.
Consequently, the excess
atmospheric pressure
outside the can causes it to
collapse inwards.
1
2
34
5

32. TRY THIS AT HOME