KENDRIYA VIDYALAYA NO.I, JIPMER CAMPUS, PUDUCHERRY – 605 006
Name of the Teacher with e-mail ID
: Smt. G. Bhanumathy, bhanurajan@yahoo.com
Designation
: TGT(Science)
Name of the KV with complete address : K.V.No.I, JIPMER Campus, Puducherry 605 006
Whether awarded by KVS.NCERT/CBSE/any other: Yes
1.
Title of the Innovation / Project : Studies on Thrust and Pressure in Liquids
2.
Background / rationale / the setting for the project / innovation:
Physics as an independent subject is introduced in Class IX, and many students feel that
the subject is full of difficult concepts. This feeling is all the more true because very little
contact is made with experiences in their daily life.
To make learning physics easier and enjoyable the author has selected the topic, “THRUST
IN LIQUIDS”,(a topic in Class IX), and has devised simple experiments to show how they lead to
the basic laws.
It was also hoped that at the end of this innovative project, the students will be able to
appreciate that even simple studies on thrust in liquids, became the sources of an astonishing
variety of technological applications, ranging from simple manometers and vacuum pumps to
aircraft!
Before introducing the concept of Thrust in liquids, the more familiar concept of “Force”
in solids was first introduced and discussed. The concept of Thrust and its relation to force had
to be properly understood. The innovative project was so designed as to make the students
understand clearly the more appropriate use of thrust rather than force in liquids. This
objective was planned and carried out as follows:
1.In activity I, the idea of thrust was introduced, making use of the familiar force concept.
Although this activity appears to be simple, the squeezing force on the bottles can be
made quantitative, if efforts are made for a proper calibration. A simple procedure was
followed for this purpose. Known weights were placed on the body of the water-filled
bottles , and the depressions produced in the plastic bottles were marked by a waterproof marker. It was then reasonable to assume that if the same depression is observed
by a squeezing force, it then corresponds to that particular weight. After this is done,
fundamental observations that Thrust is naturally defined as Force per unit area and is a
more appropriate parameter were learnt by the students.
2.The next two activities, II and III, were so designed as to probe into the nature of thrust
in liquids. The activities were designed using materials which are easily available. Water
was chosen as the liquid in these activities.From this activity the students were able to
answer the question, “how to measure the thrust?”It was clear that thrust in a liquid can
be measured by a device called manometer. Another important concept, namely, that
thrust is omnidirectional in static liquids(Pascals law) was also demonstrated.In all these
three activities the water column was at rest, or “static”.
3.It was considered interesting and appropriate to see the effects on the concept of
thrust when the liquid is set in motion. The activities IV and V assembled with easily
available materials were set up. The main observation here was that the thrust depends
on the area of cross section, unlike the case in static state, leading to the Bernoulli’s law.
4.The next three activities ,namely VI ,VII, VIII demonstrated the two applications:vacuum
pump and the aerodynamic lift.
To conclude, by performing all these innovative activities, the author strongly feels that
students had “hands on experience”. This made the learning of Thrust in Liquids
mucheasier to understand and at the same enjoyable. Another highlight of these
innovative activities is that the students were also able toappreciate the practical values
of the basic concepts.
The author wishes to mention that the total expenditure incurred to carry out in school
was less than RS.500.
3.
Elaboration /details of the project / Innovations:
STUDIES ON THRUST AND PRESSURE IN LIQUIDS
I.GENESIS OF THE PROJECT
Physics as an independent subject is introduced in Class IX, and many students feel that the
subject is full of difficult concepts. This feeling is all the more true because very little contact is
made with experiences in their daily life.
To make learning physics easier and enjoyable, the author has selected the topic, “THRUST IN
LIQUIDS” (a topic in Class IX), and has devised simple experiments to show how they lead to
the basic laws.
It was also hoped that at the end of this innovative project, the students will be able to
appreciate that even simple studies on thrust in liquids, became the sources of an astonishing
variety of technological applications, ranging from simple manometers and vacuum pumps to
aircrafts!
Before going to the concept of “THRUST IN LIQUIDS”,the more fundamental concept of “Force”
is first introduced and discussed.
The students were asked to state what keeps a bicycle moving or a weighing machine read
their weights. They all answered,”Force is needed”. It was then pointed out that the
magnitude of the force in the first case is measured by the acceleration of the moving bicycle,
whereas in the second case by the compression of the spring present inside the weighing
machine.
These statements were expressed in the mathematical form respectively as:
F=m × a or F(N)= m(kg) × a (m/s2) (first case)
F=stress × cross sectional area of the spring x strain or
F(N)=Stress(N/m2) × area of CS (m2) × Strain (second case)
So for solids the concepts of force appears to be easier to understand.
When a liquid is next considered, the concept of “THRUST” is first introduced.
This is defined as, Force/Unit area, instead of directly the force.
The question arises whether, like in static ,or moving solids, the concept of “THRUST” plays a
similar role in static or moving liquids.
To clarify this question, simple experiments were devised to show the behaviour of thrust in
static and moving liquids. In the subsequent sections the details of the project are discussed.
II. Preparatory work for the Project:
The project consisted of devising eight activities.
For devising simple experiments with liquids in Static state,(Activities I,II and III) the following
apparatus were mobilised.
 Bucket(15 lit), two plastic bottles, one with a larger diameter and the other with a
smaller diameter, fitted with mouths at the two ends.
 Horizontal pipe (PVC or glass) closed at the both ends and having holes of different
diameters along its length. Also required are glass tubes to fit into these holes to
measure the liquid level.
 Horizontal tube with varying cross sections and having holes drilled along its length, and
glass tubes to fit into these holes are again required.
For devising simple experiments with liquids in flowing (dynamic) state,(Activities IV and
V) the following apparatus were mobilised.
 Horizontal tube with a uniform cross section, open at both ends cross section and holes
of different diameters along its length, glass tubes to fit into these holes to measure the
liquid level were provided.
 Horizontal tubes with varying cross sections, open at both ends, holes drilled along its
length glass tubes to fit into these holes.
 Activity VI:long straight PVS pipe with a side tube, a pipette and a plastic bag.
 Activity VII: A thin strip of paper
 Activity VIII: A card board lamina shape like a bow/whale.
The required human help was provided by the participating students themselves.
4. Methodology adopted
Activity I: This is an extension of the “empty bottle” activity given in the NCERT text
book.(Page 140)
Two bottles (plastic) one with larger diameter of 60mm and the other with a smaller
diameter of 20 mm, were taken. Two mouths were made in each bottle. The bottles
were first filled completely with water removing airbubbles and the two mouths closed.
The bucket was filled with water and the students were asked to immerse the plastic
bottle in the bucket and remove slowly the stopper at the bottom. Normally, the bottle
will float. Students were asked to apply gentle pressing of the plastic bottle , exposed
above the water level in the bucket. Then they were asked to immerse the bottles
slowly and continuously into the bucket and observe whether the bottle get harder to
squeeze.
Then the top mouth was also open and the students were asked to record the
observation. Fig 1 depicts the scheme of the experiment.
Activity II:
This consists of a horizontal pipe/tube about 30 mm in diameter, closed at both ends.
To this tube a number of vertical tubes were attached. These tubes are of different
diameters and shapes. To observe carefully the behaviour, atleast the vertical tubes
must be of glass or transparent plastic. Water was poured through one of the vertical
tubes until the levels rise in all of them.Fig(2) gives the details of this activity.
Activity III
There is a slight difference. The horizontal tube was made with varying cross sections.
Again water was poured through one of the vertical tubes. Students were asked to
record the heights of the liquid column in the various vertical tubes in both the activities.
Fig(3) shows the details of the activity
Activity IV: Arrangements are the same as in Activity II, except that water was made to
flow. For this one end of the tube was connected to the laboratory tap and the other
end to the drain as shown in Fig(4).The rate of flow of water could be adjusted by
controlling the valve of the tap. Students were asked to record the heights of the liquid
column in the various vertical tubes.
Activity V: Arrangements are the same as in Activity III, except that water was made to
flow. All the manometer tubes were made of the same diameter. The rate of flow is
adjusted by manipulating the degree of opening of the tap. Students were asked to
record the interesting changes seen in the heights of the liquid column in the various
verticaltubes.
Fig (5) explains the details.
Activity VI: .A long straight PVC pipe with a side tube and a pipette was taken. Pipette
was connected to a water tap. A plastic bag was connected to the side tube as shown in
the Fig(6).When water flows through the jet, the plastic bag was seen to shrink.
The reason for this was explained to be due to low pressure created around the jet.
Consequently, air from the plastic bag rushes to fill the low pressure region, thereby
creating a low pressure or vacuum in the plastic bag. Thus plastic bag was seen to shrink.
The students immediately noted that the observation is similar to the lowest height of
the water column at the smallest diameter of the horizontal flow tube as in Activity V.
Activity VII: Students were asked to blow air on a strip of paper along its surface as
shown in the figure( 7). In this activity the behaviour of the paper strip before and after
blowing of air to be carefully noted.
Activity VIII: A bow shaped or a whale shaped cardboard disc was suspended at the
two ends by means of thread as shown in the Fig(8) and the air was blown by a table fan.
The students were asked to record their observations.
What the students were able to learn from these activities are now elaborated below.
IV RESULTS:
The Various observations made and the inferences deduced are now summarised.
Activity I: For both the bottles of larger and smaller diameter, the squeezing force
increases with the depth. However, increase in the squeezing force for the smaller
diameter was less than for the larger diameter at a particular depth.
The squeezing force could be made reasonably quantitative by “calibrating” amount of
depression of the bottle with known weight placed on the bottle before the experiment.
The result could be depicted graphically as shown below.
Depth
It was found that the ratios,
length PB/ area of the larger tube and lengthPA /area of the smaller tube
were equalwithin experimental errors.
While the numerator is the measure of force, the denominator is the area of cross section,
and the ratio F/A being equal. It means that Thrust is a more useful quantity for liquids. It
also shows that liquids have uniform value of
thrust( in N/m2)at a given depth/level.
Therefore THRUST is a more fundamental quantity for liquids than force.
The students then compared Force and Thrust with mass and density and understood that
just as density is more fundamental than mass, thrust is more fundamental than force in
liquids. Thrust is also called Hydrostatic Pressure and it is omnidirectional.
Activities II and III: In both the experiments , as long as the water is static ,the liquid levels
in all the tubes were found to be the same.
Students were next asked to analyse the situation by computing the upward thrust in
each tube, which is balanced by the weight of liquid column. The condition for
equilibrium as stated below was used.
the upward force by the liquid by the thrust of the liquid =downward force due to the
weight of the liquid column.
For each tube, the upward force due to the thrust of the liquid column is given by:
Thrust × area of cross section of the tube.
The downward force in the same tube is due to gravity and is given by weight of the
liquid column.
Weight of the liquid column=h ×A × d × g
Where h=height of the liquid column(m), A=area of cross section of the tube(m 2),
d=density of water (kg/m3) and g= acceleration due to gravity(m/s2).
From this equilibrium relation, since the area of the tube cancels out, all tubes have the
same height independent of the cross section.
Pressure or Thrust = h d g(N/m2) (or Pascal in SI units)
The important observation that the pressure is proportional to height led to the
development of manometers to measure pressure or thrust due to liquids. Incidentally,
an interesting fact was pointed out to the students. At the deepest part of the Pacific
ocean, the hydrostatic pressure is nearly 10,000 atmospheres. Adequate protection
devices are therefore needed for very deep-sea divers.
As an exercise, the students were asked to read about Torricellian mercury manometer,
and the creation of man-made vacuum for the first time in a mercury manometer.
So far, the behaviour of thrust or pressure in a static water column was studied. To
arouse their curiosity, the question was put to them: what would be the behaviour of
thrust if the water is made to flow, instead of remaining static?
So, the attempts were made to answer this question by once again devising simple
experiments.
Activity IV: Main observations were
a)The height of the water column in all the tubes was still the same.
b) However, compared to the static case the height of water column showed a slight,
but the same decrease, in all the tubes.
c) The decrease was more pronounced when the speed of the flowing water increases.
Activity V:
Very interesting changes in the levels were observed . Specifically, in the widest
diameter of flow tube , the level is the highest and in the smallest diameter of flow
tube ,the level is the lowest. In a flowing pipe the pressure or thrust decreases with the
diameter. The students were made to understand that if the diameter of the flowing
pipe is made sufficiently small, pressure less than the atmospheric pressure, can be
achieved.This point was demonstrated in the next activity.
Activity VI:
By performing this experiment, it was observed that thrust or pressure depends on the
cross section of the flowing tube. Thus, by making water to flow through an orifice
students realised that it is possible to devise a vacuum pump.
Activities VII and VIII:
The last two activities showed how a clever modification of the type of flow can produce
the same effects, namely the pressure difference.
The main idea is to use path differences in flow instead of diameter differences. For
this, it was obvious that air is a better and more convenient choice than water.
A paper strip bent downwards became straight and even lifted upwards when air was
blown. This is due to the air in the shorter path(similar to tube of smaller cross section)
which creates a low pressure region. The surrounding air rushes to the low pressure
region and hence the paper strip is lifted. Finally, the students were shown how by
designing the bow/whale shaped lamina hanging freely in air was lifted upwards, when
the air was blown from one end.
Students were posed with a question whether the effects are the same, if instead of
static lamina and moving air as in the experiment, the lamina is made to move in static
air. To perform experiments to answer this question, more elaborate arrangements are
obviously needed. They were told how the experiment showed that the effects are the
same. This led to the development of a flying machine and an aircraft.
FIG 1
In this experiment, two plastic bottles having mouths at the top and bottom and
completely filled with water are immersed in a bucket filled with water. The bucket can
be around 15 lit of capacity. The students get an idea of the property called “Thrust” by
squeezing the bottles immersed to various depths. They also become aware of the
dependence of squeezing force on the area of the plastic bottles. A rough idea of
squeezing force can be obtained by “calibrating” the depression in the water filled bottle
against static weight before the experiment.
Fig 2: Shows schematically how the thrust in liquids can be measured by a liquid column.
This also establishes that the thrust in a static water column is the same at a particular
height.
FIG 3
Fig 3: In this experiment, the horizontal tube containing the standing water is made with
varying cross sections. This demonstrates that the thrust is the same as indicated by the
equal heights of the water column.
FIG 4
Fig 4: The water in the horizontal tube of uniform cross-section is now made to flow.
The manometers are of different diameters. Water is drawn from the tap in the Science
Laboratory and by adjusting the valve in the tap, the velocity of flow of the water can be
varied.
FIG 5
Fig 5: Same as in the previous figure, except that the horizontal tube is of variable crosssections and that all the manometers are of the same diameter.
Fig 6: Demonstration of a vacuum pump by a water jet. The drawing of air from the
plastic tube attached to the side and its gradual collapse can be observed when water
flows through the vertical tube at sufficient speed.
Figures 7 & 8
Fig 7: Observing the “pressure effect” produced by blowing air on a paper strip. The
bent paper strip straightens and is lifted upwards.
Fig 8: Demonstrating the lift produced by the Bernoulli principle, by using a cardboard
lamina, shaped in the form of a bow. An air blower or a table fan is needed to clearly see
the lift.
5. Conclusion
In the present project, the author has attempted to make text book concepts understand more
easily and also enjoyable to learn. The topic of “Thrust in liquids” has been chosen as an
example in this innovative project. By devising innovative experiments, the students were
made to learn and appreciate the basic ideas as well as the impressive practical applications.
The main results obtained are:

To realize that “Thrust" is as better concept for liquids and gases than “Force”,
and that thrust is measured in N/m2.

TO show that Thrust in liquids is measured by a device called manometer.

To explain how by using a mercury manometer Torricelli created perfect
vacuum for the first time.

Demonstration of Pascal’s law that thrust is omnidirectional in liquids and
gases.
Turning now to dynamic state of water(flowing water)



Thrust in the uniform cross section tube was the same at all points.
Thrust is, however, lower than in the static case.
With a tube of varying cross section, the behaviour of thrust during the flow
was demonstrated. It was seen that thrust varies with cross sectional area of the tube
in which water is flowing.

Thrust or Pressure is lowest at the smallest area of cross section.

It was stated that if the cross section is made sufficiently small, one can
achieve a pressure less than the atmospheric pressure.

A simple vacuum pump with flowing water was demonstrated. It was also
mentioned that this device is called a water jet pump and this principle is used in
perfume bottles as well as in chemical industries.
By using air instead of water, and showing that the path difference of the fluid (namely
air), instead of variable cross section(as in activity V) leads to pressure difference and
what is called “aerodynamic lift. Two simple experiments were designed to observe
clearly the aerodynamic lift. It was mentioned how these simple experiments led to
profound technological developments.
In the opinion of the author, the experiments described can easily be reproduced in any
school laboratory.
Before concluding, some of the limitations faced in this project, are now mentioned.
Making actual measurements of Thrust in static water, thrust in flowing water for
various velocity of flow and measuring the aerodynamic lift for various flow speeds and
masses of the lamina demanded more time and effort.Further, more sophisticated
equipment(such as strain gauges and anemometers)are needed. For class IX, only
mention to various laws such as Bernoulli’s law, Euler’s law and Pascal’s law was made,
as going into the details were considered to be advanced for them
6. Acknowledgement:
The Author expresses her gratitude to the Principal Kendriya Vidyalaya No.1, JIPMER
Campus, and to the Kendriya Vidyalaya Sangathan, for encouragement and support.
She is indebted to Miss. K.Dhivya for her ready help and support in the preparation of the
report.
Last, but not least, it is a pleasure to thank all the students for their enthusiastic
participation which made this innovative project a joyful experience.