3.0 forces and pressure

Physics Module Form 4 Chapter 3 Forces and Pressure GCKL 2011

3.1 UNDERSTANDING PRESSURE

Define pressure. Pressure is defined as the _________ applied on a unit __________ ______.
F
P= where F = Force (N)
A 2
A = surface area (m )
P = Pressure (_______)
1 Pa = ___ N m-2

Example : The diagram shows a wooden block of dimensions 8 cm × 10 cm ×
12 cm. Its weight is 12 N. On which side should the wooden block
be placed to produce a maximum pressure exerted on the table.
What is value of this pressure ?
On minimum area i.e. 8 cm X 10 cm

P = Weight (F) .
Minimum Area (A)
= ___12____
(0.08)(0.10)

= ……………

Problem Solving 1. Calculate the pressure on a wall when the palm of a hand with area 150
cm2 is pushed against the wall a force of 210 N.

2. A teacher who weights 637N has a foot with a surface area of 200 cm2.
When he stands with one foot, calculate the force applied per cm2.

Relationship between Situation 1 : Figure 3.1a, press a thumbtack into a piece of wood with your
pressure and force, thumb where as Figure 3.1b, hit using the hammer.
pressure and surface
area

Figure 3.1(a) Figure 3.1(b)
(a) Observe Figure 3.1a and Figure 3.1b and state one comparison.
______________________________________________________

(b) Which one is received a higher force?
______________________________________________________

(c) State the relationship between pressure and force.
_________________________________________________

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Situation 2 : A 500 g cardboard box placed on a table in two different
orientations.

Figure 3.2a Figure 3.2b.
(a) Calculate the pressure exerted on the table for
i) figure 3.2a ii) figure 3.2b.

(b) Which figure shows that the higher pressure is exerted to the table?
_____________________________________________________

(c) State the relationship between pressure and surface area.
______________________________________________________

Factor that affecting 1. Ways to increase pressure are :
pressure.
- Force a) ____________ force ; b) ____________area of contact.
- Surface Area
2. Ways to decrease pressure are :

a) ____________ force ; b) ____________area of contact.

The applications of a) High pressure.
pressure in our daily
life. Tools like knives, chisels, axes and saws have sharp cutting edges. The
surface area of contact is _______ When a force is applied on the tool, the
small area of contact will produce a _________ pressure to cut the
material.

b) Low pressure.

The flat base of each metal pole of a tent has a ___________ surface area
to _____________ the pressure exerted on the ground. The poles will
not sink into the ground because of the flat bases.

Exercise
1. A cylinder has a mass of 12 kg and a 2. Wind blows normally on a wall at a
cross-sectional area of 200 cm2. What pressure of 200 kPa. If the wall has
is the pressure acting at its base? an area of 5 m2., what is the force
acting on the wall?
A. 6 kPa B. 9 kPa
C. 12 kPa D. 15 kPa A. 40kN B. 800kN
C. 1000KN D. 1200kN

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3. A 250 N force is applied at an angle 30o to the 4. A balloon is pressed with a force of 2.4 N
surface of a block. The surface is a square of using a finger. Find the pressure exerted on
sides 10 cm. What is the pressure exerted on the balloon if the area of fingertip is 2.0 x
the surface? 10-4 m2

A. 1.25 kPa B. 2.50 kPa
C. 25.0 kPa D. 12.5 kPa

5. The diagram below shows a concrete block of dimension 1.5 m x 2.0 m x 3.0 m.
Its weight is 60N. Calculate (a)maximum pressure, (b)minimum pressure:

a) maximum pressure b) minimum pressure

6. A Perodua Myvi with a mass of 900 kg. Air pressure for each tyre is 2 x 105 Pa.

(a) What is the meaning of pressure ?

____________________________________________________________________

(b) Calculate the area in contact with the ground for each tyre.

c) Zamani drives his car to his school with a distance of 10 km and find his car tyre become harder
than usual. Explain why this is happen?
________________________________________________________________________

________________________________________________________________________

________________________________________________________________________

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7. Experiment : Based on the diagram below, complete the suitable planning experiment.

a) Inference :

b) Hypothesis :

c.i) Aim :

ii) Variables  Manipulated : ……………………………

 Responding : ……………………………

 Constant : ……………………………
iii) Apparatus
& material
iv) Diagram

v) Procedure …………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
………………………………………………………………………………………….
vi) Result

vii) Analyse
data

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3.2 UNDERSTANDING PRESSURE IN LIQUIDS

Characteristics 1. Pressure in liquids acts in __________ directions.
of pressure in
liquid

2. Draw the jet of water Pressure in liquids _______
from the container. with depth.
.

3. Draw the water level in the container.
Shape and size _________
influence the pressure

4. The hole in both containers are at the same height. Draw the jet of water and oil.

water
oil

- The higher the density the _________ the pressure.

Formula for The pressure in a liquid is the product of depth, density and gravitational acceleration.
pressure in
liquid P = Pressure ( Pascal)
P= x x
ρ = density ( ______ )
g =_____________
h = height (m)

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Example :1. An air bubble is at a depth of 5 m below the surface of a lake . What
is the pressure of water on the bubble if the density of the water is
1000 kg m-3?
P=hρg
= 5 x 1000 x 10
= _________ Pa

2. The figure shows a high tin with a length of 100 cm is filled to the
full with water.

If the pressure caused by the water at point P is 8000 Pa, What is the
value of L?(Density of water = 1000 kg m - 3 )

Experiment : Pressure and density in liquids

a. Inference :
b. Hypothesis :
c. Aim :
d. Variables i) Manipulated : ……………………………
ii) Responding : ……………………………
iii) Constant : ……………………………
e. Apparatus
& material
f. Procedure
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………
…………………………………………………………………………………………

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…………………………………………………………………………………………
g. Result

h. Analyse : Graph of ……………………………………………………………………………..
data ………………………………………………………………(draw on graph paper )

i. Conclusion :

j. Precaution :

Applications of 1. 2.
pressure in
liquid

Exercise

1. Which of the following factor does 4. The density of water is 1000 kg m-3.
not influence the pressure of a liquid?
A. Depth
B. Acceleration due to gravity
C. Density
D. Volume

2. Mercury has density of 13600 kg m-3.
If the pressure of mercury is 650 kPa,
what is the depth from its surface? Find the density of liquid X in kg m-3?
A. 4.0 m B. 4.8 m
C. 5.8 m D. 6.4m

3. Water has density equals to 1 g cm-3.
What is its pressure at a depth of 12 m
from the surface?
A. 80 kPa B. 100 kPa
C. 120kPa D. 140kPa

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5. A container contains two layers of liquids. An oil layer 45cm thick floats on 60 cm of water. The
densityof oil is 750 kg m-3 and water is 1000 kg m-3. ( g = 10 m s-2 )
a) What is the pressure exerted on water by the oil layer?

b) what is the total pressure exerted on the bottom of the container?

6. The pressure at the bottom of the dam is 2.4 x 106 Pa. What is the depth of water if the density of
water is 1 000 kg m-3.

7. Figure below shows a cross-sectional area of the structure of a dam.

Wall Water

Observe the thickness of the wall at the top and bottom part of the dam in the figure above.
Based on the observations:

(a) State one suitable inference that can be made. [ 1 mark ]

(b) State one appropriate hypothesis for an investigation. [ 1 mark ]

(c) With the use of apparatus such as a thistle funnel, beaker and other apparatus, describe
an experimental framework to test your hypothesis.

In your description, state clearly the following:
(i) aim of the experiment,
(ii) variables in the experiment,
(iii) list of apparatus,
(iv) arrangement of the apparatus,
(v) the procedure of the experiment which include the method of controlling the
manipulated variable and the method of measuring the responding variable,
(vi) the way you would tabulate the data,
(vii) the way you would analysis the data. [ 10 marks ]

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UNDERSTANDING GAS PRESSURE AND
3.3
A T M O S P H E R I C P R E S S U R E
Kinetic molecular Gases are :
theory  Made of …………………………………..
 Move ……………………………………..
 Far apart with a very ………………….. attraction.
 Collide each other and collide with the wall of container with
……………
 The average kinetic energy ……………………. when the temperature
……………….

Gas Pressure The gas pressure in a container is caused by the ………... of gas molecules with
the ……… of the container.

Gas pressure can be measured by using 2 types of instrument known as :
(a) …………. gauge (consists of a semi-circular or C-shaped copper tube that
tends to straighten if more and more gas is pumped (compressed) into it).

(b) …………. (consists of a U-tube about 1 m in height. About 50% of the
volume of the U-tube is filled with liquid such as mercury or water).

Atmospheric The atmospheric pressure is caused by the downward ……….. exerted by the air
pressure ( the weight of the atmosphere) on the Earth’s surface.

The greater ……… from the sea level, the ……… the atmospheric pressure.

The atmospheric pressure at sea level is 1 atmosphere ( 1 atm)

1 atmosphere(atm) = ……… mm Hg = ……………..Pa = 1000 milibar

Find this:
If density,ρ of mercury (Hg) = 13600 kg m-3 and density of water = 1000 kg m-3.

1 atm = __________ m of water

Barometer is an instrument to measure atmospheric pressure. There are 2 types
of barometer:
(a) ................................ (is made of a partially vacuum sealed metal box).

(b) …………………. (is made of a long glass tube about 1 meter in length fully
filled with mercury and then inverted (turned upside down) into a bowl of
mercury).

Example 1: The atmospheric pressure is 760 mm Hg. What is the value of the
atmospheric pressure in Pascal? [ Density of mercury, ρ (Hg) = 13 600 kg m-3 ]

h = 760 mm = 76 cm = 0.76 m
Atmospheric pressure, P = h ρ g = (0.76)(13600)(10)

= ………………. Pa

3-9


Manometer
PA = _____ = Patm = 76 cm Hg

1 PC = PA + PHg PD = _____ + _____
D

= (76 + 4) cm Hg = ______ cm Hg
C

= 80 cm Hg

ρHg = 13600 kg m-3 ; ρwater = 1000 kg m-3

h cm PA = _____ = Patm = 76 cm Hg
2 C D water

PC = PA + PHg PD = _____ = 80 cm Hg

= (76 + 4) cm Hg

= 80 cm Hg
PD = ____ cm water

PA = Patm = 76 cm Hg

PC = PA + PHg
3 Gas
flow
= (76 + ___ ) cm Hg

= ________ cm Hg

Pgas = PD = ______

= ______ cm Hg

Simple Barometer 1. Figure shows apparatus set up which
is used to measure atmospheric
pressure.
[Mercury density = 1.36 x 104 kg m-3]

Calculate the pressure at point Q in Pa.

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2. If the atmospheric pressure is 76 cm
Hg, what is the pressure of the trapped
air P?

Applications of 1.Siphon
atmospheric
pressure. ………………………………….

…………………………………..

…………………………………..

…………………………………..

2. Vacuum cleaner
………………………………….

…………………………………..

…………………………………..

…………………………………..

Exercise ( g = 9.8 m s-2 ; ρHg = 13.6 x 103 kg m-3 )

1. What is the pressure of the gas 2. Which instruments is meant for
trapped inside the J-tube, in Pa unit? measuring atmospheric pressure?
A. Carburettor
B. Siphon
C. Fortin’s Barometer
D. Hydrometer

Which of the following is not true about
3.
atmospheric pressure?
A. Atmospheric pressure acts in all
directions.
A. 1.19 x 105 Pa
B. Atmospheric pressure decreases with
B. 1.90 x 105 Pa
distance from Earth’s surface.
C. 2.19 x 105 Pa
C. Atmospheric pressure can only measure
D. 2.90 x 105 Pa
in Pa or N m-2.

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4. Diagram below shows a set up of apparatus for measuring atmospheric pressure.

Diagram shows a set up of apparatus for measuring atmospheric pressure.
(a) What is the name for the instrument?
………………………………………………………………………

(b) Determine the atmospheric pressure as measured by the instrument ,
(i) in the cm Hg unit (ii) in the Pa unit

(c) State the change of length of the mercury column above the mercury surface

(i) The tube is raised by 10cm ...……………………………………………………….
(ii) The surrounding temperature increases ……………………………………………..
(iii) The instrument is brought to the peak of a mountain ………………………………..
(iv) Water vapor is brought to the vacuum region ………………………………………

PASCAL’S PRINCIPLE
3.4
Definition Pascal’s Principle state that pressure exerted on an ………… fluid is transmitted
…………………………………. to every part of the fluid.

Water spurts out at the ________
speed because the pressure act at
all the point are _______ .

Pascal’s
Principle

A B

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Remember, Pressure at both side are same, PA = PB
P=F So that ; FA = FB
A AA AB

a) When 20 N of force is applied at piston A, FB produced is

FB = FA AB = 20 x 8.0  FB = ________
AA 0.47

b) When piston A is moved downward for 2 m, how far the load has been move up.
Volume of liquid transmitted from A to B are equal.

VA = VB ; where as V = Ah
AAhA = ABhB

hB = ______

Application Gather information about ;
of Pascal’s
Principle 1. Hydraulic brake system

2. Automobile hydraulic lift.

Exercise
1. Figure below shows a hydraulic jack. 2. Which of the following device is
Piston A and piston B have cross- based on the Pascal’s Principle of
sectional areas 5 cm2 and 100 cm2 pressure transmission?
respectively. If mass of 3kg is placed A. Hydrometer
on piston A, what is the maximum B. Car’s hydraulic brake
weight that can be lifted by piston B? C. Bunsen burner
D. Fire extinguisher

The input piston of hydraulic press is
3.
pushed down 2.0 cm from the original
position. If the cross sectional area of
input piston is 5.0 cm2 and of the
output piston is 50 cm2 , how much
A. 300N B. 600N will the load be raised up?
C. 800N D. 900N A. 0.2 cm B. 1.0 cm
C. 1.5 cm D. 1.8 cm

3-13


4 A hydraulic jack which is used for lifting a car at a service centre for motor vehicles. The
hydraulic jack made up of two pistons X and Y of cross-sectional area 0.02 m2 and 18 m2
respectively. When the compressor is switched on, a force of 200 N acts on piston X.

(a) Name the physics principle applied in the hydraulic jack.
_____________________________________________________________________

(b) Explain how the car is lifted
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________

(c) State two advantages of using oil as the hydraulic fluid of water.
_____________________________________________________________________
_____________________________________________________________________

(d) Calculate the pressure exerted on piston Y.

(e) Calculate the maximum weight of a load that can be lifted by piston Y.

(f) If piston X move down by 45 cm, what is the distance moved by piston Y?

5. Cross section area of piston A and piston B are 4000 cm2 and 6 000 cm2 respectively.
i) Calculate the ratio of the force acting on piston A with the force acting on piston B

ii) If the piston B lifts a car 2.50 m high, how far to the right should piston A move?

iii) Why the system less effective when there is air bubbles in the piston.
………………………………………………………………………………………………….

6. Given that the cross-sectional area of smaller piston and larger piston are 20 cm2 and 60 cm2
respectively. If the smaller piston is pushed with a force of 15 N.
Calculate;
a) i) Pressure exerted on smaller piston. ii) Pressure exerted on larger piston

b) the ratio of pressure acted on the smaller piston and larger piston

c) the force experienced by the larger piston

3-14


ARCHIMEDES’ PRINCIPLE
3.5

Definition Archimedes’ Principle: When an object is __________ or __________ immersed in a
fluid, the upthrust or _____________ on it is equal to the ___________ of fluid displaced.

Relationship Simple activity to show the presence of buoyant force
between ; 1. Determine the actual weight of plasticine and the apparent weight of the plasticine
in water.

Actual weight = ………………………
1. FB with
liquid Apparent weight = ………………….
displaced (The weight of plasticine in water)

Loss in weight = …………………….

Weight of = Buoyant force
Water Displaced

Volume of liquid displaced = Volume of ______ ________

2. FB with 2. Floating object
Weight of
floating
object F
W

Floating Buoyant force = weight of object

Buoyancy FB = buoyant force (N)
Force, FB FB = Wliquid displaced ; W = mg ; m = V ρ ρ = density (kgm-3)
g = 10 ms-2
FB = ________ V = volume (m3)

Example 1. An object of density, 40 g cm-3 and mass 500 g is immersed in a liquid of density
2 g cm-3. Calculate;
a) the volume of liquid displaced b) the mass of the liquid displaced

c) the buoyant force experienced by the object (g =10 m s-2)

3-15


Application 1. Submarine
of
Archimedes’
Principle

2. Hot Air Ballon

3. Hydrometer

Exercise
1. A ship of mass 80000 kg floats on the 2. The diagram shows an object partially immersed
sea surface. If the density of the sea in a liquid. What is the mass of the object? ( g =
water is 1250 kg m-3, what is the 10 m s-2 )
volume of the displaced sea water?

A. 6.4 m3 B. 64 m3
C. 640 m3 D. 800 m3

A. 2.0 N B. 20 N
C. 0.020 kg D. 200 g

3. The diagram shows an object immersed 4. A body of density 800 kg m-3 floats in liquid A
in two liquids of different densities. and liquid B as shown. If the density of liquid B is
1000 kg m-3 , what is the possible density of
liquid A?
A. 790 kg m-3
B. 905 kg m-3
Which of the following related F1 and F2 C. 1000 kg m-3
correctly? D. 1050 kg m-3

A. F1 < F2 B. F1 = F2 C. F1 > F2

3-16


5. (a) A fisherman finds that his boat is at different levels in the sea and in the river, although
the boat carries the same load. The density of sea water is 1 025 kg m-3 and of river water
is 1 000 kg m-3.

Figure 1 and 2 illustrate the situation of the boat in the sea and in the river.

(i) What is meant by density?
___________________________________________________________________

(ii) Using Figure 1 and 2, compare the levels of the boat and the volumes of water
displaced by the boat.

………………………………………………………………………………………
………………………………………………………………………………………..
………………………………………………………………………………………..
………………………………………………………………………………………..

Relating the mass of the boat with its load, the volume of water displaced and the
density of the water, deduce a relevant physics concept.

…………………………………………………………………………………………
…………………………………………………………………………………………

(iii) Name the physics principle that explains the above situation.

…………………………………………………………………………………………
b) A submarine can sail on the sea surface and under the sea. Explain how a submarine on the surface
submerges.

(3 marks)
c) Figures 3.5(c) and 3.5(d) below illustrate the working principle of a hydrometer.
The depth to which the test tube sinks depends on its surrounding liquid.

3-17


Draw a diagram that shows the design of your hydrometer and in your explanation, emphasis
the following aspects:
(i) the stability of the hydrometer
(ii) the sensitivity of the hydrometer
(iii) the ability to measure a wide range of densities of liquids
(iv) the calibration of the hydrometer. [10m]

3.6 B E R N O U L L I ’ S P R I N C I P L E

Definition In a steady flow of fluid, the __________in the fluid is ________ when the velocity of the
fluid is high and vice versa.

Reminder :
1. Bernoulli’s Principle is obeyed only for moving fluid .
2. Fluid will move from high pressure into the low pressure.

Figure below shows an _________ The upper region of the aerofoil has _________ air
AEROFOIL velocity than the lower region of the aerofoil.
By Bernoulli’s principle, the lower region has _________ pressure than the upper region
of the aerofoil. This causes a _________ _______ _____ on the aerofoil.

Situation 1.
that Water flows through a uniform tube
involved from _____ pressure to a _____ pressure
with area. Fluid pressure decreases linearly;
Bernoulli’s
Principle PA ____ PB____ PC
VA ____ VB____VC

Therefore hA ____ hB ____hc

Water flows through a non uniform tube,
2. Venturi tube
the velocity of the fluid;
V__ > V__ > V___
VC is the highest because water is forces to
flow through a constriction at A, hence it
speed up.
Pressure of fluid;
P __ > P__ > P__

3-18

Therefore; h __ > h___ >h___


3.
When the air is not flowing the pressure is
same at every point. When the air is
allowed to flow in, the velocity of the fluid;
V__ > V____ > V___
Because the cross-sectional area at B is the
smallest.
4. Pressure of fluid;
P___ > P___ >P_____

Therefore; h___ h____ > h____

4.

Water flows around the ping pong ball at _____ speed
through the filter.
Higher atmospheric pressure produces a __________
which is larger than _________ of ping pong ball.
Ping pong ball is pushed ________ by upward force and
does not drop.

Exercise
1. If the height of the fluid in tube L is h1, 2. Aeroplane wings act as aerofoils. What is the
and that is tube M is h2, which of the funtion of an aerofoil?
following is true?

A. h1 > h2 because v1 > v2
B. h1 < h2 because v1 < v2
A. To raise up the aeroplane.
C. h1 > h2 because v1 < v2
B. To increase the speed of air flow at the surface
D. h1 < h2 because v1 > v2
of wing.
C. To reduce the air resistance and the drag force.

3. Which of the following instruments is not 4. Which of the following relates two quantities in
based on Bernoulli’s principle? Bernoulli’s principle?
A. Bunsen burner A. Velocity and force
B. Car carburettor B. Pressure and momentum
C. Hydraullic jack C. Velocity and pressure
D. Flight of an airplane D. Force and momentum

5. A boy standing by the road side. When a big lorry passed in front of him, the boy feels a pull towards
the lorry. This phenomenon can be explained by
A. Pascal’s principle
B. Bernoulli’s principle
C. Archimedes’ principle

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6. As a researcher in a boat manufacturing company. , you are assigned to study metal characteristics
used to make the boat.
You are given four choices of metals P, Q, R and S. The table below shows
the characteristics for the four metals.

Metal Shape Density Specific Heat Strength
Kgm 3 Capacity
P Streamlined 900 Low High
Q Oval 452 High High
R Circle 387 Low High
S Streamlined 500 High High
Table 1
Based on table 1
i) Explain the suitable characteristics of the metal to be used as the material to make the boat.
ii) Determine the most suitable metal to be used as the material to make the boat and give your
reasons. ( 10 marks)

7.

Name the principle involved in this situation.

3-20

3.0 forces and pressure

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