3.0 forces and pressure

Physics Module Form 4 Chapter 3 Forces and Pressure GCKL 2011

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U N D E R S T A N D I N G P R E S S U R E

Define pressure. Pressure is defined as the _________ applied on a unit __________ ______.

P = A

F where

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

pressure and force,

pressure and surface

area

Situation 1 : Figure 3.1a, press a thumbtack into a piece of wood with your

thumb where as Figure 3.1b, hit using the hammer.

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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F = Force (N) A = surface area (m

2)

P = Pressure (_______)


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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

pressure.

- Force

- Surface Area

1. Ways to increase pressure are :

a) ____________ force ; b) ____________area of contact.

2. Ways to decrease pressure are :

a) ____________ force ; b) ____________area of contact.

The applications of

pressure in our daily

life.

a) High pressure. 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

cross-sectional area of 200 cm2. What

is the pressure acting at its base? A. 6 kPa B. 9 kPa C. 12 kPa D. 15 kPa

2. Wind blows normally on a wall at a pressure of 200 kPa. If the wall has an area of 5 m

2., what is the force

acting on the wall? 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

surface of a block. The surface is a square of

sides 10 cm. What is the pressure exerted on

the surface? A. 1.25 kPa B. 2.50 kPa C. 25.0 kPa D. 12.5 kPa

4. A balloon is pressed with a force of 2.4 N

using a finger. Find the pressure exerted on

the balloon if the area of fingertip is 2.0 x

10-4

m2

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 10

5 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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UNDERSTANDING PRESSURE IN LIQUIDS

Characteristics

of pressure in

liquid

1. Pressure in liquids acts in __________ directions.

2. Draw the jet of water from the container. .

3. Draw the water level in the container.

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

- The higher the density the _________ the pressure.

Formula for

pressure in

liquid

The pressure in a liquid is the product of depth, density and gravitational acceleration.

P = x x

Shape and size _________

influence the pressure

water

oil

Pressure in liquids _______

with depth.

P = Pressure ( Pascal) 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 = h ρ g = 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

data

: Graph of …………………………………………………………………………….. ………………………………………………………………(draw on graph paper )

i. Conclusion :

j. Precaution :

Applications of

pressure in

liquid

1. 2.

Exercise

1.

Which of the following factor does not influence the pressure of a liquid?

A. Depth B. Acceleration due to gravity C. Density D. Volume

4.

The density of water is 1000 kg m

-3.

Find the density of liquid X in kg m

-3?

2.

Mercury has density of 13600 kg m

-3.

If the pressure of mercury is 650 kPa, what is the depth from its surface?

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 10

6 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.

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 ]

Water Wall


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U N D E R S T A ND I N G GA S P R E S S U R E AN D

A T M O S P H E R I C P R E S S U R E

Kinetic molecular

theory

Gases are :

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

pressure The atmospheric pressure is caused by the downward ……….. exerted by the air

( 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

3.3


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Manometer

1

2

3

Simple Barometer

1. Figure shows apparatus set up which

is used to measure atmospheric

pressure. [Mercury density = 1.36 x 10

4 kg m

-3]

Calculate the pressure at point Q in Pa.

PA = _____ = Patm = 76 cm Hg PC = PA + PHg PD = _____ + _____ = (76 + 4) cm Hg = ______ cm Hg = 80 cm Hg

ρHg = 13600 kg m-3

; ρwater = 1000 kg m-3

PA = _____ = Patm = 76 cm Hg

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

= (76 + ___ ) cm Hg

= ________ cm Hg

Pgas = PD = ______

= ______ cm Hg

D

C

h cm

water D C

Gas

flow


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2. If the atmospheric pressure is 76 cm

Hg, what is the pressure of the trapped

air P?

Applications of

atmospheric

pressure.

1.Siphon

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

trapped inside the J-tube, in Pa unit?

A. 1.19 x 10

5 Pa

B. 1.90 x 105 Pa

C. 2.19 x 105 Pa

D. 2.90 x 105 Pa

2.

3.

Which instruments is meant for measuring atmospheric pressure? A. Carburettor B. Siphon C. Fortin’s Barometer D. Hydrometer Which of the following is not true about

atmospheric pressure? A. Atmospheric pressure acts in all

directions. B. Atmospheric pressure decreases with

distance from Earth’s surface. C. Atmospheric pressure can only measure

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

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

3.4


3-13

Remember, P = F A

Pressure at both side are same, PA = PB So that ; FA = FB 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

of Pascal’s

Principle

Gather information about ;

1. Hydraulic brake system

2. Automobile hydraulic lift.

Exercise

1. Figure below shows a hydraulic jack. Piston A and piston B have cross- sectional areas 5 cm

2 and 100 cm

2

respectively. If mass of 3kg is placed on piston A, what is the maximum weight that can be lifted by piston B?

A. 300N B. 600N C. 800N D. 900N

2.

3.

Which of the following device is based on the Pascal’s Principle of pressure transmission? A. Hydrometer B. Car’s hydraulic brake C. Bunsen burner D. Fire extinguisher The input piston of hydraulic press is

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

will the load be raised up? A. 0.2 cm B. 1.0 cm C. 1.5 cm D. 1.8 cm


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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 m

2

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 cm

2 and 6 000 cm

2 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 cm

2

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


3-15

ARCHIMEDES’ PRINCIPLE

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

between ;

1. FB with

liquid

displaced

2. FB with

Weight of

floating

object

Simple activity to show the presence of buoyant force

1. Determine the actual weight of plasticine and the apparent weight of the plasticine

in water.

Actual weight = ……………………… Apparent weight = …………………. (The weight of plasticine in water) Loss in weight = ……………………. Weight of = Buoyant force Water Displaced

Volume of liquid displaced = Volume of ______ ________

2. Floating object

Buoyancy

Force, FB

FB = buoyant force (N) FB = Wliquid displaced ; W = mg ; m = V ρ ρ = density (kgm

-3)

g = 10 ms-2

FB = ________ V = volume (m

3)

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)

Floating Buoyant force = weight of object

F W

3.5


3-16

Application

of

Archimedes’

Principle

1. Submarine

2. Hot Air Ballon

3. Hydrometer

Exercise 1. A ship of mass 80000 kg floats on the

sea surface. If the density of the sea water is 1250 kg m

-3, what is the

volume of the displaced sea water? A. 6.4 m

3 B. 64 m

3

C. 640 m3 D. 800 m

3

2. The diagram shows an object partially immersed

in a liquid. What is the mass of the object? ( g =

10 m s-2

)

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

3. The diagram shows an object immersed

in two liquids of different densities.

Which of the following related F1 and F2

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

4. A body of density 800 kg m-3

floats in liquid A

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

C. 1000 kg m-3

D. 1050 kg m-3


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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.


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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]

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

Definition

AEROFOIL

In a steady flow of fluid, the __________in the fluid is ________ when the velocity of the

fluid is high and vice versa.

Figure below shows an _________ The upper region of the aerofoil has _________ air

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 that involved with Bernoulli’s Principle

1.

2. Venturi tube

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

Water flows through a uniform tube

from _____ pressure to a _____ pressure

area. Fluid pressure decreases linearly;

PA ____ PB____ PC

VA ____ VB____VC

Therefore hA ____ hB ____hc

Water flows through a non uniform 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__

Therefore; h __ > h___ >h___

3.6


3-19

3.

4.

4.

Exercise 1. If the height of the fluid in tube L is h1,

and that is tube M is h2, which of the

following is true?

A. h1 > h2 because v1 > v2 B. h1 < h2 because v1 < v2 C. h1 > h2 because v1 < v2 D. h1 < h2 because v1 > v2

2. Aeroplane wings act as aerofoils. What is the

funtion of an aerofoil?

A. To raise up the aeroplane. B. To increase the speed of air flow at the surface

of wing. C. To reduce the air resistance and the drag force.

3. Which of the following instruments is not

based on Bernoulli’s principle? A. Bunsen burner B. Car carburettor C. Hydraullic jack D. Flight of an airplane

4. Which of the following relates two quantities in

Bernoulli’s principle? A. Velocity and force B. Pressure and momentum C. Velocity and pressure 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

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.

Pressure of fluid;

P___ > P___ >P_____

Therefore; h___ h____ > h____

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.


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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

Kgm3

Specific Heat

Capacity Strength

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.

Date post:	21-Jan-2015
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3.0 forces and pressure

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