Chapter 3 force and pressure Asal

7/27/2019 Chapter 3 force and pressure Asal

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CHAPTER 3 : FORCES AND PRESSURE

By the end of this lesson you should be able to :

3.1 Understanding pressure

* Define pressure and state that P =AF

* Describe applications of pressure

* Solve problems involving pressure

3.2 Understanding pressure in liquids

* Relate depth to pressure in a liquid* Relate density to pressure in a liquid

* Explain pressure in a liquid and state that P = hg

* Describe application of pressure in liquids

* Solve problems solving pressure in liquids

3.3 Understanding gas pressure and atmospheric pressure

* Explain gas pressure

* Explain atmospheric pressure

* Describe applications of atmospheric pressure* Solve problems involving atmospheric pressure and gas pressure

3.4 Applying Pascals Principle

* State Pascals Principle* Explain hydraulic system

* Solve problems involving Pascals Principle

3.5 Applying Archimedess Principle

* Explain buoyant force

* Relate buoyant force to weight of the liquid displaced

* State Archimedess Principle* Describe applications of Archimedess Principle

* Solve problems involving Archimedess Principle

3.6 Understanding Bernoullis Principle

* State Bernoullis Principle

* Explain that a resultant force exist due to a difference in liquid pressure

* Describe applications of Bernoullis Principle* Solve problems involving Bernoullis Principle


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CHAPTER 3 : FORCES AND PRESSURE

3.1 PRESSURE

1. Pressure is defined as the magnitude of the force acting perpendicularly to a

surface per unit area of the surface

Pressure, P =AArea

FForce

,

,

2. The unit for pressure is Nm-2 or Pascal (Pa)

(1 Nm-2 = 1 Pascal)

Example:

A cylindrical of mass 120 kg has a cross-sectional area of 200 cm2. Find the

Pressure exerting on the ground when it placed vertically. (g = 10 ms-2)

Solution:

Pressure, P =AArea

FForce

,

,=

A

mg=

10200

10120

x

x

= 60 000 Nm-2

3. High pressure is useful for cutting, sewing, hammering nails into wood and

giving grips s in studs on the soles of soccer boots and spikes on the soles of a

runners shoes.

a. A knife can cut hard materials easily because its sharp edges of smaller

area can exert a greater pressure.b. Studs of the soles of soccer boots give better grip while running on the

field

c. The high pressure causes the pin to go into the board


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4. Low pressure is useful when walking or moving on soft snow and soft round.

a. The large surface area of the skis prevents the skier from sinking intothe snow

b. Camels have large flat feet so they do not sink into the soft sand

c. Excavators have specials wheels called caterpillar tracks to move oversoft ground.

d. A wide shoulder pad of a heavy bag will reduce pressure exerted on the

shoulder of a person carrying the bag.

3.2 PRESSURE IN LIQUIDS

1. Pressure in liquid act in all directions. This can be illustrated as in figure below

by filling a plastic bag with water and poking several holes in various part of it.When squeezed, water squirts out in all directions.


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2. For a given liquid, its pressure increases with depth, h.

3 Pressure in liquid increases with density, .

4 Pressure in liquid increases with gravitational field strength, g.

5. The pressure acting on the fish is due to the water column above it.

The column of liquid exerts a pressure on the base due to its weight.


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Activity : Study the relationship between density and pressure in a liquid.

( Practical book page 54 )

Notes : * A thistle funnel covered with a rubber sheet immersed in

a cylinder filled with a liquid* The height, l of water column in the U-tube represents

the liquid pressure in the thistle funnel.

Example :1. A huge metal tank is used to store a certain type of oil. What is a

pressure exerted by the oil at a depth of 3.5 m if the density of the oil is

840 kg m-3 and g is 10 ms-2 .Solution :

P = h g= 3.5 x 840 x 10

= 29 400 Pa.

6. For a given liquid, its pressure does not depend on the shape of containers.


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7. Applications of pressure in liquids

a) Dam holds water at high altitude to increase water pressure so

as to supply water to generate electricity.b) Water is stored in water tank at higher level so as to supply water

at a great pressure.

c) Submarine is built with thick wall to be able to move under the surfaceof the sea with greater pressure..

d) In order for the dam to be able to sustain stronger force acting on it

At greater depth, the structure of the dam needs to have wider base.

3.3 Understanding Gas Pressure and Atmospheric Pressure

1. A gas pressure is the force exerted on a surface by air molecules per unit area

of a surface.


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2. The earth is surrounded by a thick layer of atmosphere.

3. Atmospheric pressure is the pressure exerted by the atmospheric on the surface

of the Earth as well as all objects on Earth.

4. The value of atmospheric pressure is

1 atmosphere = 1 x 105 Pa.

= 760 mm Hg

= 10 m water

3.3.1 Activities to show the existence of atmospheric pressure

1. A glass is completely filled with water and then covered with acupboard. It is then inverted. The cardboard does not fall

Atmospheric pressure support cardboard

2. A little hot water is poured into a bottle. The water in it vaporizes.

Then the bottle closed. Cold water is poured on it, the water vapourinside the bottle condenses and its form partial vacuum inside the bottle.

The atmospheric pressure outside which is greater then compress the bottle.

A plastic bottle can crushed due to atmospheric pressure

3. The gas pressure inside a container depends on the rate of collision of gas

molecules with the walls.4. At higher temperature, molecules move faster and the rate of collision of the

gas molecules with the walls increases. Hence, pressure increases.

5. Atmospheric pressure due to the height of air column above a point.Hence as the altitude increases, the atmospheric pressure decreases.


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3.3.2 Instruments for measuring atmospheric pressure and gas pressure

A. Measuring atmospheric pressure

a) A simple mercury barometer

* The height of the mercury column, h = 76 cm, represents the atmosphericpressure.

* The atmospheric pressure, PP = h g

= 0.76 m x 1.36 x 105 kg m-3 x 10 N kg-1

= 101 300 Pa

b) Aneroid barometer

* Consist of a flexible metal can be which most of the air inside the can hasbeen taken out of it.

* When the surrounding pressure increases, the top of the can is squeezed

down slightly causing the pointer to move along the curved scalemeasuring the corresponding pressure.

* The spring prevents the flexible metal can from collapsing completely


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by the atmospheric pressure.

B. Measuring gas pressure

a) Manometer

* Consist of a U-Tube that is filled with a liquid like water, oil or mercury

* When both ends of the tube is exposed to atmospheric pressure,

both level are the same.* When one end is connected to a gas supply, the difference in level, h

gives the pressure of the gas, P

where P = Atmospheric pressure + h g

b) Bourdon Gauge

* The movement of a gas into a hollow copper tube causes itto straighten slightly.

* A system of lever and gear will enable a pointer to move along

the round scale indicating the pressure of a gas measured.


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Example 1 :

The figure below shows a simple barometer used in the laboratory to measure

atmospheric pressure.

The length of the glass tube is 100 cm. The height of a vacuum column is 10 cmand the height of the mercury column is 76 cm.

a) What is a value of the atmospheric pressure in cm Hg ?b) What is the pressure at point X in cm Hg ?c) If the density of a mercury is 1.36 x 10 kg m-3 and

the g = 10 m s -2.

Calculate the pressure at X in Pascal.

Solution :a) Atmospheric pressure = 76 cm Hg.

b) Pressure at point X = ( 76 + 14 ) cm Hg

= 90 cm Hg.c) Pressure at point X in Pascal

P = h g

= 0.90 m x 1.36 x 105 kg m-3 x 10 m s -2.= 1.224 x 106 Pa.

Example 2 :

A mercury manometer with one end attached to a gas supply measuresa difference in the level of mercury of 32 cm as shown in figure below.


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Calculate the pressure of a gas supply in

a) cm Hgb) pascal

( Atmospheric pressure = 76 cm Hg, g = 10 m s -2.

and density of mercury = 1.36 x 105

kg m-3

)

Solution :

a) P gas = Atmospheric pressure + Pressure the column of mercury.= ( 76 + 32 ) cm Hg

= 108 cm Hg.

b) P gas = 1.08 m x 1.36 x 105 kg m-3 x 10 m s -2.= 1.44 x 105 Pa.

3.3.3 Applications of atmospheric pressure.

a) Rubber plunger/ rubber suction cup does not fall off

* When we push a rubber plunger against the wall, air is pushed out

from the plunger.

* The atmospheric pressure inside the plunger decreases.

* The atmospheric pressure outside is now greater than inside the plunger.* It is able to support the weight of a plunger.

b) Atmospheric pressure helps in drinking water.


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* We are able to suck water from a tin/glass because the higher

atmospheric pressure on the outside helps to force the drink upthe straw which has a lower pressure inside it.

c) Sucking water by using syringe

* When a piston of a syringe is pulled upwards, a partial vacuumis created in the syringe.

* The atmospheric pressure outside which greater will force the water

to flow up through the nozzle of the syringe.

c) Condensed milk is being poured out of its can.

* The presence of a second hole is to enable air to flow into the can

* Results in the pressure of air in the can same as atmospheric pressure.

* The atmospheric pressure will force the milk out of the lower hole.

Chapter 3 force and pressure Asal

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