8/14/2019 Forces & Pressure

1/20

FORCES & PRESSUREFORCES & PRESSURE

PRESSURE , PRESSURE IN LIQUIDS -GAS &PRESSURE , PRESSURE IN LIQUIDS -GAS &

p = F/A Pp = F/A P == h gh g ATMOSPHERICATMOSPHERICPRESSUREPRESSURE

PascalPrinciplePascalPrinciple

Archimedes PrincipleArchimedes Principle

Bernoullis PrincipleBernoullis Principle

8/14/2019 Forces & Pressure

2/20

Understanding pressureUnderstanding pressure

(1)A ladys high heel shoe with a small base exerts a higher

pressure than a man shoe with a flat base. P = F / A.

Unit for pressure is Nm-2 or Pascal,Pa.

Walking over deep snow is difficult and tiring because

your feet sink at every steps. Skies or snowshoes make

it much easier because of the larger surface area of

contact. The skies causes the pressure becomes

smaller.

8/14/2019 Forces & Pressure

3/20

i), which of the ways is more suitable to

move on the muddy road?

Figure(a) and Figure(b) show two ways in which a

gardener moves a wheelbarrow on a muddy road.

8/14/2019 Forces & Pressure

4/20

pressure in liquidspressure in liquids

1.1. Liquids have pressure because of their weights.Liquids have pressure because of their weights.2.2. The pressure in liquid acts equally in all directions andThe pressure in liquid acts equally in all directions and does notdoes not depend ondepend on

thethe shape or sizeshap

e or size of the container,also does not depend on the surfaceof the container,also does not depend on the surface

area.area.

3. The pressure in liquid depend on :3. The pressure in liquid depend on :

- force of gravity- force of gravity

-depth of the liquid-depth of the liquid

- density of the liquid unit = Pascal- density of the liquid unit = Pascal

8/14/2019 Forces & Pressure

5/20

Gas PressureGas PressureThe basic assumptions of kinetic molecular theoryThe basic assump

tions of kinetic molecular theory

1.1. Gases are made of tiny,individual particles.Gases are made of tiny,individual particles.

2.2. Gas particles move rapidly and randomly in straight-line motion.Gas particles move rapidly and randomly in straight-line motion.Particles collide with one another and with the walls of theParticles collide with one another and with the walls of the

container in elastic collisions (no overall loss or gain of energy)container in elastic collisions (no overall loss or gain of energy)3. The average kinetic energies of gas particles increase as the3. The average kinetic energies of gas particles increase as the

temperature increases.temperature increases.4.The4.The collision of gas moleculescollision of g

as molecules with the wall of thewith the wall of the

containercontainerproduces

produces change of momentum or impulsivechange of momentum or impulsive

forceforce

So the gas molecules exert a pressure on the inside ofSo the gas molecules exert a pressure on the inside of

the container becausethe container because

pressure is force per unit area ( P =pressure is force per unit area ( P = FF ))

AA

8/14/2019 Forces & Pressure

6/20

8/14/2019 Forces & Pressure

7/20

Measuring Gas PressureMeasuring Gas Pressure

ManometerManometer

PPgasgas = P= Patmosferaatmosfera + h+ h

Pgas = Patmosfera - hPgas = Patmosfera - h

8/14/2019 Forces & Pressure

8/20

Bourdon GaugeBourdon Gauge

When the gas supply is connected to a Bourdon gauge,

the pressure in the curved metallic tube will try to straighten it.

Hence the pointer will rotate.

The magnitude of the gas pressure can be read off the

scale of the gauge.

8/14/2019 Forces & Pressure

9/20

Atmospheric pressureAtmospheric pressure

1.1. Atmospheric pressure is the pressure exerted by theAtmospheric pressure is the pressure exerted by the

atmospheric on the surface of the earth and all objects onatmospheric on the surface of the earth and all objects onearth.earth.

2. Atmospheric pressure is influenced by the height of an object2. Atmospheric pressure is influenced by the height of an object

above the sea level ( altitude).above the sea level ( altitude).

3. Hence as the altitude increases , the atmospheric pressure3. Hence as the altitude increases , the atmospheric pressure

decreasesdecreases because the higher it is from the surface of thebecause the higher it is from the surface of theEarth , the lower is the density of air.Earth , the lower is the density of air.

4.4. We do not experience the atmospheric pressure at sea levelWe do not experience the atmospheric pressure at sea level

because the pressure of body equal to the atmosphericbecause the pressure of body equal to the atmospheric

pressure.pressure.

5. The atmospheric pressure at sea level is approximately5. The atmospheric pressure at sea level is approximately

1 atm = 1x 105 Pa = 76 cm Hg = 10 m of water.1 atm = 1x 105 Pa = 76 cm Hg = 10 m of water.

8/14/2019 Forces & Pressure

10/20

Measuring atmospheric pressureMeasuring atmospheric pressure

1.Fortin Barometer1.Fortin Barometer

If the vertical height of the mercury is h cm ,If the vertical height of the mercury is h cm ,

thefore the atmospheric pressure reading isthefore the atmospheric pressure reading is

h cm mercury .h cm mercury .

2. Aneroid Barometer2. Aneroid Barometer

8/14/2019 Forces & Pressure

11/20

PASCALS PRINCIPLEPASCALS PRINCIPLE

Pascals principle states that when pressure is appliedPascals principle states that when pressure is applied

to an enclosed fluid, the pressure will be transmittedto an enclosed fluid, the pressure will be transmitted

equally throughout the whole enclosed fluid.equally throughout the whole enclosed fluid.

Applications of Pascals PrincipleApplications of Pascals Principle

(1)Automoboile hydraulic lift(1)Automoboile hydraulic lift

Automobile hydraulic lift is used to lift a vehicle in aAutomobile hydraulic lift is used to lift a vehicle in a

workshop.workshop.

8/14/2019 Forces & Pressure

12/20

(2) Hidraulik Jack(2) Hidraulik Jack

When the handle is pushed, valve P is closed and valve Q opens so

the pressure can be sent to the larger piston. The large piston will

rise.When the small piston is pulled out, valve Q closes and valve Popens so that the oil in the tank enter the hydraulic cylinder as a

result of atmospheric pressure.

By moving the push-pull handle a number of times ,

the large piston can be raised carrying a heavy load.

8/14/2019 Forces & Pressure

13/20

8/14/2019 Forces & Pressure

14/20

Archimedes principle.Archimedes principle. Archimedes Principle state that When an object isArchimedes Principle state that When an object is

immersed in a fluid (a liquid or a gas) ,the buoyant forceimmersed in a fluid (a liquid or a gas) ,the buoyant force( upthrust force) on the object is equal in size to the( upthrust force) on the object is equal in size to the

weight of fluid displaced by the object.weight of fluid displaced by the object.

Buoyant Force = Weight of fluid displace

= mg (note : F = ma)

= Vg (note : = m

)

V

Thus FB = V g

8/14/2019 Forces & Pressure

15/20

Application of Archimedes PrincipleApplication of Archimedes Principle

Hydrogen filled balloonHydrogen filled balloon

Hot-air balloonHot-air balloon

8/14/2019 Forces & Pressure

16/20

HydrometerHydrometer An hydrometer is an instrumentAn hydrometer is an instrument

used to measureused to measure

the density of a liquid.the density of a liquid. The hydrometer floatsThe hydrometer floats

higher in a liquid of higherhigher in a liquid of higher

density.density.ShipShipThe buoyant force actingThe buoyant force acting

on the ship is large enoughon the ship is large enough

because the ship is hollowbecause the ship is hollow

and the volume of waterand the volume of water

displaced by the ship isdisplaced by the ship is

sufficiently large. ( Fsufficiently large. ( FBB == V g)V g)

8/14/2019 Forces & Pressure

17/20

Bernoullis principle.Bernoullis principle.

Bernoullis principleBernoullis principlestatesstatesthatthat : The pressure of: The pressure ofaa movingmoving

liquidliquiddecreases as the speed of the fluid increases anddecreases as the speed of the fluid increases andvice versa.vice versa.

The conditions which enable the BerrnoullisThe conditions which enable the Berrnoullis

Principle to usePrinciple to use

(1)(1) The fluid is incompressible and flows easilyThe fluid is incompressible and flows easily (2)(2) The fluid is streamline or in layers,i.e. there are noThe fluid is streamline or in layers,i.e. there are no

eddies or turbulenceeddies or turbulence

8/14/2019 Forces & Pressure

18/20

22..

3.3.

8/14/2019 Forces & Pressure

19/20

Use of Bernoullis Principle in Everyday LifeUse of Bernoullis Principle in Everyday Life

A person who stands near a railway feels like falling intoA person who stands near a railway feels like falling into

it when suddenly a train moves with a high speedit when suddenly a train moves with a high speedpasses him. It is because the velocity of the air in front ofpasses him. It is because the velocity of the air in front ofhim increases. According to Bernoullis Principle , thehim increases. According to Bernoullis Principle , thepressure of the moving air decreases as the speed of thepressure of the moving air decreases as the speed of theair increases. The higher atmospheric pressure behindair increases. The higher atmospheric pressure behind

pushes him forward.pushes him forward. When two speed bots move faster and closely to eachWhen two speed bots move faster and closely to each

other, an accident may be occurred. It is because theother, an accident may be occurred. It is because thewater moved at a very high velocity between the boats.water moved at a very high velocity between the boats.

According to Bernoullis Principle , the pressure of theAccording to Bernoullis Principle , the pressure of themoving air decreases as the speed of the air increases .moving air decreases as the speed of the air increases .

The higher water pressure on either sides of the boatsThe higher water pressure on either sides of the boats

caused its closer to each other .caused its closer to each other .

8/14/2019 Forces & Pressure

20/20

((3)3) A table-tennis player often gives the ball top spinA table-tennis player often gives the ball top spinenabling a hard drive to be hit over the net and land onenabling a hard drive to be hit over the net and land on

the table. A ball hit with spin swerves considerably in itsthe table. A ball hit with spin swerves considerably in itspath.path.

-therefore a pressure difference between the upper

and lower surfaces which deflects the ball,

originally hit from right to left in downwardsdirection