Pressure Force Area

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Pressure

16th December 2015


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Recap: What do these words mean?

• Force:

– a push or a pull

• Surface Area:

– a 2-D space over a surface

• Pressure:

– a force acting over a given surface area

– P = F/A


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Units for Pressure, Force and Area

• Force:

– measured in Newtons (N)

• Area: – measured in square metres (m2)

•Pressure: – measured in Newtons per square metre (N/m2)

– this is called a Pascal (Pa)


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What is Pressure?

• Pressure = Force ÷ Area

• Recall that force is weight of an objectacting over a surface area and this causes

pressure. Hence;

Pressure = Weight ÷ Area


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Units of Pressure

• Pressure is measured in Pascals (Pa).

• We sometimes see pressure beingmeasured in N/m2 or N/cm2

•1 Pa is the same as 1N/m

2


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What is Pressure?

• Any object that possesses weight, whether atrest or not, exerts a pressure upon the surfacewith which it is in contact.

• The magnitude of the pressure exerted by anobject on a given surface is equal to its weight

acting in the direction perpendicular to thatsurface, divided by the total surface area of contact between the object and the surface.


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Force of object acts perpendicular to

the surface area of contact


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Sensible shoes?• The force (weight) of our body

pushing down will exert a pressure.

• The surface area of our shoes incontact with the ground will

affect the pressure.

• These stiletto shoes willconcentrate the force over a

small area.

• These shoes will give a larger pressure and would not besuitable for walking on soft

ground like sand.

Surface area


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More sensible shoes?• These boots have a large surface area.

• The weight of a person is spread out

over the larger surface area.

• The pressure exerted by the person is

smaller than if they were wearing

stiletto shoes.

• A small pressure means you will less

likely sink into soft ground.


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Examples of where a large area is

needed to reduce pressure.

Caterpillar tracks on diggers and on tanks

Tyres on a tractor

Skis and snowboards

snowshoes


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Examples of where a small area is

needed to increase the pressure

• Nails

• Needles

• Kitchen knives

• Scissors

• Drawing pins

• Spade


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How to Measure Pressure

• Manometer

•Elastic Transducers

• Bourdon Gauge

• Electrical Pressure Transducers


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Manometer

With both legs of a U-tube

manometer open to the

atmosphere or subjected to the

same pressure, the liquid

maintains the same level in

each leg, establishing a zero

reference.


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Manometer

•With a greater pressureapplied to the left side of a U-tube manometer, the liquidlowers in the left leg andrises in the right leg.

• The liquid moves until theweight of the liquid, asindicated by h, exactly

balances the pressure appliedat P2.


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Measuring Pressure – Bourdon Gauge

•

A Bourdon gauge uses a coiled tube, which, as it expands due to pressure increase causes a rotation of an arm connected to the tube.

• In 1849 the Bourdon tube pressure gauge was patented

in France by Eugene Bourdon.


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What is Atmospheric Pressure?

• Atmospheric pressure is the pressure exerted by the weight of air in the atmosphere of Earth.

• Gravity pulls the air molecules toward theearth, giving them weight.

• The weight of the air molecules all around usover a given surface area is called the air pressure or atmospheric pressure.


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The air is made up of molecules.


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Illustration of Atmospheric Pressure

Surface area thatweight of air is

acting on

Force = Weight of Air

= Mass x Gravity

Atmospheric Pressure is:

Weight of Air / Surface Area

Gravity


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Atmospheric Pressure

Gas pressuredepends on both

density and

temperature.

Adding airmolecules

increases the

pressure in a

balloon.

Heating the airalso increases

the pressure.


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Air pressure isequal in all

directions.Pressure = force per unit area

Because air is a fluid, force applied in one direction is distributed

equally in all directions.

Thus the downward pull of gravity on air molecules produces air

pressure in all directions.


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Torricelli’s barometer

used a glass columnsuspended in a bowl of

mercury. The pressure

of the air molecules

pushed the mercury upinto the glass tube.

The weight of the mercury in

the tube was equal to the

weight of the air pressing

down on the mercury in the

dish.


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As atmosphericpressure

increases…

The mercury in

the tube rises.


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a) When a straw is put into aglass of orange soda, the

pressure inside and outsidethe straw is the same, sothe liquid levels inside andoutside the straw are the

same.

b) When a person sucks onthe straw, the pressure

inside the straw is lowered.The greater pressure on thesurface of the liquidoutside the straw pushesthe liquid up the straw.

Drinking Straws


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Suction Cups

The cardboard does not fall

and the water remains in the

glass even though it’s not

supported by anything.

The force caused by the

atmospheric pressure acts onthe surface of the cardboard is

greater than the weight of the

water in the glass.

Li id P


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Liquid Pressure• Liquids are made up of particles

• The particles can move in all directions

• The pressure exerted by a liquid acts in all directionstoo

• Liquids with a high density will exert a higher pressure

• As you go deeper, the pressure increases

• This is due to the weight of all the water above pushingdown


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Water Pressure and Body Pressure

• Water pressure

acts in all

directions on this

fish

• The fish’s body

pushes back with

an equal and

opposite pressure


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Submarine and Pressure

Air is vented out by opening the

top of the plastic tubing, to allow

water to “flood” the submarine.

Air is blown into the plastic

tubing, to push water out of the

submarine, so it can float

upwards.


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How Submarine WorksValves are used to

control flow of water

into or out of the

exterior of the

submarine.

By allowing theballast tank to fill up

with water, the lower

valves will be open, as

more water fills up,the submarine will

start to submerge.

The upper valves are

also open to vent air

out.

Lower valves

Upper


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How Submarine Works

Valves are used tocontrol flow of air

into or out of the

exterior of the

submarine.

To surface the

submarine, the upper

valves must open to

allow air into the

ballast tank to push

the water down pass

the lower valves.

Upper

P ti Q ti


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Practice Questions


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Pressure Force Area

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