Forces in Fluids

Don’t forget, gases act like fluids

Pressure in fluids is a pushing force occuring equally in all directions.

(Pascal)

Pressure is when a push, squashes something

Walking pushes on the ground, if the ground is soft it gets squashed and we leave footprints

If the ground is very soft we leave deep footprints

because of pressure.

Our footprints will not be as deep if we spread out the weight causing less PRESSURE.

Which shoes do you think will push hardest on the

floor?

Fluids are made up of particles that are moving and their collisions have forces which we call pressure.

Air pressure is equal in all directions and is the sum of all of the forces from all of the collisions (balanced forces) from the particles in the air.

Pressure• Pressure changes are due to changes in Force or Area

(Remember this formula)• Pressure = Force/Area• Area = L x W

Barometric Pressure

• Air is made up of molecules of gases (nitrogen, oxygen, argon, water vapor, etc.).

• Because these gases have mass, air is pulled toward the center of Earth by gravity

Image from the University of Illinois WW2010 PROJECT

Barometric Pressure• As you go up in

altitude, there is less air above you.

• Less air means less mass and less weight pushing down the surface.

So, Barometric pressure decreases as youE

leva

tion

Pre

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re (

mil

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go up in the atmosphere

Image from the University of Illinois WW2010 PROJECT

Torricelli’s BAROMETER used a glass tube filled with Hg suspended in a bowl of Hg. The pressure of the air molecules pushing on the bowl pushed the Hg up into 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.

As atmospheric pressure increases…

The mercury in the tube rises.

Barometric Pressure

Aneroid Barometer

• This is an Aneroid Barometer, which contains an expandable air chamber that changes size as air pressure changes.

• A needle attached to the air chamber moves around the scale as air pressure changes

Standard Pressure• Scientists realized that gas properties varied with changes in pressure so they created a STANDARD PRESSURE to use with gases at 0˚C. A volume of 22.4 Liters/mole accompany these conditions.

Ways to measure Standard

Pressure(0˚C)• 1 Standard Atmosphere is equal to:

• 760 mm Hg• 1013 millibars• 101.3 kPa (kiloPascals)

1kPa=1000N/m2

• 14.7 pounds/in2

• 760 Torr• Dimensional analysis will be used

to convert from one system to another

Pascal’s Principle

• Force applied to a confined fluid will increase pressure in ALL directions

• If a 10 N force is applied to a piston with 1 cm2 area, what is the force absorbed on a 00 100 cm2 piston?

• F1/A1 = F2/A2• 10N/1cm2 = F2/100cm2

• 1000 N

Natural Hydraulics• Sea Stars have a

hydraulic system connected to their tube feet which provide their locomotion and sucker action

• Turgor pressure in plants.

Buoyancy

Buoyancy and Swimming

• Buoyancy is why most people float. Your body displaces water. When the mass of the water you displace is more than your mass, you float.

• Because of buoyancy you weigh very little in water.

Buoyancy• Buoyant Force –

Force that acts against gravity causing objects to seem lighter.

• Fluid trying to enter the space of an object pushes up on object.

• Based on the difference between density of the fluid and the object

Buoyancy

• Ballast tanks can be filled with water or air to change the density of the boat.

Submarine action• Positive buoyancy – density

less than water – floats on top or is rising

• Neutral buoyancy – density is equal to water – remains at constant depth

• Negative buoyancy - sinks

Archimedes’ Principle• The apparent mass loss of the object when submerged in a fluid is equal to the mass of the displaced fluid

Sample Archimedes Problem

• What will be the mass of a glass cube when submerged in water. It is 20 cm on each side. The density of the glass is 1.3g/cm3 .

• Volume=LxWxH=(20cm)3 = 8000cm3

• Mo=8000cm3 x 1.3g/cm3 =10400g• Mw=8000cm3 x 1.0g/cm3 = 8000g• Mo(in water) 2400g

Another Archimedes Problem

• A 3.43Liter leaded glass sample was unearthed by an archeologist. The density of the glass was 2.23g/ml. What will the mass of the glass be if submerged in water?

• Volume of glass: • Mass of glass: • Mass of displaced water: • Mass of glass(submerged):

4218.9g