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Force in Fluid Systems

4 states of matter

Flip chart activity

SolidLiquidGasPlasma

Fluids

A fluid can be a liquid or a gas

Hydraulic Systems

A fluid system that uses a liquid as the fluid

Pneumatic

A fluid system that uses a gas as the fluid

DensitySee page 30 for the Density Table 1.4

Density = mass / volume

Ex : 2 silver cubes

Rearranging Equations Density = mass / volume

m

D V

D = m / Vm = D V V = m / D

Table: Density VariableEnglishSIDensityDlb-masskg yd3m3

Massmlb-masskg

VolumeVyd3m3

Density Ex:What is the density of an object that has a mass of 30 kg and a volume of 60 m3?

Density = mass / volumeD = 30 kg / 60 m3D = 0.5 kg/ m3

VolumeA Rectangular Object

Volume = length X width X height

m3, ft3

Pressure Mathlab

Water

Density of water = 1 g / cm3

Water Weight Density = 62.4 lb / ft3

Hydrometer an instrument that measures density

Specific gravity = density of a substance / density of water

There is no unit Ex: Density of lead is 11.34 g/cm3

Specific gravity of lead is 11.34

Table: Specific Gravity VariableEnglish SISpecific gravitygsp none noneDensity of aDs lb/ ft3 g/cm3 Substance

Density of waterDw lb/ ft3 g/cm3

The Tale of ArchimedesProblem:Did the goldsmith steal some of the gold that was to be used in the Crown?????

The King was sure some of his gold was stolen.If Archimedes did not discover the answer Archimedes would pay with his life.

Buoyant ForceThis is an upward force on an object thats equal to the weight of the fluid displaced determining whether an object sinks or floats

Density and BuoyancyIf you are putting solids in water (1g/ml), if the density is:1 g/ml or less float1 g/ml or more sink

Apply this pattern to other liquids

Density/buoyancy Activity Specific Gravity Mathlab

PressurePressure = Force / Area

AREA

Area = length X width

m2, ft2

Table: Pressure VariableEnglishSIPressure P PsiN/ m2

Force FlbN

Area Ain2m2

Pressure Example

A woman has stiletto heels on she weighs 120 lb and the heel is 0.5 X 0.5. How much pressure would the heal exert if all the womans weight is on one heel?

Area= 0.5 X 0.5 = 0.25 in2

P = F / A = 120 lb / 0.25 in2 = 480 psi ( psi= lb/ in2)

More than enough to puncture skin or the aluminum of an airplane

Pascals PrincipleA change in pressure at any point in a confined fluid is transmitted undiminished throughout the fluid ex: barometer

Total Pressure = gage pressure + atmospheric pressure

atmospheric pressure- many units 30 inches of mercury OR 14.7 psi

Table: Total Pressure Variable English EnglishTotal pressure P totpsi inches

Gage pressure P g psi inches

Atmospheric P atm14.7 psi 30 inPressure

Total Pressure ExampleA car tire needs a gage pressure of 30psi. What would be the total pressure on the tire?Total pressure = gage pressure + atmospheric pressureAtm press= 14.7 psi

Tot press= 30 psi + 14.7 psi = 44.7 psi

Fluid HeightPressure = water weight density X height

This can be used for any substance as long as you know the weight density

Fluid HeightvariableEnglish SIPressure Plb/ft2 kg/m2

Water wgt rw62.4 lb/ft31000 kg/m3Density

Fluid height h ft m

Pressure in TanksThe tank with the greatest fluid height has the greatest pressure (figure 1.26 pg 42)

If two tanks have the same height but different diameter the pressure at the bottom is the same(figure 1.28 pg 43)

Even if the tank funnels down to a small bottom surface, the pressure remains the same for the same depth of fluid

Pressure in Tanks

Fluid Height ExampleA water tower is 300 ft high. What should the water pressure gage read at the bottom of the tower?

Pressure = water weight density X height

Pressure = 62.4 lb/ft3 X 300ftPressure = 18,720 lb / ft2