201301 hydrostatics

7/29/2019 201301 Hydrostatics

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Hydrostatics(Part I)Jan. and Feb. 2013.

Marcela Duhne


2/23

Hydrostatics The study of water which is not moving or

at rest.


3/23

Pressure The force exerted by water on each

square meter of some object submerged

in water.

Force per unit area.

Pressure acts in any direction.


4/23

PressureUNITS:

kN/m2

Pascals (Pa)

Blaise Pascal (16201662)

1 Pa = 1 N/m2

One Pascal is a very small quantity and sokilo-Pascals are often used:

1 kPa = 1 kN/m2


5/23

Pressure


6/23

Force and pressure aredifferent


7/23

Pressure and depth


8/23

Pressure and depth

The pressure on some object under water isdetermined by the depth of water above it.

One may calculate pressure using thepressure-head equation:

p = pressure (N/m2) r = mass density (kg/m3)

g = gravity constant = 9.81 m/s2

h = depth of water

Pressure-head equation = Ecuacin fundamental de la esttica de fluidos.


9/23

Pressure and depth


10/23

Pressure is the same in alldirections

Pascals Principle:

A change inpressure at anypoint in anenclosed fluid atrest is transmitted

undiminished to allpoints in the fluid.


11/23


Pascals Principle:

http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/Pascals_principle.html


12/23


Applying Pascals Principle:

Balloon under water / Scuba Divers

The siphon Principle of the hydraulic jack and

hydraulic press

Force amplification in the braking

system of most motor vehicles. Used in artesian wells, water towers,

and dams.


13/23

The hydrostatic paradox

The force applied will depend on the area of the base ofeach container.


14/23

The hydrostatic paradox

Is the pressure the same at the bottom ofthe three buckets? (yes)

How can this be proved? (Sum of forces)


15/23

Pressure Head Engineers often refer to pressure in terms

of metres of water rather than as a

pressure in kN/m2.

Why use metres or head measurements? Differences in ground level can affect the

pressure in a pipeline. Having the samedimensions allows engineers to add (orsubtract) changes in ground level topressure values.


16/23

Pressure Head

Which is the pressure in meters of water?

Pressure = 3 m head of water


17/23

Pressure Head

Calculating pressure head in mercury.

1. Calculate the density of mercury.

2. Use the pressure-head equation tocalculate the head of mercury.


18/23

Pressure Head

Calculating pressure head in mercuryContinue:

The depth of mercury required to createthe same pressure as 3 m of water is only

0.22 m.

Why?


19/23

Atmospheric Pressure

The pressure of the atmosphere is allaround us pressing on our bodies.

At high altitudes atmospheric pressure isless than at the earths surface (oceanlevel).

Examples:

Airplane

Deep sea diversthe bends.


20/23

Atmospheric Pressure

Evangelista Torricelli (16081647), a pupilof Galileo Galilei (15641642), began to

understand about atmospheric pressureand to study the importance of vacuums.

What happens with the vacuum in abottle or bag when opened?


21/23

Mercury barometer One of the instruments

used to measure

atmospheric pressure. It was developed by

Evangelista Torricelli in1643.

Fortin barometer- theintroduction of a verniermeasuring scale tomeasure accurately.

* Average at sea level

*


22/23

Atmospheric PressureUnits ofMeasurement Calculate atmospheric pressure when the

reading on a mercury barometer is 760 mm ofmercury. What would be the height of the

column if the same air pressure was measuredusing water instead of mercury?

or = 1 atmor = 101,325 Pa


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Atmospheric PressureUnits ofMeasurement

A pipeline is operating at a pressure of 3.5bar. Calculate the pressure in metres

head of water.

201301 hydrostatics

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