7/29/2019 201301 Hydrostatics
1/23
Hydrostatics(Part I)Jan. and Feb. 2013.
Marcela Duhne
7/29/2019 201301 Hydrostatics
2/23
Hydrostatics The study of water which is not moving or
at rest.
7/29/2019 201301 Hydrostatics
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.
7/29/2019 201301 Hydrostatics
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
7/29/2019 201301 Hydrostatics
5/23
Pressure
7/29/2019 201301 Hydrostatics
6/23
Force and pressure aredifferent
7/29/2019 201301 Hydrostatics
7/23
Pressure and depth
7/29/2019 201301 Hydrostatics
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.
7/29/2019 201301 Hydrostatics
9/23
Pressure and depth
7/29/2019 201301 Hydrostatics
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.
7/29/2019 201301 Hydrostatics
11/23
Pressure is the same in alldirections
Pascals Principle:
http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/Pascals_principle.html
7/29/2019 201301 Hydrostatics
12/23
Pressure is the same in alldirections
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.
7/29/2019 201301 Hydrostatics
13/23
The hydrostatic paradox
The force applied will depend on the area of the base ofeach container.
7/29/2019 201301 Hydrostatics
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)
7/29/2019 201301 Hydrostatics
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.
7/29/2019 201301 Hydrostatics
16/23
Pressure Head
Which is the pressure in meters of water?
Pressure = 3 m head of water
7/29/2019 201301 Hydrostatics
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.
7/29/2019 201301 Hydrostatics
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?
7/29/2019 201301 Hydrostatics
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.
7/29/2019 201301 Hydrostatics
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?
7/29/2019 201301 Hydrostatics
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
*
7/29/2019 201301 Hydrostatics
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
7/29/2019 201301 Hydrostatics
23/23
Atmospheric PressureUnits ofMeasurement
A pipeline is operating at a pressure of 3.5bar. Calculate the pressure in metres
head of water.