Static Fluids
Fluid – any substance that takes
the form of the container in which
it is held. (gases and liquids)
Activating Question #1
• Which weighs more?
A bucket full of water
or a bucket full of air?
Explain.
• Density =
mass/volume
m

V
Finding the weight of a fluid
• Weight = mg… but finding the mass of a fluid
can be challenging.
• Finding weight of a fluid…
m
 
V
m  V
mg  Vg
weight  Vg
Activating Question #2
• Which exerts more force on the ground? A
person standing on 1 foot? Or the same
person standing on 2 feet?
• Both exert the SAME force on the ground.
• The person on 1 foot exerts more
Pressure on the ground.
• Pressure = Force/Area
Pressure
• Since pressure depends on force and
area, this is why a person can lay on a bed
of nails relatively safely, but cannot lay on
a single nail.
• The area of a bed of nails is larger so the
pressure is smaller on the body.
• The area of a single nail head is small, so
the pressure is large… ouch
• https://www.youtube.com/watch?v=acRnK
nsddwc
Pressure due to a fluid
• Pressure = force/area
• So.. Pressure = weight of fluid/area
P
Vg
A
V
but...  height
A
so...P  gh
Pressure due to a fluid
• Absolute Pressure = Pressure due to the
depth of the fluid and added to the
atmospheric pressure on top of the fluid.
• Absolute Pressure = P0+ρgh
• Gauge Pressure = pressure just due to
height of liquid
• Gauge Pressure = ρgh
Activating Question #3
• If I have a beaker of water on a scale, will the
weight of the system change if I just touch the
top of the water?
• The weight of the water increases just by
touching the water… why?
• Pascal’s Principle: The pressure of a fluid acts
equally in all directions.
• The added pressure of touching the water on the
top is also exerted on the bottom of the beaker
and therefore increasing the weight.
Pascal’s Principle Mathematically
• Pin = Pout
Fin Fout

Ain Aout
For Hydraulic Lifts, the ratio of F/A remains constant. So
a little bit of force over a small input area, will cause a
large force out if it is over a large area. (lifting a car on a
hyraulic lift)
Buoyancy
• Any object that is submerged in a fluid
experiences a buoyancy force.
• This is a force that pushes upwards on an
object that is submerged in a fluid.
• What does the amount of buoyant force on
an object depend on?
Archimedes’ Principle
• When an object with a volume, V, is placed
in a fluid, it displaces a volume of V of the
fluid. (a bathtub filled to the top, when you
put your foot in, it overflows).
• Archimedes’ Principle: The buoyant force
on an object is equal to the weight of the
fluid displaced.
Archimedes’ Principle
Mathematically
FB  weight _ of _ fluid _ displaced
FB   fluidVobject g
Now the applications…
• Pressure Units…
• Standard units are N/m2… also known as
Pascals (Pa)
• Atmospheres of pressure (Atm)
– 1 atm = 1x105 Pa
• There’s also… Torr, bar, mm-Hg, mm-H20,
PSI…
Barometers
• Looking at the diagram, explain how a
Torricelli barometer functions.
• A glass tube is completely filled with Hg
and then inverted into a bowl of Hg. If the
tube is long enough, the level of the
mercury will drop, leaving a vacuum at the
top of the tube.
• Some calculations…
• P = ρgh = (13.6x103)(9.8)h = 1x105
• h = 0.76m
Check for understanding
• Calculate the height of the tube needed for
a water barometer…
Suction Cups
• Describe how a suction cup functions.
• Check for understanding: A NASA intern
suggests using suction cup shoes for
astronauts to walk along the outside of a
spacecraft. Gently tell this intern what is
wrong with his solution.
Water in a straw
• When placing your finger over a straw that
is submerged in water, the water stays in
place when removed from the water…
why?
Hydraulic Lifts
Airlocks
• Mr. Roth’s airlock… how does it work?
Siphoning
• How can we get the water in the bucket to
the water in the sink using the plastic
tubing?
Units for Density
• Standard units are kg/m3
• CHECK FOR UNDERSTANDING:
• Covert 1 g/cm3 into kg/m3
Coke vs. Diet coke submerged
in water
• Huh? Why?
Is the crown made of gold?
• When a crown of 14.7kg is submerged in
water, an accurate scale reads only
13.4kg. Is the crown made of gold?
Hydrometer
• A hydrometer is a tool
used by brewers to
determine the relative
density of their wort
compared to water.
Check for understanding
• A particular hydrometer is weighted at the
bottom with a mass of 45.0g. It is 25.0cm
long and has a cross-sectional area of
2.00 cm2. How far from the weighted end
should the 1.000 mark be placed?