Fluids

1
Chapter 13 14

• Fluids
• (Liquids, Gases, Plasmas

2
Fluids vs Liquids

• Any substance that can freely flow from one place
  to another is called a fluid.
• Liquids, gases and plasmas are all fluids
• Liquids fluid that is difficult to compress
• Gases fluid that is easy to compress
• Plasmas gas that has been electrified or ionized

3
Pressure

• Pressure is the ratio of force to the area over
  which that force is distributed.
• Pressure force/area
• Units N/m2 PascalPa
• Or atmospheres, bars, millimeters of mercury.

4

• If the two blocks in the figure below are
  identical which one produces the greatest force
  on the table?
• Which orientation produces the greatest pressure
  on the table?

5

• Demo - balloon
• Demo - soccer ball
• Demo - pressure points?
• Ex Bed of nails, high heels, rounded corners

6
Atmospheric Pressure

• PF/A
• The force is weight of the atmosphere
• Patm 1.01105 Pa 1atm

7
Unbalanced pressures

• When there is a pressure difference between two
  sides of an object the object experiences a force
  that points from high pressure to low pressure.
• Discuss pressure on all sides of an object
• Demo - Suction cups
• Discuss collapsing a 55gal drum

8

• How much force does the atmosphere apply to 1m2
  of land?
• 1N0.225lbs
• Demo - Meter stick

9
Pressure in a liquid

• Depends on the depth of the liquid and the
  density of the liquid
• Derive P density g depth
• Total Pressure
• Ptot Patm Pliq
• Ptot Patm density g depth
• Demo Cartesian diver
• Discuss the bends depth charges

10
Which lake exerts more pressure on its dam?

• Why is the base of the dam thicker?

11

• Its important to remember that pressure depends
  on depth of the liquid, not the volume.
• The system will be in equilibrium when the
  pressure at the bottom of each tube is equal

12
Barometer

• The pressure inside and outside the tube must be
  equal at the base of the tube
• Inside the tube you have a vacuum at the top so
  the pressure is just due to the liquid
• Outside you have the atmospheric pressure
• What if the barometer was filled with water?

13
Buoyancy

• Buoyancy is the upward pointing force that a
  fluid exerts on an immersed object it is a
  consequence of pressure increasing with depth.
• The fluid can by liquid or gas
• Ex water, air

14
Archimedes Principle

• An immersed object is buoyed up by a force equal
  to the weight of fluid it displaces.
• FBdfluidVg

15

• A block of iron and a block of aluminum each of
  identical size and shape are placed into a water
  filled container. Which one experiences the great
  buoyant force?
• If the aluminum block is replaced by wood?
• What is the buoyant force on a 10 ton ship
  floating in fresh water?
• -Salt water?
• -Mercury?

16
What makes an object sink or float?

• The relative sizes of the buoyant force and the
  weight. It can be shown this depends on the
  density of the object and the fluid it is in.
• An object more dense than a fluid in which it is
  immersed will sink.
• An object less dense then the fluid in which it
  is immersed will float.
• An object having a density equal to the density
  of the fluid of which it is immersed will neither
  sink nor float.

17
Flotation

• A floating object displaces a weight of fluid
  equal to its own weight.
• Discuss loading a ship with cargo and flotation
  level

18
Pascals Principle

• Pascals Principle says that a change in pressure
  at any point in an enclosed fluid at rest is
  transmitted undiminished to all points in the
  fluid.
• Cartesian diver part 2

19
Bernoullis Principle

• When the speed of a fluid increases, the pressure
  of the fluid decreases.
• Demo Scantron
• Discuss airplane wings, curveballs, roof being
  blown off James Bond (ships)