Sub Unit 1.2

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Sub Unit 1.2 Fluid Force

• Pressure in Fluid Systems

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Objectives

• Describe the four states of matter.
• Define density and pressure
• Explain why pressure in a fluid depends on depth
  in the fluid
• Explain why an object submerged in a fluid
  experiences a buoyant force
• Predict whether an object will sink or float in a
  given fluid.
• Explain how a force can be multiplied in a
  hydraulic lift.
• Explain where atmospheric pressure comes from.
• Describe how a barometer measures atmospheric
  pressure
• Explain the difference between absolute and gage
  pressure.

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States of Matter

• Matter can exist in four states
• Solid (ice)
• Liquid (water)
• Gas (steam vapor)
• Plasma (Extremely hot ionized atoms)

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Fluids

• Fluids are materials that can flow, has no
  definite shape of its own, and conforms to the
  shape of its container.
• Liquids
• Gasses
• Fluid systems use both liquids (hydraulic) and
  gasses (pneumatic) to operate mechanical devices.

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City Water System
Hydraulic system under pressure
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Density and Pressure

• Density is how much mass is contained in a given
  amount of space.
• Amount of matter per unit of volume.

• mass

• Density

(rho)

• volume

• m

• r

• v

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Units

• mass

• Density

• volume

• English

• SI

• kg

• (slugs)

• lb

• g

• or

• 3

• 3

• ft

• 3

• m

• cm

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Example

• The mass of 1 cm3 of water has a mass of 1g
  therefore the density is 1g/1cm3.

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Weight Density

• Weight Density is the comparison of an objects
  weight to its volume

• weight

• Weight Density

• volume

• weight

• lb

• N

• r

w

• 3

• V

• ft

• 3

• m

Units
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Weight Density of Water

• Water has a weight density of 62.4 lb/ft3.

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Pressure

• A force applied over a surface is pressure.

• force

• Pressure

• area

Units

• F

• N

• lb

• P

• 2

• 2

• A

• ft

• m

English
SI
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Pressure Units

• N/m2 1 Pascal (Pa)
• 1000 Pa 1kilopascal (kPa)
• lb/in2 psi (pounds per square inch)

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Pressure and Depth

• Pressure increase with depth because of the
  additional weight of the fluid above.

• Pressure

• weight density

• height

• x

• r

• h

• P

• x

w
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Buoyancy and Archimedes Principle

• Ptop rw x h
• Pbottom rw x (h d)
• F P x A
• Ftop Ptop x A (rw x h) x A
• Fbottom Pbottom x A rw x (hd) x A
• Fbuoyant Fbottom Ftop rw Ad
• Ad Vbrick
• Fbuoyant rw x Vbrick weight of water
  displaced

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Archimedes Principle

• An object immersed in a fluid has an upward force
  exerted on it equal to the weight of the fluid
  displaced by the object.
• Note the buoyant force is based on the weight of
  the fluid displaced not on the weight of the
  object.

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Pascals Principle

• A change in pressure at any point in a confined
  fluid is transmitted undiminished throughout the
  fluid.

• P F / A or
• F P x A
• A p r2

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Atmospheric Pressure

• The weight of the air above an area.
• At sea level, a column of air extending up
  through the atmosphere, with a cross sectional
  area of 1m2, encloses about 10,000 kg of air.
• This air weighs about 1 x 105 N
• Therefore, atmospheric pressure is about 105 Pa
  or 100kPa at sea level.
• Decreases with altitude
• This is why your ears pop (equalization)

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Atmospheric Pressure

• Barometer instrument used for measuring
  atmospheric pressure.
• At sea level the average atmospheric pressure is
  101.3 kPa 760mm of mercury one atmosphere
  14.7 psi 2117 lb/ft2

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Absolute and Gage Pressure

• Absolute pressure is the total pressure measured
  above zero (perfect vacuum).
• Gage pressure is the pressure measured above
  atmospheric pressure.
• Absolute pressure gage pressure atmospheric
  pressure
• Suppose a tire gage measures the pressure of a
  tire to be 30 psi
• Absolute pressure 30 psi 14.7 psi 44.7 psi
• The air inside the tire pushes out with a
  pressure of 44.7 psi. The atmosphere pushes in
  with a pressure of 14.7 psi. The difference is
  30 psi the gage pressure.

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Pressure is a Prime Mover

• Pressure acts like a force to cause movement.

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Equilibrium
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Summary

• Matter can exist in four states solid, liquid,
  gas, and plasma.
• Liquids and gases are called fluids.
• The density of a substance is its mass per unit
  volume.
• The density of water is 1g/cm3.
• Weight density is weight per unit volume.
• Pressure is force divided by the area over which
  the force acts.
• We treat pressure as a scalar.
• In SI units, pressure is measured in pascals,
  where 1 Pa 1 N/m
• Pressure increases with depth in a fluid.

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Summary

• For a given fluid, the pressure does not depend
  on the size or shape of the container.
• When an object is submerged in a fluid, an upward
  force is exerted on the object caused by the
  pressure difference between the top and the
  bottom of the object. This force is called a
  buoyant force.
• The buoyant force exerted on a submerged object
  equals the weight of the fluid displaced by the
  object.
• A pressure applied to a confined fluid is
  transmitted throughout the fluid.
• Atmospheric pressure is caused by the weight of
  the air above a given area.
• Atmospheric pressure can be measured with a
  barometer.
• Absolute pressure is the sum of the gage pressure
  and atmospheric pressure.