Sub Unit 2 – Fluid Force Pressure in Fluid Systems Objectives



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Sub Unit 1.2 – Fluid Force

  • Pressure in Fluid Systems

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.

States of Matter

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

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.

City Water System

  • Hydraulic system under pressure

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 =
  • volume
  • m
  • v
  • r =
  • (rho)

Units

  • English
  • SI
  • mass
  • Density =
  • volume
  • ft
  • lb
  • m
  • kg
  • 3
  • 3
  • cm
  • g
  • 3
  • or
  • (slugs)

Example

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

Weight Density

  • Weight Density is the comparison of an object’s weight to it’s volume
  • weight
  • Weight Density =
  • volume
  • r =
  • V
  • w
  • weight
  • lb
  • N
  • ft
  • 3
  • m
  • 3
  • Units

Weight Density of Water

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

Pressure

  • A force applied over a surface is pressure.
  • force
  • Pressure =
  • area
  • F
  • P =
  • A
  • N
  • lb
  • ft
  • 2
  • m
  • 2
  • English
  • SI
  • Units

Pressure Units

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

Pressure and Depth

  • Pressure increase with depth because of the additional weight of the fluid above.
  • weight density
  • height
  • Pressure =
  • r
  • h
  • w
  • P =
  • x
  • x

Buoyancy and Archimedes’ Principle

  • Ptop = w x h
  • Pbottom = w x (h + d)
  • F = P x A
    • Ftop = Ptop x A = (w x h) x A
    • Fbottom = Pbottom x A = [w x (h+d)] x A
  • Fbuoyant = Fbottom – Ftop = w Ad
  • Ad = Vbrick
  • Fbuoyant = w x Vbrick = weight of water displaced

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.

Pascal’s 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 = r2

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)

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

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.

Pressure is a Prime Mover

  • Pressure acts like a force to cause movement.

Equilibrium

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.

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.


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