Physics 108 lecture 7: Pressure

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Physics 108 lecture 7 Pressure

• What is Pressure?
• Pressure comes from force that is spread out over
  an area.
• Common units of pressure (conversions are in the
  books cover)
• Pa Pascal Standard metric unit
• bar bar 100,000 Pa (close to one atm)
• mmHg Millimeters of mercury blood and barometric
  pressure
• atm atmospheres the pressure of the atmosphere
• psi pounds per square inch tire pressure, air
  compressors
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• Types of pressure
• Absolute pressure - the pressure including the
  atmosphere or surroundings
• - barometric (atmospheric) pressure is an example
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• Gauge pressure - the pressure excluding the
  atmosphere or surroundings
• - tire pressure and blood pressure are examples
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• The equation for pressure is
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Example (pressure)Calculate the pressure
exerted on the ground if you have all of your
weight (55kg) momentarily supported by a) one
high heel of area 1.2 cm2 b) the front portion of
a tennis shoe of area 65cm2.
Given                    Path                  
Want           Conversions/Equations
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What is Density?

• Density is defined as mass per unit volume.
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• Density is important in understanding the
  pressure exerted by fluids.
• More dense fluids produce greater pressures
  inside them.
• Common units of density are
• kg/m3 kilograms per cubic meter
• g/cm3 grams per cubic centimeter
• g/L grams per liter
• lb/ft3 pounds per cubic foot
• Densities of some common substances are on page
  301.
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• The equation for density is
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Example (density)Butter (roughly equivalent to
fat) comes in bulk boxes that measure 14 inches
on a side and weight 64 pounds. Estimate the
density of fat using this data.
Given                    Path                  
Want           Conversions/Equations
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Pressure and Depth Relationship

• Pressure when submerged below the surface of a
  fluid
• increases with depth
• increases with fluid density
• Imagine a column of fluid with
• height h, area A, density ?
• The volume of that fluid column is
• The weight force of the fluid column is
• The pressure due to the weight force of the fluid
  column is
• The total pressure at the bottom of the fluid
  column is

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Example (pressure and depth) The world record
for tandem SCUBA diving depth is 350m. What is
the pressure at this depth?
Given                    Path                  
Want           Conversions/Equations
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Pascals Principle (Hydraulics)

• Pressure applied to a fluid is transmitted
  equally to all other points including the walls
  of the container for the fluid.
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• Lets see how equal pressures in different areas
  of the fluid can be used as a force multiplier
  for things like hydraulic lifts.
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Example (hydraulics)You use a syringe to inject
fluid into a patients arm. The plunger and IV
line diameters are 6.0mm and 2.0mm respectively.
If you apply a force of 12N to the plunger, what
is the force out the IV? How much pressure have
you applied?
Given                    Path                  
Want           Conversions/Equations