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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

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Sub unit 1 2 fluid force l.jpg

Sub Unit 1.2 – Fluid Force

Pressure in Fluid Systems


Objectives l.jpg
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 =

  • English

volume

SI

(slugs)

kg

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 object’s weight to it’s 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

lb

N

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

x

height

P =

r

h

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 (h+d)] 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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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 = 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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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 ofwater 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.