Physics I Lesson 6: Fluids

CRB Which of the following are the defining characteristics of all fluids?

I. Flow when forces are exerted on them.

II. Can take the shape of their container.

III. Be less dense than their solid counterparts.

(A) I only

(B) I and II only

(C) II and III only

(D) I, II and III

(B) I and II only

Two defining characteristics of fluids are that they flow and conform to their container's shape.

Note that most fluids are less dense than their solid counterparts, but there are exceptions like water!

CRB True or false? Both solids and fluids can withstand forces perpendicular to their surfaces and Shear (tangential) to their surfaces.

False. Both solids and fluids can withstand forces perpendicular to their surfaces, however only solids can withstand Shear (tangential) forces.

Those Shear Forces are what would make the fluids flow!

Which of the following are fluids?

- solids

- liquids

- gases

Both liquids and gases are considered fluids because they flow when a force is exerted on them and can take the shape of their container.

What is the key difference then between a gas and a liquid?

Liquids are incompressible, while gases are compressible.

What is Pascal's principle (relationship between pressure in and pressure out)? How do the forces in and out relate to this concept?

Pressure in = Pressure out

Because of this, the forces are related to the area: F1/A1 = F2/A2 = Pressure in = Pressure out

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CRB Imagine I have a half-full, closed bottle of soda. According to Pascal's Principle, if I try to squeeze this bottle, to which of the following places would the change in pressure be transmitted?

I. The liquid soda

II. The air above the soda

III. The soda bottle

(A) I only

(B) I and II only

(C) I and III only

(D) I, II and III

(D) I, II and III

If I tried to squeeze a half-full, closed bottle of soda, then each of the following would have the change in pressure affect it equally:

I. The liquid soda

II. The air above the soda

III. The soda bottle

Incompressible fluids in closed container will distribute the increased pressure to all of the fluid and the container's walls!

CRB True or false? This incompressibility of fluids is used in Hydraulic systems to obtain a Mechanical Advantage.

True. This incompressibility of fluids is used in Hydraulic systems to obtain a Mechanical Advantage.

A liquid is in a horseshoe shaped test tube with one side large than the other. If a force of 9.67 N is spread across the liquid with an area of 1.89 m^2, what would the force of the liquid be on the other side of the test tube, which has an area of 13.24 m^2?

(A) 45.63

(B) 67.74

(C) 89.40

(D) 112.89

(B) 67.74

F1/A1 = F2/A2

(9.67)/(1.89) = F2/(13.24)

F2 = approx. 70 N (actual: 67.74)

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Which of the following is equal to 1 Pascal (1Pa)?

(A) m*s/N

(B) N/ (m*s)

(C) N/ m^2

(D) N/(m^2*s)

(C) N/ m^2

One Pascal is equal to one Newton per square meter. Remember, pressure is a force over an area!

What is the equation for pressure of a fluid at a given depth in terms of density (with no atmosphere)?

Pressure = ρhg

ρ = density

h = depth from top of fluid

g = acceleration due to gravity (9.8 m/s^2)

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What is the pressure of water in a vacuum at a depth of 4.78 m?

(A) 46,844 Pa

(B) 57,391 Pa

(C) 89,648 Pa

(D) 104,589 Pa

(A) 46,844 Pa

ρH2O = 1000 kg/m^3 <- You should have this memorized!

Pressure = ρhg

Pressure = (1000)(4.78)(9.8)

Pressure = 49,000 pascals (actual: 46,844)

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What is the equation for specific gravity? How does this relate to the percent of an object that will be below the surface if floating in water?

Specific Gravity = (ρ of object)/(ρ of water)

Specific gravity = percent of object below water when floating.

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CRB Imagine a seven-layer density column, where there are distinct phases of fluids, with water around the middle. Would the fluids above or below water have a higher Specific Gravity?

The fluids below water would have a higher Specific Gravity, because they are more dense!

What is the equation for buoyancy force?

Buoyancy Force = Weight of liquid displaced (in N) = Vρg

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CRB Which of the following about Buoyancy Force are true?

I. The volume of fluid displaced is equal to the volume of the object submerged in the fluid.

II. The Buoyancy Force can also be called Archimedes' Principle.

III. The Buoyancy Force can also be influenced by extreme ambient pressures.

(A) I only

(B) I and II only

(C) II and III only

(D) I, II and III

(B) I and II only

Each of the following about the Buoyancy Force are true:

I. The volume of fluid displaced is equal to the volume of the object submerged in the fluid.

II. The Buoyancy Force can also be called Archimedes' Principle.

CRB True or false? My buoyant force in a fluid with a low specific gravity ( <1)will be greater than my buoyant force in water.

False. My buoyant force in a fluid with a low specific gravity ( <1)will be LESS than my buoyant force in water.

This is because that buoyant force is dependent upon the weight of the fluid displaced, so a more dense (higher specific gravity) fluid will have a stronger buoyant force!

An object with a weight of 97.89 N is submerged in water, and has a net force of 46.57 N acting on it. What is the volume of the object?

(A) 5.67⋅10^3

(B) 9.88⋅10^1

(C) 2.43⋅10^-1

(D) 4.75⋅10^-3

(D) 4.75⋅10^-3

Weight of object = 100 N

Net force = 50 N

Buoyant force = 50 N

ρ of water= 1000kg/ m^3

Buoyant force = Vρg

46.57 = V (1000)(9.8)

V = 4.9⋅10^-3 m^3 (actual: 4.75⋅10^-3 m^3)

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What is the equation of continuity? What is the purpose of this equation?

v1A1 = v2A2

v1 = velocity in

A1 = area in

v2 = velocity out

A2 = Area out

Compares flow of fluid in a pipe of varying cross sectional area.

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If a fluid is flowing at 2.09 m/s through an area of 8.79 m^2, what is the velocity of the water when the pipe squeezes down to an area of 3.42 m^2?

(A) 3.64

(B) 5.37

(C) 8.33

(D) 14.60

(B) 5.37

v1A1 = v2A2

(2.09)(8.79) = (3.42)(v2)

v2 = 5 m/s (actual: 5.37)

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CRB Absolute (Hydrostatic) Pressure combines both the ambient pressure and the pressure the fluid exerts. Write the equation for Absolute Pressure out.

P = Po + ρgh

P = Absolute Pressure (Pa or N/m^2)

Po = Pressure at Surface (usually equivalent to atmospheric pressure) (Pa)

ρ = Density (kg/m^3)

g = Acceleration due to Gravity (9.8 m/s^2)

h = Height (m)

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CRB Compare Absolute Pressure and Gauge Pressure.

Absolute Pressure includes both the Ambient Pressure and the pressure a fluid is exerting.

Gauge Pressure is the difference between Absolute Pressure and the Atmospheric pressure.

CRB True or false? The Ambient Pressure and Atmospheric Pressure are always equal, so Gauge Pressure Pgauge = ρgh.

False. The Ambient Pressure and Atmospheric Pressure are often equal, but there can be other ambient pressures, like in a vacuum or inside of a pressure cooker.

CRB Write out the equation for Gauge Pressure.

Pgauge = P - Patm = (Po + ρgh) - Patm

Pgauge = Gauge Pressure (Pa)

P = Absolute Pressure (Pa)

Patm = Atmospheric Pressure (Pa)

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CRB In terms of liquids, compare Cohesion and Adhesion.

Cohesion is an attractive force felt between the liquid molecules at the surface of a liquid.

Adhesion is the attractive force between the liquid molecules at the surface of a liquid and the molecules of another substance.

CRB Another notable characteristic about fluids is that their surface tension can help them form a very thin, strong surface layer. Would this be caused by adhesion or cohesion?

Since this surface tension is dependent on a strong attraction between the liquid molecules, they can be attributed to Cohesion.

CRB In a test tube, you can see both Concave and Convex Menisci, depending on the liquid involved. Which force, adhesion or cohesion, would be responsible for each meniscus type?

Concave menisci are characterized by the fluid being more attracted to the walls of the tube, so they are dominated by Adhesion.

Convex Menisci are characterized by fluid being more attracted to itself, so they are dominated by Cohesion.