Chapter 4 - Fluids

Question 1

Density

Answer 1

• ρ = m / V
• units for density are kg/m³
• mL = cm³

Question 2

Density of Water

Answer 2

1000 kg / m³ or 1 g / cm³

Question 3

Weight of Volume

Answer 3

F_g = ρVg

frequently used in buoyancy problems

Question 4

Specific Gravity

Answer 4

SG = ρ / 1 (g/cm³)

• will determine whether an object will sink or float

Question 5

Pressure

Answer 5

• pressure is the force excerted over a given area

P = F/A

• units for pressure are pascals (Pa)
• 1 Pa = 1 N/m²
• 1.013 x 10⁵ Pa = 760 mmHg = 760 torr = 1 atm

Question 6

Absolute (hydrostatic) Pressure

Answer 6

total pressure that is excerted on an object that is fully submerged in a fluid

P = P₀ + ρgz

P₀ - the pressure outside of the fluid

ρ - density of the object

z - depth of the object

Question 7

Gauge Pressure

Answer 7

P_gauge = P - P_atm = (P0 + ρgz) - P_atm

Question 8

Pascal’s Principle

Answer 8

a change in pressure will be transmitted undimished to every portion of the fluid and to the walls of the containing vessel

P₁ = P₂

F₁d₁ = F₂d₂

Question 9

Surface Tension

Answer 9

causes liquid to form a thin but strong layer like “skin” at the surface of a liquid

Question 10

Archimedes’ Principle

Answer 10

• this principle deals with buoyancy of an object placed in fluid
• a body/object placed in water will be buoyed upwards by a force equal to the weight of the fluid that displaces

F_buoy = ρ_fluidV_{fluid displaces}g = ρ_fluid V_submerged g

Question 11

Adhesion

Answer 11

the attractive force that surface molecules feels towards different molcules

Question 12

Cohesion

Answer 12

the attractive force that a molecule has for other molecules that are similar to itself

Question 13

Meniscus

Answer 13

• this demostrates the adhesion properties of a fluid.
• when a meniscus is formed the adhesive forces are greater than those of cohesion
• a backwards (convex) meniscus will form when the cohesion forces are greater than the adhesion forces

Question 14

Viscosity

Answer 14

• is the resistance for a fluid to flow
• units for viscosity (Pa x s = N x s/ m²

Question 15

Inviscid

Answer 15

a fluid that has no viscosity

Question 16

Laminar flow

Answer 16

• smooth and orderly flow of fluids; fluids that flow parallel to each other
• MCAT always assumes laminar flow

Question 17

Poiseuille’s Law

Answer 17

• used to calculate the rate of flow within a given pipe or confined space

Q = πr⁴ΔP/8ηL

• most often the relationship between radius and pressure gradient is tested on the MCAT
  
  • the relationship between the two is inversley exponential to the fourth

Question 18

Critical Speed

Answer 18

• when the flow of a fluid exceeds the critical speed it will cause the flow to become tubulent and form eddies at the end
• V_c = N_R* η / ρD
• N_R* - is Reynold’s number which i s determined by factors such as size, shape, and surface roughness

Question 19

Flow rate

Answer 19

is constant for a closed system and is independent of changes in cross-sectional areas

(volume per unit time)

Question 20

Continuity Equation

Answer 20

this equation is used to calculate the flow rate of a fluid

Q = V1A1 = V2A2

Question 21

Bernoulli’s Equation

Answer 21

P₁ + 1/2ρv₁ ² + ρgh₁ = P₂ + 1/2ρv₂ ² + ρgh₁

• this equation states thatthe sum of static and dynamic pressure will be constant throughout a closed system