4. Fluids

Question 1

Pascal’s principle

Answer 1

any EXTERNAL pressure on a liquid is distributed throughout liq equally; input pressure = output pressure

Question 2

Pressure formula, what are units of Pa? Is it a vector or scalar?

Answer 2

Force/Area (formula reflects Pascal’s principle); Pa = N/m^2. Scalar

Also: V = A1d1 = A2d2, where d=displacement

And: W = F1d1 = F2d2

Question 3

External pressure vs internal pressure formula

Answer 3

P_in = P_out vs P = rhoheightgravity

Question 4

Density of water. How to find specific gravity and what does it tell you?

Answer 4

1000 kg/m^3 = 1 g/cm^3
1000 L in 1m^3

Rho of substance/rho of water

SG is like a percentage (ex: density of ice = 0.92 g/cm^3, hence its SG = 0.92 —> 92% of ice is under water, while 8% is floating up top. SG<1 —> float in water , SG>1 —> sink in water, SG=1 —> fully submerged but not sink

Question 5

Conversion b/w atm, Pa and mmHg

Answer 5

1 atm = 101,325 Pa = 760 mmHg

Question 6

Bernouilli’s eqn

Answer 6

W + PE + KE for in = W + PE + KE out

replace work w/ press. and mass of PE/KE w/ rho

Question 7

Flux aka flow rate formula

Answer 7

Q = vol in/t

Question 8

Continuity aka volumetric flow rate eqn. How does Q apply to closed systems?

Answer 8

Q = Ainvin=Aoutvout

Q=flow rate
A=cross sect area (A = r^2)
v=linear speed, vel

Fluids flow faster in tighter passages and flow slower in wider passages. Flow rate = constant in closed system and independent of changes in cross sectional areas

Question 9

Know Poiseulle’s Law formula

Answer 9

Flow rate aka flux = Vol/t = (deltaPpiR^4)/(8etaL)

Question 10

Bernoulli vs Poiseulle

Answer 10

when vel inc –> pressure fluid exerts on walls dec => Venturi effect; Bernoulli = typical for incompressible fluids vs flow rate Q = directly related to pressure differential (P1-P2) Poiseulle = typical for viscous fluids

Question 11

Venturi effect

Answer 11

constriction in pipe results in faster fluid vel and lower pressure

Question 12

Higher viscosity coeff means? Know coeffs of water @ 0 and 20 degrees C vs air

Answer 12

higher viscosity; 1.8 mPa*s aka 1.8 cP, 1 cP vs 0.018 cP

Question 13

mPa*s to cP conversion (units of eta)

Answer 13

1 mPa*s = 1 cP

Question 14

Know how to find viscous force

Answer 14

Fv = eta*A*(v/d)
d = depth of fluid, v = vel of fluid, A = area of surface in contact w/ fluid

Question 15

Static fluid pressure vs absolute/hydrostatic pressure vs gauge pressure vs incident/ambient pressure

Answer 15

P=rhogh vs P = P_0 + rhogh, total pressure exerted on object submerged in fluid, absolute pressure depends on density of fluid and not object’s, P_0 = ambient/incident pressure vs Pgauge = P-Patm, diff b/w absolute pressure inside and atm pressure outside (ex: diff b/w absolute pressure inside tires and atm pressure on tires), gauge pressure depends on density of fluid and not object’s vs pressure at the surface (I guess you can assume pressure @ surface = atmospheric pressure unless otherwise)

Question 16

How to find buoyancy force from Archimedes principle

Answer 16

Buoyancy force (which is always an upward force caused by the pressure difference above and below the object) = weight of DISPLACED FLUID

F = rho of fluidvol of fluid displacedg = rho of fluidvol of submerged objectg

Question 17

Laminar flow vs turbulent flow

Answer 17

Smooth and orderly; more viscous fluids –> dec in vel –> more laminar flow vs rough and disorderly, produces eddies - swirls of fluid of diff sizes occurring downstream; less viscous fluids –> inc in vel –> more turbulent flow

Question 18

Critical speed vs boundary layer

Answer 18

Speed at which turbulence can arise during unobstructed fluid flow vs a thin layer of fluid adjacent to the wall showing laminar flow when critical speed is exceeded

Question 19

Critical speed formula

Answer 19

v = (N_Reta)/(rhoD)

N_R=Reynolds number, a dimensionless constant; depends on size, shape and surface roughness of objects within fluid
eta =viscosity of fluid
rho=density
D=diameter of tube

Question 20

How does a backwards/convex meniscus occur?

Answer 20

When cohesive forces > adhesive forces

Question 21

Hydrostatics = study of…?

Answer 21

Fluids at rest —> liquids and gases at rest

Question 22

How to dec surface tension of fluids?

Answer 22

inc temp; dec surface tension also dec surface area of fluids as well

Question 23

Describe mechanisms of a simple pendulum. How is inverted pendulum diff from reg pendulum?

Answer 23

period and freq = indep of mass, acc due to gravity determines freq. inverted pendulum has minimum KE when it reaches equil and higher non-gravitational force to restore to equil, both reg and inverted pendulum have same amount of grav torque