Announcements 9/3/10

Prayer I now know everyone’s names!! (maybe) Any new people? Please see me ASAP. Don’t forget about office hours)

a. Mine today are 2-3 pm (in underground lab)b. TA’s today are 4-5:30 pm (also in

undergrnd lab)

Reading question (graded):

Which of the following is NOT one of the four assumptions we make when we model “ideal fluids?”

a. The fluid is incompressibleb. The fluid is masslessc. The fluid is nonviscousd. The flow is irrotational e. The flow is laminar

Disclaimer: Viscosity exists

Assumption: “the fluid is nonviscous”

What does “nonviscous” mean?

Viscosity is most significant when…

Viscosity causes a l____ in p________

The power of viscosity:http://www.youtube.com/watch?v=W3YZ5veN_Bg

Reading quiz (graded):

Water flows from a pipe with large diameter into a pipe with smaller diameter. The speed of the water in the small tube will be _________ the speed in the large tube.

a. greater thanb. less thanc. equal to

The pressure of the water in the small tube will be _________ the pressure in the large tube.

a. greater thanb. less thanc. equal to

The Bernoulli Effect

When a constantly flowing fluid has regions of different speed in its flow, the pressure of the fluid will be lowest in regions where the speed is fastest.

What this doesn’t mean…

Why would the fluid change speed? The “garden hose equation”, aka Eqn of C_____ Terminology: “volume flow rate”

Demos

Bernoulli red fluid Floating ball Chimney effect Ball in funnel Cards & wooden block

Video: Elder Nelson, April 1997 conference

Quick writing

In the reading assignment for today, Ralph noticed two different equations both labeled "Bernoulli's Equation". One said,

P1 + ½ v12 + gh1 =  P2 + ½ v2

2 + gh2

whereas the other said, P + ½ v2 + gh = constant

He asks you how they can both be the same equation when they look so different? And what it’s the value of the constant in the second equation, anyway? What should you tell him?

Why does this happen?

View #1: pressure on walls caused by molecules bouncing off walls in transverse direction.

View #2: you’re a molecule right there

in what direction is the net force?

View #3: energy & work, per volume (yields “Bernoulli Equation”)

Thought questions (ungraded):

Water flows from the little pipe into the big pipe. Ignore any friction or height change. The volume flow rate on the right will be ______ on the left

a. greater thanb. the same asc. less than

The speed on the right will be ____ times the speed on the left.

a. 1/9b. 1/3c. 1d. 3e. 9

Worked Problem:faucet

Pmain3m

r2

r1

The faucet of radius r2 = 2 cm puts water out at 15 liters/minute. The pressure at the opening of the faucet is about 1 atm. The water main (r1 = 6 cm) is 3 m below the faucet.

a. What is the speed of the water in the narrow pipe?

b. What is the pressure in the main?Answers: 0.199 m/s; 1.307E5 Pa = 1.290 atm

Airplane Wings

Principle 1: deflection

Principle 2: Bernoulli

http://www.av8n.com/how/htm/airfoils.html

“airfoils”

Curve balls

Ball moving to the right (i.e. air moving to the left), with topspin

Demo

Ping pong!