Download Lecture 3 - fluid motion - BYU Physics and Astronomy

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)
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Reading question (graded):
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Which of the following is NOT one of the
four assumptions we make when we model
“ideal fluids?”
a. The fluid is incompressible
b. The fluid is massless
c. The fluid is nonviscous
d. The flow is irrotational
e. The flow is laminar
Disclaimer: Viscosity exists
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Assumption: “the fluid is nonviscous”
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What does “nonviscous” mean?
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Viscosity is most significant when…
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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 than
b. less than
c. equal to
 The pressure of the water in the small tube will
be _________ the pressure in the large tube.
a. greater than
b. less than
c. equal to
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The Bernoulli Effect
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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…
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Why would the fluid change speed?
 The “garden hose equation”, aka Eqn of C_____
 Terminology: “volume flow rate”
Demos
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Bernoulli red fluid
Floating ball
Chimney effect
Ball in funnel
Cards & wooden block
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Video: Elder Nelson, April 1997 conference
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Quick writing
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In the reading assignment for today, Ralph noticed two
different equations both labeled "Bernoulli's Equation".
One said,
P1 + ½ rv12 + rgh1 = P2 + ½ rv22 + rgh2
whereas the other said,
P + ½ rv2 + rgh = 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?
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View #1: pressure on walls caused by molecules
bouncing off walls in transverse direction.
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View #2: you’re a molecule right there
in what direction is the net force?
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View #3: energy & work, per volume (yields
“Bernoulli Equation”)
Thought questions (ungraded):
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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 than
b. the same as
c. less than
The speed on the right will be ____ times the speed on the
left.
a. 1/9
b. 1/3
c. 1
d. 3
e. 9
Worked Problem:
faucet
r2
Pmain
3m
r1
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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
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Principle 1: deflection
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Principle 2: Bernoulli
“airfoils”
http://www.av8n.com/how/htm/airfoils.html
Curve balls
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Ball moving to the right (i.e. air moving to the
left), with topspin
Demo
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Ping pong!