Announcements 8/31/12
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Prayer
Class directory handouts
Any new people? Please see me after class.
From syllabus: “If a very large or very small value needs to be
written in scientific notation, as specified by the answer range,
indicate the exponent of 10 with an “e”. For example, 2.998 108
would be written 2.998e8, and 1.6 10-19 would be written 1.6e19. Do not put any spaces, commas, or “x”s in the number. Do
put in negative signs where appropriate.”
xkcd:
From warmup
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Extra time on?
a. Bernoulli’s equation (5 people)
b. Toricelli’s law (1 person)
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Other comments?
a. you should maybe show a skydiving clip to show
fluid dynamics and how adjusting yourself can
adjust where/how fast you go.
b. I'm not sure if you got the first quiz I submitted
Clicker question:
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Archimedes was charged with determining if a crown
was pure gold. One method he may have used:
suppose he balanced the crown with a lump of pure
gold outside water. After immersing, the balance
tipped as shown. The crown must:
a. Be denser than gold
b. Be less dense than gold
Clicker questions:
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In the demo, what will happen to the
upper scale when the mass is submerged.
a. scale reading increases
b. scale reading decreases
c. nothing changes
What will happen to the lower scale?
a. scale reading increases
b. scale reading decreases
c. nothing changes
Demo: scales
Clicker question:
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A cannonball is in a boat, which is in the middle of a
small lake. (Frictionless, wave-less, etc.) A person
tosses the cannonball from the boat into the lake.
What happens to the overall water level of the lake
(where the water touches the shore)?
a. It rises
b. It falls
c. It stays the same
Worked Problem
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An upside-down pyramid (side length s, height h) is
placed in a fluid. rpyramid < rfluid, so it floats. In terms of
the given quantities and fundamental constants, how far
into the fluid does the pyramid settle? (It doesn’t tip.)
s
water level
Answer: y  3 r p r f  h
h
y=?
Fluid Motion
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Disclaimer: We’ll ignore
a. compressibility
b. viscosity
c. turbulence
The power of viscosity (watch on your own):
http://www.youtube.com/watch?v=W3YZ5veN_Bg
Clicker question:
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Water flows from a pipe with large diameter into a
pipe with smaller diameter. The speed of the water in
the narrow section will be _________ the speed in the
wide section.
a. greater than
b. less than
c. equal to
The pressure of the water in the narrow section will be
_________ the pressure in the wide section.
a. greater than
b. less than
c. equal to
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”
VFR  A1v1  A2v2
Warmup question 1
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Two identical buckets… second one filled while
holding your thumb over the end of the hose.
How long to fill?
a. The same amount. The volume of water
coming out is exactly the same as before, it is
merely the speed of the water that has
changed.
Warmup question 2
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In your own words explain what the "equation
of continuity" for fluids means. Audience =
brother/sister/mother who hasn't taken physics
at all.
a. The equation of continuity is a fancy way of
saying what goes in must go out. The same
amount of water going into a hose, for
example, must go out. Now, if the opening of
the hose is larger than the ending of the hose,
lots of water is coming in within a second. The
equation of continuity states that the same
amount of water must come out within the
same time period, but since the opening is
smaller, it comes out faster.
Demos
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Bernoulli red fluid
Chimney effect
Warmup question #3: If the airstream from a
hair dryer is directed over a ping pong ball, the
ball can be levitated. Explain how this works.
Floating ball
Ball in funnel
Video: Elder Nelson’s spool
(start at 1:57, go to ~4:00)
Cards & wooden block
Piece of Paper
Quick writing (random student)
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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: you’re a molecule right there
in what direction is the net force?
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View #2: energy & work, per volume (yields
“Bernoulli Equation”)
Clicker question:
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Water flows from right to left, 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
Demo (not used)
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Does aluminum sink or float?