Announcements 9/2/11




Prayer
Signup sheet, final time
Any new people? Please see me after class.
Don’t forget to come to office hours for help on HW
a. Colton: MWF 2-3 pm (in underground lab)
b. Chris: MW 5 - 6:30 pm, F 3:30 - 5 pm (UGL)
Worked Problem

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=?
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 incompressible
b. The fluid is massless
c. The fluid is nonviscous
d. The flow is irrotational
e. The flow is laminar
The power of viscosity (watch on your own):
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

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”
VFR  A1v1  A2v2
Demos






Bernoulli red fluid
Floating ball
Chimney effect
Ball in funnel
Cards & wooden block
Piece of Paper
Quick writing

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?

View #1: you’re a molecule right there
in what direction is the net force?

View #2: energy & work, per volume (yields
“Bernoulli Equation”)
Thought questions (ungraded):


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
Airplane Wings

Principle 1: deflection

Principle 2: Bernoulli
“airfoils”
http://www.av8n.com/how/htm/airfoils.html
Curve balls

Ball moving to the right (i.e. air moving to the
left), with topspin
 Ping pong demo!
Worked Problem:
faucet
r2
Pmain
3m
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