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Unformatted text preview: September 30, 2013 our later use, waves. Week 1: ﬂuids (ch 12)
dV Density: dm = ρ dV
if it’s constant, ρ = m/V 3 ρwater ≈ 10 kg/m
4 ρlead ≈ 10 kg/m
ρneutron star ρblack
Monday, September 30, 2013 3 3 ρconcrete ≈ 2ρwater
ρgold ≈ 2ρlead Just for fun,
≈ 10 (kg/m )
black holes: hole 18 3 19 3 ≥ 1.8 × 10 (kg/m )(Msun /M ) 2 FAQ, cont.
• Q:“Last time you told us the densities of water, lead, neutron stars, etc. Do we need
to know all that data for the exams?” A:
No! Any needed data will be given to you
on the exam. No need to memorize stuff. • The 3 x 5 card is for equations etc. If you
forgot an equation, ask the TA and they
might agree to write it on the chalkboard
for you (at their discretion). Monday, September 30, 2013 Red scantron,
Grove general store
Or buy from TA
or classmates before
quiz (for $$$$)
Monday, September 30, 2013 Pressure
dF = −p nda
(area) p = dF⊥ /da if constant, p = F⊥ /A Ex: hydraulic press: pressure of ﬂuid inside is almost
constant (more soon), so p = F1 /A1 = F2 /A2
F2 = F1 · (A2 /A1 ) F1 Like a lever, can lift heavy object.
Wby1 = F1 ∆x1 = p∆V1 ∆ V 1 = ∆ x 1 A1 Won2 = F2 ∆x2 = p∆V2 ∆ V 2 = ∆ x 1 A2 Same (no loss) if ﬂuid is incompressible: ∆V1 = ∆V2
Monday, September 30, 2013 “Gauge” Pressure
= Pressure ABOVE atmospheric pressure. E.g.
pbike tire ≈ 100psi pcar tire ≈ 20psi Estimate area of each tire in contact with the ground
(200)/(100) = 1(in)
Car: (4000)/(20) = 50(in)
Monday, September 30, 2013 A=F/p
OK, makes sense
MKS: 1P a = 1N/m Atmospheric pressure
At sea level, air pressure is around 101,000 Pa = 1atm.
This is around 15 psi. E.g. on your hand, this is a force
of around 150 lbs. Comes from the weight of all the air
molecules in the atmosphere above us. It doesn’t push
our hand down because the same pressure is below
our hand, pushing it up.
mg Monday, September 30, 2013 Pressure and depth
Pressure in a ﬂuid increases as depth is increased,
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This note was uploaded on 02/11/2014 for the course PHYSICS 2C taught by Professor Hicks during the Fall '09 term at UCSD.
- Fall '09