Tue Oct 19

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Phys 2101
Gabriela González
Density: mass per unit volume
ρ = Δm/ΔV. If uniform, ρ = m/V
SI units: kg/m3
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Pressure: force per unit area
p = F/A
SI units: N/m2 = Pa
Non-SI units: 1 atm = 1.01x105 Pa = 760 torr =760 mm Hg
1 atm = 14.7 lb/in2 (psi)
3
Pressure: force per unit area p = F/A
SI units: Pa = N/m2 = (kg m/s2)/m2 = kg/(ms2)
Non-SI units: 1 atm = 1.01x105 Pa 1.01 hPa = 760 torr =760 mm Hg = 29.9 in Hg
1 atm = 14.7 lb/in2 (psi)
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2
Pressure in fluids at rest increase with depth, due to weight of fluid
on top:
F = F0 + mg = F0 + (m/V)gV = F0 + ρghA
p = F/A = p0 + ρgh
A=area
h
Pressure decreases when climbing mountains, and increases when diving:
In both cases, be careful with pressure changes!
5
•  How deep in the sea is the pressure twice as much the
pressure at the surface?
•  Would the answer be different in a mountain lake?
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3
http://auto.howstuffworks.com/pressure-gauge.htm
How long should a mercury barometer
be to measure 29 psi tire pressure?
7
A change in the pressure applied to an enclosed
incompressible fluid is transmitted undiminished to every
portion of the fluid and to the walls of its container.
Hydraulic lever:
Fi/Ai=F0/A0
F0=Fi (A0/Ai)
A force amplifier!
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4
http://auto.howstuffworks.com/auto-parts/brakes/brake-types/brake3.htm
9
When a body is submerged
in a fluid, a “buoyant” force Fb
from the surrounding fluid
acts on the body, directed
upward and with a magnitude
equal to the weight of the
fluid that has been displaced
by the body.
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5
What does a balance read when it is measuring the
weight of a submerged body?
N
mg
N
Fb
mg
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Use a block of (real) gold to balance
the crown in a lever: we then know
the weight of the crown is the same
as the gold block. If the crown is
made out of gold, the crown and the
block will also have the same
volume, and produce the same
buoyant force when the lever is
submerged, so the equilibrium
should still hold. If it doesn’t, we
know the crown is not made out of
gold.
In the figure shown, is the crown made out of a more or less dense material?
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