Transcript Physics 106P: Lecture 1 Notes

Physics 101: Lecture 23
Fluids
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Today’s lecture will cover Textbook Sections 11.1-11.5
Density
Pressure
Pascal’s Principle
Physics 101: Lecture 23, Pg 1
Physics 101: Density
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Density = Mass/Volume
  = M/V
 SI unit: [kg/m3]
Densities of some common things (kg/m3)
 Water
1000
 ice
917 (floats on water)
 blood
1060 (sinks in water)
 lead
11,300
 Copper
8890
 Mercury 13,600
 Aluminum 2700
 Wood
550
 air
1.29
 Helium
0.18
Physics 101: Lecture 23, Pg 2
Pressure
Pressure that a fluid or gas exerts on a surface:
P = F / A
SI unit: [N/m2= Pa]
F: force acting perpendicular to surface
A: Area of surface
Atmospheric presure:
1.01 105 Pa
Physics 101: Lecture 23, Pg 3
Pressure and Depth
Barometer: a way to measure atmospheric pressure
p2 = p1 + gh
p1=0
patm = gh
Measure h, determine patm
p2=patm
h
example--Mercury
 = 13,600 kg/m3
patm = 1.05 x 105 Pa
 h = 0.757 m = 757 mm = 29.80” (for 1 atm)
Physics 101: Lecture 23, Pg 4
Concept Question
Suppose you have a barometer with mercury and a barometer with
water. How does the height hwater compare with the height hmercury?
1. hwater is much larger than hmercury
2. hwater is a little larger than hmercury
3. hwater is a little smaller than hmercury
4. hwater is much smaller than hmercury
CORRECT
water is much less dense than
mercury, so the same amount of
pressure will move the water
farther up the column.
Pa  gh
p1=0
p2=patm
h
Pa
h
g
 mercury  13.6  water
so, h water  13.6 h mercury
Physics 101: Lecture 23, Pg 5
Concept Question
Is it possible to stand on the roof of a five story (50 foot) tall house
and drink, using a straw, from a glass on the ground?
1. No
CORRECT
2. Yes
Even if a person could completely remove all of the air
from the straw, the height to which the outside air
pressure moves the water up the straw would not be high
enough for the person to drink the water.
Physics 101: Lecture 23, Pg 6
Concept Question
p=0
pa
Pa  gh
h
Pa
h
g
Evacuate the straw by sucking
How high will water rise?
no more than h = Pa/g = 10.3m = 33.8 feet
no matter how hard you suck!
Physics 101: Lecture 23, Pg 7
Summary
• Density
• Pressure
P2 = P1 + gh
Pascal’s Principle
Physics 101: Lecture 23, Pg 8