Homework:

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Exam 3 Results
Average 58 (73) 66 - 80 A (84) 58 -
65 B (65) 50 - 57 C (63) 42 - 49 D (39) 0 -
41 F (33)
Solutions are posted outside my office.
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Pressure Density

• For next time
• No reading assignment
• Homework
• Problems SP77, SP78, and SP79

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Question

• How do you find area?

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Suppose you want to buy a farm. The farm realtor
shows you two farms. Both are the same price and
the quality of the land is the same in every
respect. Therefore, you decide to buy the bigger
one. Suppose you obtained an aerial photograph
of the region and were able to draw in the
boundaries of the two farms as shown in the
figure below. Determine which is the bigger farm.
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A L ? W
Farm 2
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A L ? W
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(No Transcript)
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Volume

• The volume of an object is the number of unit
  cubes (1 cm3) needed to make a duplicate of the
  object.

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The 75/25 rule
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Volume

• The volume of an object is the number of unit
  cubes (1 cm3) needed to make a duplicate of the
  object.
• When an object is submerged in a liquid, the
  object displaces an amount of liquid equal to its
  volume.

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Story of Archimedes

• When Hiero of Syracuse decided to place a golden
  crown in a temple to the gods, he measured out a
  certain mass of gold to the artisan to be used in
  the making of the crown. The artisan returned a
  beautifully wrought crown to Hiero. The mass of
  the crown was identical to the mass of the gold
  Hiero had given the artisan. However, Hiero had
  reason to suspect that some of the gold had been
  replaced with silver in the making of the crown.
  

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• Hiero, unsure how to detect the theft, asked
  Archimedes for help. As the story goes, as
  Archimedes was lowering himself into a tub to
  bathe, he observed that the more of himself he
  lowered into the tub, the more water ran out of
  the tub. When he made that observation,
  Archimedes supposedly leapt from the tub and ran
  down the street shouting Eurhka, eurhka.

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• His observation led him to propose the following
  test. Archimedes took a lump of gold and a lump
  of silver, each of which had the same mass as the
  crown. He first placed the silver lump into a
  vessel full of water and measured the water which
  ran out. He then placed the gold lump into a
  vessel full of water and again measured the water
  which ran out. He finally placed the crown into
  the water and measured the water which ran out.

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Density

• Its just the amount of mass in a unit of
  volume.
• Density of water ?
• 1.0 g/cc
• or, 1000 kg/m3

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Sample problems

• A 50 cm3 piece of magnesium has a mass of 87 g.
  What is the mass of a 6 cm3 piece of magnesium?
• Which has the larger density, a cup full of water
  or a lake of water?
• Which has the greater density, a kg of lead or a
  kg of feathers?

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(No Transcript)
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Pressure in a fluid

• Pressure in a fluid is the magnitude F of the
  force acting perpendicular to a surface divided
  by the area A of the surface.
• What should matter?
• The pressure at any point in a fluid depends on
  the depth of the point beneath the surface and on
  the density of the fluid.

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Jeffs first diving invention
Too much pressure!
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(No Transcript)
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P2 A P1 A mg
P2 A P1 A ?(Vol) g
P2 A P1 A ? h A g
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P2 P1 rgh
Patm 101 325 N/m2 Patm 101 325 Pa Patm 101
kPa Patm 14.7 lbs/in2
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• CRUSH THE CAN