SOME HEAVY THOUGHTS

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SOME HEAVY THOUGHTS

• On the flotation of objects

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mass/volume density

• Mass how much matter is there
• Volume how much space is there
• Density how much matter is packed into how much
  space

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Density of H2O lab

• Most student graphs looked like this
• jagged appearance in the beginning is because
  even tiny measurement errors had proportionately
  greater affect at these small volumes

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Density of H2O lab

• Some student graphs looked like this
• Probably due to wrong technique

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Density of H2O lab

• A few student graphs looked like this
• Probably due to wrong technique

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Density of H2O lab

• Perfect data would look like this
• Water is what the metric system is based on
• DH2O 1.000 g/mL

• DENSITY REMAINS CONSTANT AT A GIVEN TEMPERATURE
  AND PRESSURE

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3 Liquids Lab

• Were the liquids separated because they were
  chemically different?
• Perhaps oil, water and something else?

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3 Liquids Lab

• RED LIQUID corn starch H2O dye
• corn starch is a quite massive molecule compared
  to water
• this increases the relative density of this fluid
  mixture
• gravity pulls most strongly on the corn starch

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3 Liquids Lab

• BLUE LIQUID NaCl H2O dye
• table salt (NaCl) is not a huge molecule but is
  more massive than water
• densitysaltwater gt densityyellow liquid
• densitysaltwater lt densitycorn starch

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3 Liquids Lab

• YELLOW LIQUID H2O dye
• densityfreshwater dye only SLIGHTLY more than
  pure water (1.001 g/mL)

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How does this translate on the graph?
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20 g/10 mL 2 g/mL .gravity pulled the most
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20 g/10 mL 2 g/mL .gravity pulled the most
10 g/10 mL 1 g/mL .gravity pulled medium
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20 g/10 mL 2 g/mL .gravity pulled most
10 g/10 mL 1 g/mL .gravity pulled medium
5 g/10 mL 0.5 g/mL .gravity pulled least
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So how do OBJECTS float?
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• If an object displaces more than its own weight
  in the water before it is completely submerged
• it will float!
• EXPLANATION
• water displaced by the block is pushed out to the
  sides and up
• water level goes up
• the displaced waters own weight pushes back on
  the block, because the water is pulled by gravity
  too
• Weight of the water displaced by the wood
  weight of the wood ? OBJECT FLOATS (is buoyant)

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Equal volumes feel equal buoyant forces
0.45 g/cm3
2.7 g/cm3
11.3 g/cm3
but different densities have different weights
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• Oak block sits deeper in the water (and therefore
  displaces more of the water) than does a block of
  pine.
• It is heavier for its size ? denser
• Oak molecules ? more closely packed together than
  pine molecules
• Oak is pulled down more strongly than the pine
  and so displaces more water

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So how DO steel ships float?

• Consider these two equal masses of steel

ship weight 10,000 kg
ball weight 10,000 kg
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Shape is everything!

• Shape of bowl its greater surface area higher
  effective volume
• Higher effective volume lower average density
• Ship displaces more water than the ball does

10,000 kg
10,000 kg
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Shape is everything!
10,000 kg
10,000 kg
Effective volume
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Shape is everything!

• displaced water forceship weightship
• ship floats
• displaced water forceball lt weightball
• ball sinks

10,000 kg
10,000 kg
Effective volume
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What if the liquid is NOT pure water?

• All things are still true, we just have to add up
  the forces at play!

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What if the liquid is saltwater?

• Saltwaters buoyant force is greater than
  freshwaters because it is being pulled more
  strongly downward, so it resists being displaced.

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What if the liquid is oil?

• Oils buoyant force is less than freshwaters or
  saltwaters because it is being pulled less
  strongly downward, so it is more easily displaced.

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credits

• http//www.explorelearning.com/index.cfm?methodcR
  esource.dspDetailResourceID17
• http//www.physics.mun.ca/jjerrett/buoyant/buoyan
  t.html