Properties of Liquids: Density and Buoyancy

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Properties of Liquids Density and Buoyancy
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Definitions

• Density
• The mass per unit volume of a material.

Buoyancy The ability of a fluid to exert an
upward force on a floating object.
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Density
Densities of Common Materials

• The symbol for density is usually D and has units
  of g/cm3.
• Density is a physical property of a material.
• Often, a materials density is used to identify
  it.

Water 1.00 g/cm3
Aluminum 2.7 g/cm3
Steel 7.9 g/cm3
Gasoline .70 g/cm3
Mercury 13.6 g/cm3
Air .0013 g/cm3
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Density
m

• Solids, liquids, gases, and other phases of
  matter all have densities.
• Solids are most dense, liquids next dense, and
  gases least dense
• MOST of the time, density is inversely related to
  temperature.

D

V
Which weighs more, A pound of feathers or a
pound of lead? Which is more dense?
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Calculating Densities
Mass of the Sun 2 x 1030 kg
Volume of the Sun 1.4 x 1027 m3
Average Density of the Sun 1.4 g/cm3
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Calculating Densities
Mass of the Earth 6 x 1024 kg
Volume of the Earth 1.1 x 1021 m3
Average Density of Earth 5.5 g/cm3
Does it make sense that the Earth is more dense
than the Sun?
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A Density Problem
Does this help?
Metal Density g/cm3
Aluminum 2.70
Gold 19.30
Copper 8.63
Steel 7.87
Silver 10.40
Chromium 7.10
Lead 11.30

• You found this ring on the track at your school.
  You wondered what it was made of.
• You calculated its density to be approximately
  7.9 g/cm3.
• What is it made of?

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Forces

• Newtons Laws
• 1.) The law of inertia An object in motion stay
  in motion until it is acted upon. Or, an object
  at rest stays at rest until acted upon.
• 2.) F ma
• 3.) When two bodies interact by a force, the
  force on the first body is equal in magnitude and
  opposite in direction to the force on the second
  body.

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Archimedes Principle

• The buoyant force
• exerted on an object
• submersed in a fluid is
• equal to the weight of
• the fluid displaced.

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But what does that mean?!?

• Fb DVg
• Fb Buoyant force
• V Volume of fluid displaced
• D Density of fluid
• g Acceleration due to gravity

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Why does it float?

• For an object to float
• Wobject Fbuoyant

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Buoyancy Calculations

• The polar bear on ice
• A 500 kg polar bear sees a slab of ice floating
  by and tries to hop on. The ice has a volume of 4
  m3 and a density of .92 g/cm3. Seawater has a
  density of 1.035 g/cm3. Does the polar bear float
  away happily?

Not this time! Fb 4140 N lt Wbear 4900N
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Activity

• You will determine the density of a liquid and
  use that density to identify the liquid.
• Good luck!

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Questions?