Chapter 14 Buoyancy

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Chapter 14Buoyancy
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Buoyancy in a Liquid

• The downward weight of an object (force due to
  gravity) is not as great as an upward force on
  that object called the buoyant force or buoyancy

Weight
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Buoyancy in a Liquid

• The force of pressure is greater at the bottom of
  the object than the force of pressure exerted on
  the top of the object.

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• Therefore, the buoyant force is due to the
  pressure differences between the top and the
  bottom of object.

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• Buoyancy arises from the fact that
• fluid pressure increases with depth
• the increased pressure is exerted in all
  directions
• there is an unbalanced upward force on the bottom
  of a submerged object

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Buoyant Force Equals Weight of Liquid Displaced

• any object placed in water displaces a certain
  amount of water
• think about how the water level in the bathtub
  rises when you get in
• you can use the weight of the displaced water to
  determine the buoyant force

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Buoyant Force Equals Weight of Liquid Displaced

• What is the buoyant force on the weight?
• To calculate, figure out the weight of the
  displaced water.

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Archimedes Principle

• Archimedes lived over 2000 years ago in Greece.
  He discovered the fact that
• The buoyant force on an object is equal to the
  weight of the fluid displaced by the object.

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Sink or Float?

• The buoyant force determines whether an object
  will sink or float.
• If the buoyant force is greater than the weight
  of the object, then the object will float
• If the buoyant force is less than the weight of
  the object, then the object will sink.

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Sink or Float?

• An object floats when it displaces a volume of
  fluid whose weight is greater than or equal to
  its own weight.
• An object will float in a fluid if the density of
  that object is less than the density of the fluid.

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Why is the tip of the iceberg the only part seen
out of the ocean?
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Isostasy

• The buoyant force of the ocean pushes the
  iceberg upwards, but the volume of the ice is
  only slightly less than the same volume of salt
  water it displaces, so almost 90 of the iceberg
  remains submerged.

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Why do some objects sink while others float?

• Density (the objects mass divided by its volume
  - how much space it takes up)
• In order for an object to float, the water it
  displaces must weigh more than the object itself
• Or to put in density terms, the object must have
  a density lower than the density of the water

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If the density of water is 1g/cm3, then . . .

• will wood (D 0.8 g/cm3) float?
• will aluminum (D 2.7 g/cm3) float?
• will steel (D 7.8 g/cm3) float?

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Didnt you just say steel would sink?!? Then
why is this ship made of steel floating on top of
the water?
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The shell of the ship may be made of steel, but
most of the space inside the hull is filled with
air that has a very low density.
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Some creatures in the sea have gas filled
bladders whose volume can be changed to adjust
for the buoyant force at various depths.
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This is how submarines work too. They can take
in sea water to submerge or discharge sea water
to rise up to the surface.
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Can you think of something else that uses changes
in the density within a chamber to make it rise
or fall in our atmosphere?