BUOYANCY

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BUOYANCY

• CE 3250
• Joe Eckhardt, Mike Firestone, Christos Melistas,
  Meghan Vohs

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What is Buoyancy?

• In simple terms, buoyancy is the ability of an
  object to float.
• Whether or not a given object will float is
  determined by the buoyant force on the object.
• This force is caused by the difference between
  the pressure at the top of the object, which
  pushes it downward and the pressure at the bottom
  of the object which pushes it upward. Because
  the pressure at the bottom of the object is
  always greater than the pressure at the top,
  every submerged object feels an upward buoyant
  force.
• The magnitude of this buoyant force is equal to
  the weight of the fluid that is displaced by the
  submerged object.

The weight of the water that is displaced by this
block of wood is equal to the buoyant force
acting on the block. (excerpt from
http//www.pbs.org)
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What is Buoyancy? (cont.)

• The difference between the buoyant force acting
  up on the object and the force due to gravity
  acting downward on the object will determine
  whether or not the object will float or sink.
• If the buoyant force is greater than the
  gravitational force, the object will float, but
  if the opposite is true, the object will sink.

Buoyancy kept the S.S. Edmund Fitzgerald afloat
in the Great Lakes until water filled its hull
causing the ships weight to exceed the buoyant
force acting upward on it. The rest is history.
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How to Calculate Buoyancy

• The buoyant force (FB) is equal to the weight of
  fluid the object displaces.
• If an object is submerged it displaces its total
  volume of fluid.
• If it is partially submerged it displaces the
  volume of fluid equal to its submerged volume.
• The weight of the fluid it displaces is the
  volume of fluid multiplied by the fluids
  specific weight.

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How to Calculate BuoyancyTotally Submerged
Object in Water

• Objects Volume 1ft3
• Specific Weight of Water (?Water) 62.4lb/ft3
• FB 1ft3 x 62.4lb/ft3 62.4lb

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How to Calculate BuoyancyObject 50 Submerged in
Water

• Objects Volume 1ft3
• Submerged volume 0.5ft3
• Specific Weight of Water (?Water) 62.4lb/ft3
• FB 0.5ft3 x 62.4lb/ft3 31.2lb

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Understanding Buoyancy

• Q An inverted spherical shell of diameter, d
  1m is used to cover a tank filled with water.
  What is the minimum weight the shell must be to
  hold itself in place?

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Understanding Buoyancy (cont.)

• A Since only half the sphere is covering the
  tank, we only need to account for half the
  volume. So, V 4/3 pr3. Since only half the
  volume is to be accounted for, the volume that is
  to be used is V 4/6 pr3. So, the volume of the
  sphere will be 4/6 p(.5m)3 .262m3. Then, the
  forces in the y-direction must be summed and set
  equal to zero. Our equation will be FB W 0.
  FB Specific weight of water (gW) volume of
  sphere. So, our equation to solve for W will be
  (9,806 N/m3)(.262m3) W 0. Solving for W,
  the minimum weight the shell must be to hold
  itself in place is 2,569.17N.

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Who Needs Buoyancy?

• Everyone needs buoyancy!
• If you swim, boat, scuba dive, or float you need
  buoyancy!!!

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Buoyancy and Swimming

• Buoyancy is why most people float. Your body
  displaces water. When the weight of the water
  you displace is more than your weight you float.
• Because of buoyancy you weigh very little in
  water.
• You can put most of your energy into movement
  because you need very little to carry yourself