Question:
Why is it that when one object has a lower density than a liquid,
not all of the object floats? For example, only 20% of a block with a density
of 0.8 g./cm.^3 floats on water with a density of 1 g./mL.
Replies:
Jordan -
The buoyancy force on an object in a fluid is a function of the mass of the
fluid displaced. In the case of fresh water with a density of 1 g/cm^3, any
object with a lesser density has a greater buoyant (upward) force than the
downward force of gravity. An object will move above the surface until mass
of the displaced liquid equals the mass of the submerged portion of the object.
Perhaps you can figure out why it is easier to swim in the Great Salt Lake than
in Lake Michigan. If you would choose to swim in a pool of mercury (silly as
it might be due to the poisonous nature of mercury) you would find it very
difficult to swim below the surface... Mercury has a density of more than 13
times that of water.
If you want to try an interesting experiment, ask your science teacher to
place an ice cube in a beaker of rubbing alcohol. Then try to explain what
you see.
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