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Name: Jordan
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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.

Larry Krengel



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