What is the relationship between the amount of water and
object displaces and the weight of the object?
Who discovered this theory?
This is called the law of bouyancy, discovered by Archimedes, and basically
any material or object immersed in a fluid will tend to rise through the
fluid if the fluid density is greater than the material density. The force
associated with buoyancy is the difference between the weight of the
displaced fluid and the weight of the immersed material.
Density is the amount of and object (weight or mass) there is an a given
volume (the amount of water the object displaces).
So objects which weigh a lot and don't displace much water (have a low
volume) will sink, while objects which are light and displace a lot of water
The relationship, then is how well the object will float. If it floats, it
is less dense than water, and if it sinks it is more dense than water.
You might think something like a ship is more dense than water, after all,
it is made of metal which is certainly more dense than water, but, the ship
is filled with air, which is much less dense than water, and makes up the
difference. That is why, when a ship gets a hole in it, it sinks. The
water pushes out all the air and makes the total density of the ship
greater. When the water, metal, and air inside the ship become more dense
than the water outside the ship, the ship will sink.
You are probably thinking of Archimedes' principle. Archimedes was a Greek
philosopher. The story goes that the king of the day wanted a new crown
made entirely of gold. After the goldsmith made it, the king was
suspicious that the goldsmith made it of iron and coated it with gold. The
king did not want to destroy the crown if in fact it were gold, so he asked
Archimedes to figure out if it was pure gold or not without destroying the
crown. Archimedes pondered this day in and day out until one day while
getting into his bath he discovered the principle that bears his name. The
tub was full to the rim, and when he sat down in it, he noticed that the
more he sank himself into the water, the more water that spilled over the
side of the tub. He was supposedly so happy to make this discovery that he
ran out into the streets naked shouting "I found it!" What he discovered is
that the amount of water displaced by an object depends on the mass of that
object (not the weight). If he knew the mass of that object, and the volume
of fluid it displaces, he could determine its density. Since the densities
of iron and gold are different, he did a test. He determined the density of
the crown and compared it to the density of pure gold to see if they were
the same. Legend says they were not the same, so the king was tricked.
An object that FLOATS displaces the amount of water that has the same weight
as the object. If it sinks, it displaces an amount of water that has less
weight than the object. This is generally attributed to an ancient Greek
known as Archimedes.
If the object is less dense than water (if it floats on water), it
displaces a weight of water equal to the weight of the object. If the
object sinks in water, it simply displaces a volume of water equal to the
volume of the object.
This is known as the Principle of Archimedes.
Richard Barrans Jr., Ph.D.
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Update: June 2012