Rather than just giving a short answer to each
question, I thought I'd discuss the topic in a
more general way so you can (hopefully!) see how
all your questions are related.
To start, let's first look at the topic of
buoyancy. According to Archimedes'
Principle, the buoyant force on a submerged
(or partially submerged) object is equal to the
weight of the fluid displaced by the object.
Here, we use the term "fluid" because this same
principle applies to both liquids and gases.
In essence, the buoyancy of an object in a fluid
depends on the relative densities (mass per
volume) of the object and the fluid. If the
object is more dense than the fluid, it will
Conversely, if the object is less
dense than the fluid it will "rise" or
"float". For objects less dense than the
fluid, the relative densities of the object and
fluid determine what fraction of the object is
Question #1 and #4
To visualize this, let's imagine a wood cork. A
wood cork in air will fall (or "sink") to the
ground because a cork is more dense than air. In
other words, the mass of the cork is greater than
the mass of air that would occupy the same volume
as that cork. However, if we instead place the
cork in water, it will now "float" because the
cork is less dense than the water. Hence we can
see that the buoyant force exerted on the object
depends on the density of the fluid. Let's now
imagine that the cork was made out of lead instead
of wood. In this case, the cork will clearly
"sink" in the water since lead is much more dense
than water. Hence, the object's material, or
more specifically its density, affects its ability
The shape of an object does affect its
ability to float. Let's talk about boats.
Some, such as cargo boats, are made of steel.
Clearly, steel is more dense than water, yet these
boats still float. The key is that only the
hull is made out of steel, while the inside is
mostly empty. The shape of the hull allows the
boat to displace a volume of water equal to the
weight of the boat. Since much of the submerged
area is air, the average density (total mass of
the boat divided by the volume of water displaced)
is less than that of water, thus allowing it to
Question #2 and #5
An object will float whenever the weight of the
object is equal or less than the buoyant force. If
the density of the object is less than the density
of the fluid it will always float. An object made
up of materials more dense than the fluid can
still float as long as the average density is less
than that of the fluid (as described in the answer
to Question #3).
Good luck at the science fair!
Click Here to return to the search form.