Chapter 4 - Density and Buoyancy
Home Page for Chapter 4
A Quick Review and How to Measure
What is Density/Calculating Density
What is Buoyancy? / Buoyancy in everyday life.
By now you should know what to find here
Buoyancy is the upward force caused by a fluid, such as water. This concept helps to explain
why some things float while other objects sink. Buoyancy is an important factor in the design
of many objects and in a number of water-based activities, such as boating or scuba diving.

The mathematician Archimedes discovered much of how buoyancy works almost 2000 years
ago. In his research, Archimedes discovered that an object is buoyed up by a force equal to
the weight of the water displaced by the object. In other words, a inflatable boat that displaces
100 pounds (45 kilograms) of water is buoyed up by that same weight of support. An object
that floats in the water is known as being positively buoyant. An object that sinks to the
bottom is negatively buoyant, while an object that hovers at the same level in the water is

This same idea helps to determine what will float in water and what will sink. If an object
weighs more than the weight of the water it displaces, it will sink. If the object weighs less, it
will float. This helps explain why a heavy ship can easily float in the water, while a much
smaller and lighter brick will sink quickly. It isn't the size or shape of an object that primarily
determines buoyancy, but the relation between an object's weight compared to the weight of
the water the object displaces.
What is Buoyancy?
How does Buoyancy work?
When you place a block of wood in a pail of water, the
level goes up.

If you could weigh the water that the wood displaces,
you would find that its weight equals the weight of the
If you had two pieces of wood that were exactly the
same size and shape, they would each displace the
same amount of water. A block of wood made of oak,
for example, sits deeper in the water (and therefore
displaces more of the water) than does a block of
The reason is that it's heavier for its size, or denser --
in this case, the molecules that make it up are more
closely packed together than the molecules that
make up the pine.
If you could somehow keep increasing the density of the
block, it would sink lower and lower into the water. When its
whose weight was equal to the weight of the block, it would, in
a sense, become weightless in the water.

Making the block just slightly denser would cause it to sink to
the bottom.
Controlling Buoyancy - The Submarine
In this cross section of a submarine you can see
how much space there is in the hull to fill with
water to change the density of the submarine.

When the total density is less than the seawater
the sub will float, when the ballasts are filled with
water the submarine is more dense than water
and the submarine will sink.
How a submarine looks in the water.
Heat causes a
density.  This
makes the lava
lamp work!