If the overall density of a composite object (one made up of more than one type of substance) is lower than the density of the fluid surrounding it, buoyant forces pushing on the object from below will be strong enough to make the object float. To determine overall density, the density of each part must be factored into the equation. The overall density of a composite object equals the total mass (i.e., the combined weight) of the two or more substances divided by their total volume.

A submarine is an example of a composite object. Its overall density will vary depending on how much air it contains at any given time. As a result, a submarine can be made to float or sink by allowing water to enter its ballast chambers or by forcing water out, thus changing the amount of air in the submarine and its mass as well. Since its volume doesn't change, the density of the submarine changes, and so does its buoyancy.

On its own, a raisin dropped into carbonated water sinks. Because it has a higher density than water, the volume of water it displaces weighs less than the raisin itself. Gas (carbon dioxide)-filled bubbles, however, rise in the denser medium of the water. You can see this happening in a cup of club soda, or underwater, when you blow air out of your mouth. Rising gas bubbles, when attached to an object like a raisin, increase the overall volume of the composite object without changing its overall weight very much. This means that the overall density of the raisin-bubble tandem decreases, and when it becomes lower than that of water, the raisin rises to the surface. As the raisin rises, some of the bubbles may burst, and without them, the raisin again sinks to the bottom of the cup. A life jacket works in the same way to help a person who might otherwise sink in the ocean float. The overall density of the life jacket-person tandem is lower than the density of the ocean water.