Buoyancy and Bubbles
Date: Winter 2011-2012
How does the buoyancy of an object depend on the presence of bubbles?
On the whole, bubbles reduce the density of water and reduce buoyancy. In fact, in teaching sailing classes we were told to warn small boat sailors about getting too close to the back of large ships and their wakes. The air bubbles in the wake close to the stern of the ship could cause real buoyancy problems!
R. W. "Mr. A." Avakian
The buoyancy of an object depends upon the mass of the volume of the fluid
displaced. Since bubbles have a large volume per unit mass, they displace a
lot of the fluid. This makes the object appear a lot lighter if bubbles are
An object will float if it weighs less than the water it displaces.
So if I put an object in a bucket that is filled to its very top with water
Then collect and weight all of the water that spills over the side of the
Then if the object weighs less than the spilled water, the object will
If the object weighs the same as the spilled water it will neither sink nor
float, it will just hang there
And if the object weighs more than the spilled water, it will sink.
Bubbles in an object that is underwater displaces water but weighs nothing.
So in that way, bubbles effect the buoyancy of an object.
US Navy submarines have ballast tanks.
When they want to dive, they fill the ballast tanks with water until the
submarine weighs more than the water it displaces.
When they want to surface they blow compressed air into the ballast tanks
which expels the water from the ballast tanks and causes the submarine to
weigh less than the water it displaces and the submarine floats to the top
of the water.
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Update: June 2012