Buoyancy

Georgia Perimeter College

Objectives

  1. Be able to define buoyancy.
  2. Describe the factors which affect the buoyancy of an object.
  3. Describe how an object's surface area affects its buoyancy.
  4. Explain the relationship between density and buoyancy.


Density is defined as the mass of a substance per unit of volume.
It is usually measured in grams per cubic centimeter (g/cm3).

The greater the mass within a given volume, the greater the density.

(In other words, if you have two objects of the same size (volume), the heavier object will have the greater density.)

The density of water is affected by its temperature, salinity, and pressure.

As the temperature of water decreases, its density increases until it reaches its maximum density at 4oC (for fresh water).
Below 4oC, the density of water DECREASES and the water begins to EXPAND. This has to do with the molecular structure of water. Water begins to crystallize and ice crystals begin to form as water approaches 0oC

The temperature of maximum density of water (or the point at which it begins to crystallize or freeze) is lowered by adding salt. (Which is why we use salt on icy roads and sidewalks. It keeps the water liquid because it must be much colder in order for salty water to freeze.)

The density of ocean water increases with increasing salinity. (Sea water is denser then fresh water.)


Buoyancy is the tendency of an object to float in a fluid.

Buoyancy is controlled by differences in density between the object and the fluid.

For example, if you have a block of ice with a density of 0.91 g/cm3, and float it in water with a density of 1.0 g/cm3, the ice will sink into the water until 91% of its mass is submerged.

Since salt water is more dense than fresh water, an object will float "higher" in salt water (less of its mass will be submerged.) This is particularly noticable when floating in hypersaline waters such as the Great Salt Lake or the Dead Sea. You can't sink!!


Air is a fluid, and also has buoyancy effects and can rise and sink depending on density differences.

Air moves vertically for one of two reasons:

  1. It is forced to rise mechanically (when it encounters a mountain, for instance), or
  2. Because of changes in buoyancy.

If an air mass is heated without changing the number of gas molecules present, the air mass will expand.  When the air mass expands, it occupies more volume but has the same number of air molecules as before.  This means that the air density is less than before.

This air mass becomes positively buoyant and will rise because it is less dense than the air surrounding it.

If the air mass is more dense than the surrounding air, the mass would sink because it would have negative buoyancy.  Cooling a parcel of air causes it to be more dense and therefore less buoyant.


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Notes written by E. Lynn Zeigler and Pamela Gore
Web page created by Cory Benson and edited by Pamela J.W. Gore
Georgia Perimeter College,

Clarkston, GA

June 16, 2000