Page created by Heather C. Hopkins
Last updated 11/25/2009
Why does a person sink when they stop swimming?
The act of swimming underwater creates lift, similar to that of an airplane's wing under flight. When a person stops swimming, they tend to sink because they lose the extra "lift," or buoyancy.
Photo of a seahorse, courtesy of
Buoyancy is an important aspect of diving. According to the Archimedes Principle, an object in a fluid is lifted upwards by a force that is equal to the weight of the fluid displaced by the object. In other words, an object is able to float depending on both the volume that it displaces, and the object's weight. Therefore, heavy ships are able to float because they displace an extremely large amount of water. Buoyancy is also the reason that objects seem to be lighter and easier to move in the water than on land.
When divers float, they are said to be positively buoyant, and when they sink, they are negatively buoyant. Buoyancy is important to divers because without it, they would sink and have a greater risk of destroying coral and fragile ocean life. In addition, when divers have too much buoyancy, they are forced to work harder to stay where they want to be underwater. It may also be dangerous if divers ascend too rapidly.
Divers maintain buoyancy with several pieces of equipment, including weight belts for negative buoyancy, and a vest called a Buoyancy Control Device (BCD) for positive buoyancy. The BCD can be inflated or deflated to allow for more or less buoyancy as the diver desires.
Therefore, divers seek the goal of being able to control their buoyancy, and their position in the water, in order to have an enjoyable and safe experience underwater.