Archimedes' principle describes the buoyancy that a body experiences when immersed into a liquid (more precisely into a fluid).
It explains the physical background behind buoyancy control in diving.
Basic knowledge for Open Water Diver* (OWD*)
When an object is immersed into water, a downward force acts on it causing the the descend of the object. This force is simply its mass.
Additionally, an upward or so called buoyant force acts on the immersed object causing its ascend. Archimedes states that this force is equal to the mass of the water displaced by the object. Hence, the buoyant force is determined by the volume of the object (and the density of the water)
Depending on the proportion of these two forces, the object descends, ascends or floats:
- If the weight of the object outweighs the weight of the displaced fluid, the object is negative buoyant and descends.
- If the object's weight is lower than the weight of the displaced fluid it is positive buoyant and ascends.
- If the weight of the object equals the weight of the displaced fluid, the object is neutrally buoyant and floats.
You can use the image at the top to illustrate Archimedes' principle: If you immerse a body with a weight of 5 kg and a volume of 2 l in fresh water, it displaces 2 kg of water. The weight of the displaced water acts as buoyancy force and it seems to weight only 3 kg. However, the weight of the object outweighs the weight of the displaced water and is therefore negative buoyant and will sink.
If we can adjust the volume of the same object to 5 liters and immerse it into fresh water the object will displace 5 liters of water weighting 5 kg. The weight of the displaced water now equals the weight of the object and it is therefore neutrally buoyant. It will hover midwater and neither sink or ascend. Assuming we can continue inflating our object to a volume of 7 liters while under water, the weight of the displaced water outweighs the weight of the object and it becomes positive buoyant and ascends to the surface.
The salinity of the water also has an impact on buoyancy. Salt water has a higher density than fresh water. This means that salt water is heavier than fresh water for the same volume. So a body experiences a higher buoyant force in salt water than in fresh water.
Knowledge for Experienced Diver** (ED**)
Difference between fresh and salt water
In addition to the volume of the displaced water, the buoyant force is also determined by its density (see section Physical derivation). Fresh water has a density of 1.0 kg/dm3, whereas the density of salt water is on average 1.035 kg/dm3. If the object from the above example is immersed in salt water, the displaced volume is still 2 l, but the weight of the displaced water is 2.07 kg. A body experiences about 3.5% more buoyant force in salt water than in fresh water, since the displaced volume of salt water is 3.5% heavier.
For the determination of the amount of lead, this means that you have to take more lead weights with you in the same equipment configuration in salt water. If the diver, including diving equipment, has a volume of 100 liters, for example, 3.5 kg more lead weights are required in salt water than in fresh water.