Physics in diving (1): Buoyancy

Why does a piece of wood float and does a rock sink? Why does a steel rod sink but will a steel hull boat float? Why do I go down when I deflate my BCD and do I go up when I inflate it?


It's all about buoyancy.........

Archimedes

But what is buoyancy exactly? Legend says that when Archimedes took a bath he discovered the buoyancy principle. He then jumped out of the bathtub and ran into the street shouting "Eureka!!!". Afterwards it was stated as: "Any object, fully or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object".

That, of course, is a very nice statement, but how does it work?

When a stone is submerged in a fluid, the buoyacy force (Fb) on the object can be calculated by multiplying the volume of the object by the density of the fluid and the gravitational force is the weight of the object:

Fb = Vo*ρf and Fg = Mo

But you know the volume and the mass of the stone, so you can also calculate the density of the stone by dividing the mass of the stone by it's volume. When comparing the density of the object and that of the fluid, there are three different states of buoyancy:

1. Positive buoyancy; The density of the fluid is higher than that of the object. This will result in the object ascending and finally surfacing.
2. Negative buoyancy; The density of the object is higher than that of the fluid. This will result in the object sinking to the bottom.
3. Neutral buoyancy; Here the density of the object and fluid are equal. The object will hover. This is the buoyancy state of a diver tries to maintain.

Diving and buoyancy

So there are three things influencing the buoyancy of an object, or a diver. These are the density of the water you dive in and your weight and volume, including your dive gear. 


Salt water has a higher density than sweet water, so a diver has a higher buoyancy in salt water than in sweet water. So in general, in salt water you need to add more weight than in sweet water.


If you have a positive buoyancy you can ad some weight to put on some extra pieces to your belt or in your BCD. If you alreaydy have a negative buoyancy you can increase your volume by wearing a thicker suit. Never just inflate your BCD to add volume, you'll need that when you underwater!

You might have noticed that when you get deeper underwater, your buancy tends to become negative; you feel like you are dragged down. This will be discussed in my next entry on pressure and (air) compression


Related subjects:
Physics in diving (2): Underwater pressure and gas compression