Sinks water

Pressure and buoyancy

Buoyancy force and weight force

If a body is in a medium, regardless of whether it is liquid or gas, the body experiences a buoyancy force \ (F _ {\ rm {A}} \). At the same time, of course, the body also experiences a weight \ (F _ {\ rm {G}} \). The interaction of these two forces determines whether a body sinks, floats, rises or swims in the medium.

Four different cases

Disregarding frictional forces, you can differentiate between four different situations, which differ due to the interplay of weight \ ({\ vec F _ {\ rm {G}}} \) of a body and the buoyancy \ ({\ vec F _ {\ rm {A}}} \) in a medium yield:

Sink: If the amount \ ({F _ {\ rm {A}}} \) of the buoyancy force is smaller than the amount \ ({F _ {\ rm {G}}} \) of the weight force, so \ ({F _ {\ rm {A}}} <{F _ {\ rm {G}}} \), then the resulting force is directed downwards and the body sinks.

Hover: If the amounts \ ({F _ {\ rm {A}}} \) of the buoyancy force and \ ({F _ {\ rm {G}}} \) of the weight force are the same, then \ ({F _ {\ rm {A }}} = {F _ {\ rm {G}}} \), then the resulting force is zero and the body floats.

Climb: If the amount \ ({F _ {\ rm {A}}} \) of the buoyancy force is greater than the amount \ ({F _ {\ rm {G}}} \) of the weight force, so \ ({F _ {\ rm {A}}}> {F _ {\ rm {G}}} \), then the resulting force is directed upwards and the body rises.

swim: The case of swimming occurs when, with the body completely submerged, \ ({F _ {\ rm {A}}}> {F _ {\ rm {G}}} \), so that the body rises. If the medium, usually a liquid, has an upper limit, the body emerges. The body no longer displaces as much liquid as when completely immersed, which leads to a decrease in buoyancy. The body emerges until the amount of buoyancy force and the amount of weight force are equal, i.e. \ ({F _ {\ rm {A}}} = {F _ {\ rm {G}}} \) applies (see Fig. 1).

Note: You should definitely distinguish between the amounts of weight \ ({F _ {\ rm {G}}} \) of the body and the weight \ ({F _ {\ rm {G, medium}}} \) of the displaced medium.

The following animation illustrates these four situations.