In: Physics
When an object is placed in a fluid a force, called the buoyant force, acts to push the object upwards. The strength of the force is equal to the weight of the volume of fluid displaced by the object.
It is
the objects volume, not its weight that determines the buoyant
force.
Take a submarine, for example, the buoyant force acts to push it to
the surface while the weight of the submarine acts to push it to
the bottom. The strength of these two opposing forces determines if
the sub rises or sinks in the water.
A submarine however, can only control its weight, since the buoyant force can only be adjusted by a change in volume. This is unlikely to happen since the hull is inflexible and designed to withstand high water pressure that would crush it at great depths
A submarine sinks if it takes in water to increase its weight
(force acting downward). It will rise if it expells water from its
tanks, therefore making itself lighter.
As it rises, the buoyant force is greater than the force acting to
push it down (weight).
As it sinks the force pushing it down (weight) is greater than the
buoyant force pushing it up.