The trick submarines use
In the previous concepts article, we learned that ships float because their hollow shape gives them a low average density. But a submarine needs to do something ships never do: choose whether to float, sink, or stay at a specific depth.
How? By changing its own density in real time.
Submarines have ballast tanks — large compartments that can be filled with either water or air. Fill them with air, and the submarine is light and buoyant. Flood them with water, and the submarine gets heavier and sinks. Let in exactly the right amount, and it hovers motionless at any depth.
That is the entire trick. No magic. No complex engines fighting gravity. Just water in, water out.
Three states of buoyancy
Every object in a fluid is in one of three states. A submarine just happens to be one of the few things that can switch between all three at will.
Object is less dense than the fluid. Buoyant force exceeds weight. It rises.
FloatsObject's density matches the fluid. Buoyant force equals weight. It hovers.
HoversObject is denser than the fluid. Weight exceeds buoyant force. It sinks.
SinksThe submarine controls which state it is in by adjusting how much water is in its ballast tanks. More water means higher average density. More air means lower average density. It is density control, applied in real time, hundreds of meters below the surface.
The formula behind buoyancy
Archimedes figured this out over 2,000 years ago, and his principle still describes every submarine, ship, balloon, and floating object on Earth. The buoyant force on any submerged object equals:
If Fb is greater than the object's weight, it rises. If equal, it hovers. If less, it sinks. The submarine controls V (by adjusting how much of it is submerged) and effectively controls its own ρ (by changing how much water it carries). That is the whole game.
Plug in density, volume, and gravity to calculate buoyant force. Watch objects rise, sink, and hover in interactive fluid simulations. The best way to make Archimedes click.
Helium balloons use the same trick
A helium balloon is essentially an upside-down submarine. Instead of water, it is surrounded by air. Helium is less dense than air, so the balloon's average density is lower than the atmosphere around it. The buoyant force pushes it up — exactly the same principle, just in a different fluid.
When the balloon rises high enough, the surrounding air becomes thinner (less dense). Eventually the buoyant force drops to match the balloon's weight, and it stops rising. Neutral buoyancy — just like a submarine hovering at depth.
Sinking, floating, and hovering are not three different physics concepts. They are three outcomes of the same equation: Fb = ρVg. Every object in every fluid follows this rule. A submarine, a helium balloon, a rubber duck, and an aircraft carrier all obey the same ancient principle.
True or False?
One question. Based on what you just read.
Same water, same steel — total control
A submarine does not fight physics to go underwater. It uses physics. By adjusting the water in its ballast tanks, it shifts between positive, neutral, and negative buoyancy. No thrust needed to dive. No thrust needed to surface. Just density management.
This is what makes Archimedes' principle so powerful: one formula explains ships, submarines, balloons, and why ice floats in your drink. Once you see it, you cannot unsee it.
Want to play with it? The Physiworld Fluids lesson lets you calculate buoyant force, change densities, and watch objects respond in real time. It is the fastest way to make this concept truly click.
Submarines control their depth by adjusting their average density — filling ballast tanks with water to sink and with air to rise. The buoyant force on any submerged object is Fb = ρVg. When this force exceeds weight, the object rises. When it is less, the object sinks. When they match, the object hovers. Ships, submarines, and helium balloons all follow the same principle.
The Fluids section covers density, buoyancy, Archimedes' principle, fluid pressure, and fluid dynamics through interactive simulations and visual challenges.