Floatation at Work

Why it matters

A submarine is the only ship whose captain chooses its density. Flood the tanks: sink. Blow them empty: rise. Match the sea exactly: hang there, silent.

The idea in plain words

Floatation's greatest hits. Tap them.

tune ρ(average) against ρ(fluid)

Why does a hydrometer sink DEEPER in kerosene than in water?

Deeper float = lighter liquid — the scale reads density upside-down

• application: submarine — ballast tanks flood (sink) or fill with air (float) — density on demand
• application: iceberg — ρ(ice)/ρ(sea) ≈ 0.9 → nine-tenths hides underwater; ships fear the invisible part
• application: hydrometer — floats deeper in lighter liquids — reads density directly; lactometer (milk), battery tester (acid)
• application: balloon — hot air or hydrogen is less dense than cold air — floatation in the ocean of atmosphere
• Kerosene is less dense
• More volume must submerge to displace the hydrometer's fixed weight

Predict first

For a submarine to HOVER at constant depth, its total weight must be…

Heavier → sinks; lighter → rises; equal → neutral buoyancy. Ballast tanks tune the weight against the fixed upthrust.

• exactly equal to the upthrust on its hull — correct
• slightly more than the upthrust
• zero

What you do

Trim the ballast: dive, then hover dead-still, then blow the tanks and surface.

Check yourself

A fish rises or sinks by adjusting its…

Nature's ballast tank — inflate to rise, compress to sink. Submarines copied fish.

Ships sailing from river to sea…

Same weight, denser liquid, less draft — the Plimsoll line marks the legal limits for each water.

A lactometer detects watered-down milk because added water…

Pure milk has a fixed density band; dilution drops it below — the float betrays the fraud. Archimedes still catching cheats.

• swim bladder volume (its average density) — correct
• fin speed only
• weight — it eats stones
• rise a little — sea water is denser — correct
• sink a little
• are unaffected
• lowers the milk's density, letting the lactometer sink deeper — correct
• raises the density
• changes the colour