Float or Sink?
The principle of floatation — why iron ships don't drown
What this lesson covers
Why it matters
An iron nail sinks instantly. A 100,000-tonne iron ship floats — and delivers your packages. Same iron. The difference is a shape that displaces an OCEAN's worth of water.
The idea in plain words
The floating contract. Tap the clauses.
weight of body = weight of displaced liquid
A 4000 kg barge floats. What weight of water does it displace?
exactly 4000 kg of water — the contract, in kilograms
- why ships float: the hull trick — shaping iron into a hollow bowl lowers its AVERAGE density below water's
- the visible bargain: draft — how deep the ship rides — grows with cargo until the displaced water matches the new weight
- the formula: fraction rule — floating fraction submerged = ρ(body)/ρ(liquid)
- floating: displaced weight = boat weight
Predict first
A floating body's weight compared to the water it displaces is…
Floating = equilibrium: weight (down) = upthrust (up) = weight of displaced water. The ship sinks exactly until the bargain balances.
- exactly EQUAL — that's the condition of floating — correct
- less than the displaced water
- greater — it presses down to stay in
What you do
Load the barge crate by crate. Watch the draft grow to keep the bargain — then find the crate that breaks it.
Check yourself
The same ship rides HIGHER in sea water than river water because…
Denser liquid honours the contract with less depth — hence ships' Plimsoll lines for sea vs fresh water.
A floating log's apparent weight is…
Floating IS being fully supported. A spring balance attached would read nothing.
Overload a boat and it sinks because…
Once under, displacement maxes out at the hull's full volume — if weight still exceeds it, down is the only direction.
- salt water is denser — less volume needed for the same displaced weight — correct
- waves hold it up
- salt makes the ship lighter
- zero — upthrust carries all of it — correct
- half its true weight
- equal to its true weight
- even fully submerged, the displaced water weighs less than boat + cargo — correct
- cargo blocks the buoyancy
- water enters and dissolves it