The Float Rule

Why it matters

Wax floats, iron sinks, and a 100,000-tonne iron SHIP floats anyway. One ratio — yours versus water's — makes the call every single time.

The idea in plain words

The rule and its famous cheat. Tap them.

ρ(body) < ρ(liquid) → float

Ice (0.92) floats in water but sinks in kerosene (0.8). Why?

Same ice, two verdicts — the LIQUID's density is half the equation

• the law: the rule — compare densities: cork 0.24 floats, iron 7.9 sinks — in water
• the ship trick: the cheat — shape iron into a hollow hull: (iron + air) ÷ big volume = average density < 1
• the twist: changing the liquid — iron SINKS in water but FLOATS on mercury (13.6) — destiny depends on both densities
• vs water: 0.92 < 1 → float
• vs kerosene: 0.92 > 0.8 → sink

Predict first

A solid floats in water when its density is…

Less dense than the liquid → floats. Denser → sinks. Equal → hangs anywhere. The ship cheats by being mostly hollow — its AVERAGE density is below 1.

• less than water's (R.D. < 1) — correct
• more than water's
• exactly equal only

What you do

Load the barge crate by crate and watch it ride lower — find the exact load where the float rule breaks.

Check yourself

An egg sinks in fresh water but floats in very salty water because salt…

Brine ≈ 1.1 g/cm³ beats the egg's ≈ 1.03 — the Dead Sea does this to swimmers.

Comparing the three states of the SAME substance, density usually runs…

Steam is ~1600× less dense than water. (Water/ice is the famous exception where the solid is LIGHTER than the liquid.)

An overloaded boat sinks when its average density…

Every crate raises mass without raising volume much — the ratio creeps past 1 and physics forecloses.

• raises the water's density above the egg's — correct
• pushes the egg up chemically
• makes the egg lighter
• solid ≈ liquid ≫ gas (the gas is thousands of times lighter) — correct
• gas is densest — it fills everything
• all three equal
• rises above the water's — correct
• falls below the water's
• stays the same but luck runs out