Skip to story
ELI5 Yahaaa

Water pushes back

Why Do Things Float?

Push water out of the way and it pushes back — sometimes hard enough to hold you up.

A boatIts weightWater pushing back — and winning
Float or sink is a contest between two forces.

  1. Step 1 of 5

    Water pushes up on anything in it.

    Your arm in waterWater pushes upwardGravity pulls down

    Lower your arm into a bath and it feels lighter. Water is squeezed by everything above it, and that pressure pushes up on whatever is submerged.

  2. Step 2 of 5

    The push equals the weight of the water shoved aside.

    1 Object goes in2 Water is pushedaside3 Push up =weight of thatwater

    Put an object in and some water has to move. The upward push is exactly as strong as the weight of that displaced water. This is Archimedes' rule.

  3. Step 3 of 5

    Lighter than the water it moves? It floats.

    Weighs less than thewater it displaces FloatsWeighs more Sinks

    If shoving aside a bucket of water gives you a bucket's worth of upward push, and you weigh less than that bucket, you rise until things balance.

  4. Step 4 of 5

    Shape changes everything.

    A steel block Displaces little water— sinksA steel ship Displaces a huge amount— floats

    A lump of steel sinks. Hammer the same steel into a hull and it encloses a huge volume of air, so it shoves aside far more water — and floats.

  5. Step 5 of 5

    It is really about density.

    Cork0.24 — floatshighWater1.00 — the lineSteel7.8 — sinks hard

    Density is weight packed into a given space. Anything less dense than water floats; anything more dense sinks. A ship full of air averages out lighter than water.

    You float better in the sea than in a pool. Salt makes water denser, so it pushes back harder.

The short version

You float when the water you push out of the way weighs more than you do.

Try it yourself

Squash a ball of foil tight and drop it in water — it sinks. Shape the same foil into a boat and it floats.

Topics

Next tiny story