Water pushes back
Why Do Things Float?
Push water out of the way and it pushes back — sometimes hard enough to hold you up.
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Step 1 of 5
Water pushes up on anything in it.
Pressure grows with depth, so the push from below wins 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.
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Step 2 of 5
The push equals the weight of the water shoved aside.
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.
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Step 3 of 5
Lighter than the water it moves? It floats.
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.
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Step 4 of 5
Shape changes everything.
The same steel, arranged to enclose air instead of fill space 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.
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Step 5 of 5
It is really about density.
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. Ice is about a tenth less dense, which is exactly the fraction of an iceberg that shows above the surface.
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 shove aside weighs more than you do — that is Archimedes' rule. It is why a steel hull floats where a steel lump sinks, and why a tenth of an iceberg shows above water.
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.