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Pressure from below

Why Does Water Push Up?

Water pressure is stronger deeper down, so the push under an object beats the push on top.

An object in waterStronger pressure underneathWeaker pressure on top
The difference is the upward push.

  1. Step 1 of 5

    Water gets heavier the deeper you go.

    Surface — 1 atmosphereno water above10 m down — 2atmospherespressuredoubles30 m down — 4atmospheresyour earsreally feel it

    Every layer of water is squashed by all the water above it. Ten metres down, the pressure is already double what it is at the surface.

  2. Step 2 of 5

    Pressure pushes in from every direction.

    A point in water Pressed from everyside

    Water does not just press down. At any point it pushes equally in all directions — sideways, upward and down.

  3. Step 3 of 5

    So the bottom of an object is pushed harder.

    On top — shallower Weaker push downUnderneath — deeper Stronger push up

    The bottom face sits deeper than the top face, so the upward push there is stronger than the downward push above. The sideways pushes cancel out.

  4. Step 4 of 5

    That difference is buoyancy.

    1 Objectdisplaces water2 Net upward push3 Equals thedisplacedwater's weight

    The leftover upward force turns out to exactly equal the weight of the water the object displaced — which is Archimedes' rule, explained.

  5. Step 5 of 5

    Big and light wins.

    Small and heavy Little push, lots ofweight — sinksBig and light Huge push, littleweight — floats

    The push depends on volume, not weight. A big hollow shape displaces lots of water and gets a huge push while weighing almost nothing.

    This is how a submarine works: flood the tanks to weigh more, blow them empty to weigh less.

The short version

Water pushes up because it presses harder at the bottom of an object than the top.

Try it yourself

Push an empty bottle down in the bath. The deeper you push, the harder it fights back.

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