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Workshop/3 min exploration

Why water pushes harder at the bottom of a dam

Why is a dam wall usually much thicker at its base?

Water presses in every direction. In a still, uniform column, pressure depends on how much water is above a point: the deeper the point, the larger the pressure. The arrows here are spaced samples of that simple relationship.

Interactive model

Explore the model and follow its moving parts.

Read the full explanation ↓
Select a part. Change a control. Follow the motion.

Gauge pressure is p = ρgh. Doubling water depth doubles pressure at the bottom, while the total horizontal force on a vertical face grows with depth squared because the pressured area also grows.

If the water depth doubles, what happens to pressure at the bottom?

Follow the explanation

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How the parts work together

  1. Read one depth

    At the surface, gauge pressure is zero. Move down and p = ρgh grows in direct proportion to depth.

  2. Compare the arrows

    The arrow at the base is longest because it represents the greatest local pressure in this column.

  3. Add the push

    Summing the triangular pressure profile gives F per metre width = ½ρgH² for a vertical rectangular face.

The main parts

Water column
Its height sets the pressure at each depth.
Dam wall
The wall receives the depth-varying horizontal pressure.
Pressure arrows
Longer arrows near the bottom show larger local pressure.

One discovery leads to another.

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