Water Pressure Depends Only on Depth, Not Container Shape





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Published on Jun 24, 2017

The pressure of water at rest in a container depends only on the depth, not the shape of the container. For each additional 10 cm of depth, the pressure increases by 1 kPa (kiloPascal). This means that the pressure is a constant at a given depth, irrespective of the size and shape of the container.

This basic principle of hydrostatics might at first seem to be counter-intuitive. It certainly does not apply to solid objects, like blocks of wood. The difference is that a liquid has no shear strength -- water cannot be bent or twisted. Instead, it transmits forces uniformly in all directions.

Water poured into any container, of any shape, quickly flows to the bottom and arranges itself with uniformly decreasing pressure from bottom to top. If you draw a "topo map" of pressure (viewed from the side of the container), the lines of constant pressure are always uniformly spaced horizontal lines, for any container shape.

The reason is that water exerts a force perpendicular to the container wall, exactly the same as it would against an equivalent shape and volume of water. Thus, any container wall, of any shape or direction, behaves just like an equivalent amount of water. The water "doesn't care" whether it's meeting a wall or more water. It's all the same to the water. So as far as water pressure is concerned, a weird and twisting container is just the same as a rectangular container with parallel vertical sides.

The force of water on the bottom of a container can exceed the weight of all the water in the container. To understand how this is possible, watch the last third of the video presentation, which uses an upside-down T-shaped container as an example, or see the separate video at https://youtu.be/JkhufvEHGLc

More science videos: Electric Circuits and the Hydraulic Analogy


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