On a school trip to a dam, Shalini notices that the concrete wall is much thicker at the bottom…
On a school trip to a dam, Shalini notices that the concrete wall is much thicker at the bottom than at the top. She says it is built that way because water pushes harder the deeper you go. Tushar says water pushes equally at every depth, and the wide base is only there to keep the wall steady, like a pyramid. Who has it right?
Who do you think is right? Pick one.
The answer: Shalini is right, liquid pressure increases with depth
Why
Tushar is correct that a wide base helps a heavy wall stay steady — but he is wrong about the water. Water at any depth is pressed on by all the water above it. Near the surface there is very little water above, so the push on the wall is small; near the bottom there are many metres of water above, so the push is much greater. Liquid pressure increases with depth. A dam wall therefore has to hold back the biggest push at its base, and engineers make it thickest exactly there. Test: make three small holes, one above another, in the side of a tall plastic bottle, fill it with water and watch — the jet from the lowest hole shoots out the farthest, while the top one barely dribbles. Shalini is right: the wall is thickest at the bottom because water pressure grows with depth.
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