6.2 Pressure in Liquids
Solids press down on what they rest on. But do liquids push too? And if so, in which directions? A few simple activities give a clear answer.
Activity 6.1 — Equal heights, equal bulges Fit a balloon over the end of each of two pipes — one narrow, one broad — and fill both with water to the same height. Although the broad pipe holds far more water (more weight!), both balloons bulge by the same amount. So the bulge is not caused by the weight of water. What, then?
Pressure Depends on Height, Not Amount The bulge is caused by the pressure of the water column — and that pressure depends on the height of the column, not on how much water there is. Equal heights give equal pressure (equal bulges), even with different amounts of water. Add more water to raise the column, and the balloon bulges more: taller column → greater pressure at the bottom.
This is the answer to the overhead-tank puzzle.
Why Water Tanks Sit High Up An overhead water tank is placed high so the water column above your tap is tall, giving high pressure and a strong stream from the tap. The higher the tank above the tap, the stronger the flow. (So someone on the first floor gets a more powerful stream than someone on the second floor, because there’s a taller column of water above them.)
Liquids push sideways too
Activity 6.2 — Water from the sides Make holes in the side of a plastic bottle and fill it with water. Water spurts out sideways from every hole. So liquids exert pressure not only on the bottom of a container but on its sides as well — in fact, liquids exert pressure in all directions. (This is also why water fountains out of a leak in a pipe.)
Ever Heard Of… A Dam’s Broad Base The base of a dam is much broader than its top. Why? Water presses horizontally on the dam’s walls, and this sideways pressure is greatest near the bottom (where the water column above is tallest). A broad, heavy base is needed to withstand that huge low-level pressure — a beautiful example of liquid pressure shaping how we build.
Science Talk Two vessels hold water to the same height but have different shapes and base areas. The pressure at the bottom of each is the same (it depends only on height) — but is the force on the two bases the same? Talk it through using Pressure = Force ÷ Area: if the pressures are equal but the base areas differ, what must be true of the forces?