Why the Tap Slows Down

The story

The kitchen tap always gushed water until, one evening, it became a sad trickle just as Riya needed to fill her bottle. Kabir thought maybe air bubbles were to blame, or that their little brother Chintu had broken it again. Suddenly, the water slowed even more, leaving only a thin stream. Chintu denied touching anything, and Kabir wondered if the tap was clogged or if someone was using water upstairs. Their grandmother, Aaji, suggested checking the water tank on the terrace. They discovered the tank was nearly empty. With less water above, there was less push to force the water down to the tap. The next day, after the tank was filled, the tap flowed strongly again. This left everyone wondering whether the same tap would flow faster on the ground floor than three floors up.

The science

The strength of water flowing from a tap depends on hydrostatic pressure, which is created by the height of water above it. When a building's overhead tank is full, the pressure at the tap is high, so water gushes out quickly. As the water level drops, the height, and therefore the pressure, decreases, causing the flow to slow down. This effect is explained by Torricelli's theorem in physics, which shows that the speed of liquid flowing out depends on the height of the liquid above the exit point. The lower the height of water in the tank, the weaker the tap's flow.

Try it

Questions students ask