Skip to content
Grades 9 and 10 Intermediate About 15 min Version 1 Awaiting review Premium

Liquid pressure and depth

A liquid presses on everything in it and on the walls of its container. The deeper you go, the more liquid there is above, so the pressure is greater: p = ρ × g × h, where ρ (rho) is the density of the liquid, g is about 9.8 N/kg and h is the depth. At the same depth the pressure is the same in every direction. A denser liquid gives a bigger pressure at the same depth. That is why dams are built thicker at the bottom.

What you will learn

  • Show that liquid pressure increases with depth.
  • Calculate liquid pressure using p = ρgh.
  • Explain why dams are thicker at the base.

Before you start

  • Know that pressure = force ÷ area.

Materials

  • Water and cooking oil

Equipment

  • Tall plastic bottle with holes at 10 cm, 20 cm, 30 cm and 40 cm, each plugged with tape
  • Stand 30 cm high and a tray to catch the water

Procedure

  1. Open the hole 10 cm below the surface and record the pressure and where the jet lands.
  2. Open the holes at 20 cm, 30 cm and 40 cm, one at a time, and record each.
  3. Fill the container with oil and repeat for one depth.

How this simulation is simplified

  • g is 9.8 N/kg and the container is kept topped up, so the depth does not change.
  • Air pressure acts on both the surface and the jet, so only the liquid's own pressure is shown.
  • The holes are 80 cm above the bench at the surface level (container 50 cm tall on a 30 cm stand).