🧪 Pressure in Solids, Liquids and Gases

Why doesn’t someone wearing snowshoes sink into the snow? Why is the bottom of a dam so thick? The answer is pressure! Pressure shows up in solids, in liquids and in gases. Let’s find out!

We put the same brick on the sand, first on one face and then on another. The weight of the brick, which is the force, is the same. But the narrow face sinks a lot into the sand, and the wide face sinks a little. So the pressure of a solid depends on the force and on the surface area.

When we put a second brick on top, the force gets bigger and the brick sinks deeper into the sand. So the bigger the force, the bigger the pressure. The bigger the surface area, the smaller the pressure. The unit of pressure is the pascal.

Maya is wearing snowshoes, and Ben is walking in boots with thin soles. Maya’s weight is spread over a wide area, so she sinks only a little. Ben’s weight presses on a small area, so he sinks a lot. The tip of a nail is sharp and the edge of a knife is thin: small area, big pressure!

Now liquids. We made three holes in a bottle: at the top, in the middle and at the bottom. From the bottom hole, the water shoots out the strongest and the farthest. Liquid pressure gets bigger as the depth gets bigger.

Liquid pressure depends on density too. In both glasses, the pressure gauge is at the same depth. Salt water is denser than water. So the needle of the gauge swings further in the salt water.

In a dam lake, the pressure of the water gets bigger with depth too. That is why the wall of the dam is thick at the bottom and thinner at the top. The wall at the very bottom takes the biggest pressure.

Gases make pressure too. The gas particles inside a balloon never stop moving in every direction, and they hit the wall of the balloon. These hits push the balloon in every direction.

The air all around us has pressure too: air pressure, also called atmospheric pressure. When Maya drinks through a straw, she sucks the air out of the straw, so the pressure inside the straw drops. The air pressure on the surface of the juice pushes the juice up the straw.

A syringe works on the same idea. When you pull the plunger, the pressure inside drops. The air pressure outside pushes the liquid into the syringe.

Pressure put on a liquid in a closed container is passed on unchanged in every direction. This is called Pascal’s principle. If we push the narrow piston with a small force, a big force appears on the wide piston, and the car lifts up. A hydraulic jack works like this.

Summary: the pressure of a solid depends on the force and the surface area. Liquid pressure grows with depth and with density. Gases and air press in every direction too. Now try it in the Pressure Laboratory!