Not just liquids but gases too!

Differences in air pressure give resultant forces

Have you seen the demonstration where the air is removed from an aluminium can? The open can sits on the bench totally at rest, but as soon as the air is removed from inside it (with some kind of air pump) the sides of the can cave in spectacularly.

At first the open can is surrounded by air inside and out. The pressure in the air on both the inside and the outside of the can is the same. The air pressure on the outside exerts a force on the outside surface of the can, acting inwards (just as the water of the pool does on your ears). The air pressure on the inside exerts a force on the inside surface of the can, acting outwards. As the two pressures are the same, the two forces on each small section of can are equal also. Each section of the can is in equilibrium as these two forces are equal and opposite.

Although the forces acting on the sides of the can are large the resultant force is zero, so nothing happens. However, when air is removed from inside the can, the can crumples because the large force due to the outside air pressure is still there, acting inwards on the outside of the can. This is a spectacular demonstration of the fact that gases, including the air which is all around us, have a pressure just like liquids.