Liquids and gases are fluids. A fluid is able to change shape and flow from place to place. Fluids exert pressure on surfaces, and this pressure acts at 90° to those surfaces – we say that it acts normal to the surface.
The Magdeburg hemispheres are two metal cups that fit together. If most of the air is removed from inside them using a vacuum pump, it is almost impossible to pull them apart again. The pressure of the atmosphere acting on their outside surface pushes them tightly together. Once the air is let back in, the pressure inside equals the pressure outside again, and the cups can easily be separated.
The effects of pressure can be seen in the collapsing can experiment. Here some water is boiled in an empty drinks can and steam fills the can. If the can is turned upside down in a trough of cold water, the steam condenses and the air pressure inside goes down. The pressure of the air outside the can suddenly crushes the can.
Atmospheric pressure changes with altitude. The higher you go:
For example, atmospheric pressure at sea level is about 100,000 Pa, but it is only about 21,000 Pa at the cruising height of an airliner.
Just like the atmosphere, liquids exert pressure on objects. The pressure in liquids changes with depth. The deeper you go:
Liquid pressure is exerted on the surface of an object in a liquid. This pressure causes upthrust. An object placed in a liquid will begin to sink. As it sinks, the liquid pressure on it increases and so the upthrust increases. For a floating object, the upthrust is equal and opposite to the object’s weight. An object will continue to sink if its weight is greater than the maximum upthrust.