What is pressure? What causes pressure?

When we talk about pressure in a scientific context, we’re really discussing the relationship between force and the area over which that force is spread. Imagine pushing down on a thumbtack with your thumb; the force of your thumb is concentrated on a very small area, allowing the tack to easily pierce through paper. In contrast, if you press a book with the same force but over its entire cover, it doesn't pierce the paper because the force is spread over a larger area.

Mathematically, this relationship is described by the formula
\( P = \frac{F}{A} \)
where
\( P \) stands for pressure, \( F \) denotes the force applied, and \( A \) represents the area. To think of it simply: the smaller the area over which a given force is applied, the greater the pressure.

When dealing with fluids—liquids and gases—pressure becomes a bit more complex but follows the same basic principles. Fluid pressure is felt from all directions rather than simply 'downward' as with solid objects. It is the result of the weight of the fluid above pressing down. Whether it's the pressure experienced by a submarine deep under the sea or water pressure in a garden hose, it is defined by the depth and density of the fluid, as well as gravity.

Deep within a fluid, molecules are being pushed upon from the fluid above, creating pressure that is exerted equally at all points in the fluid. This is often realized as hydrostatic pressure in liquids and is calculated using the formula: \( P = \rho g h \)
where \( \rho \) represents the fluid density, \( g \) is the acceleration due to gravity, and \( h \) denotes the depth of the fluid.

Understanding what leads to pressure helps to grasp the concept holistically. Several situations cause pressure: when you sit down, your body exerts force on the chair due to gravity, and the chair's surface area upon which that force is spread determines the pressure. Similarly, in fluids, pressure is caused by the weight of the fluid exerting a force over the area of the object within the fluid. Another cause of pressure can be temperature; in a closed container, heated gas molecules move faster and collide with the walls more vigorously, increasing the force over the container's surface area, and thus, the pressure.

Furthermore, atmospheric pressure, a type of fluid pressure, is caused by the air molecules being pulled towards the Earth by gravity, exerting force on any surface they come into contact with. All of these examples reflect the core force and area relationship fundamental to understanding pressure.