My answer to What is water pressure?

Answer by Arûn Râinâ:

**Pressure** (symbol: *p* or *P*) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled *gage* pressure)is the pressure relative to the ambient pressure.

Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre; similarly, the pound-force per square inch (psi) is the traditional unit of pressure in the imperial and US customary systems. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height ofcolumn of a particular fluid in a manometer.

Pressure is the amount of force acting per unit area. The symbol for pressure is *p* or *P*.The IUPAC recommendation for pressure is a lower-case *p*.

However, upper-case *P* is widely used. The usage of *P* vs *p* depends on the field in which one is working, on the nearby presence of other symbols for quantities such as power and momentum, and on writing style.

**Mathematically:**

where:

P is the pressure,

F is the normal force,

A is the area of the surface on contact.

Pressure is a scalar quantity. It relates the vector surface element (a vector normal to the surface) with the normal force acting on it. The pressure is the scalar proportionality constant that relates the two normal vectors:

**Liquid pressure**

When a person swims under the water, water pressure is felt acting on the person's eardrums. The deeper that person swims, the greater the pressure. The pressure felt is due to the weight of the water above the person. As someone swims deeper, there is more water above the person and therefore greater pressure. The pressure a liquid exerts depends on its depth.

Liquid pressure also depends on the density of the liquid. If someone was submerged in a liquid more dense than water, the pressure would be correspondingly greater. The pressure due to a liquid in liquid columns of constant density or at a depth within a substance is represented by the following formula:

**where:**

*p*** is liquid pressure**

*g*** is gravity at the surface of overlaying material**

*ρ*** is ****density**** of liquid**

*h*** is height of liquid column or depth within a substance**

**Another way of saying this ***same*** formula is the following:**

What is water pressure?

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