# Pressure

## In a nutshell

Pressure is defined to be the force per unit area. This means that it depends on the size of the force being applied and the area of the surface that the force is being applied to.

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**Equations**

#### WORD EQUATION | #### SYMBOL EQUATION |

$pressure = \frac{force}{area}$ | $p = \frac{F}{A}$ |

**Variable definitions**

#### QUANTITY NAME | #### SYMBOL | #### UNIT NAME | #### UNIT |

$pressure$ | $p$ | $pascal$ | $Pa$ |

$force$ | $F$ | $newton$ | $N$ |

$area$ | $A$ | $metre \space squared$ | $m^{2}$ |

## Pressure calculations

A fluid is either a gas or a liquid. Pressure acts in a fluid in all directions. You can calculate the pressure on the surface of a fluid if you know the force being exerted onto the fluid and the area over which the force is being exerted.

$pressure = \frac{force}{area}$

$p = \frac{F}{A}$

Pressure is measured in pascals, $Pa$. The pascal can also be written as $N/m^{2}$ as forces are measured in newtons and area in metres squared.

## Atmospheric pressure

The atmosphere is the thin layer of air that surrounds the Earth, it contains a mixture of gases and the particles of these gases collide with objects as they travel through the atmosphere.

The atmospheric pressure decreases as the height above the surface of the Earth increases, this is because the weight of the air above you is lower.

## Pressure in a liquid

For an object submerged in a liquid, the pressure on the object depends on the depth of the object in the liquid. The pressure in a liquid increases as the depth increases, this is because the weight of the liquid above is greater.

## Upthrust

When an object floats in a fluid, there is an upward force acting on it called the upthrust force and this force allows objects to float.

##### Example

*If a box is floating on water, it has its weight acting on it and an upthrust force also acting on it. The upthrust keeps the box from sinking by acting in the opposite direction to gravity. *