'Pressure' is a term often used in everyday life, for example, we frequently speak of * tire pressure, air pressure, blood pressure,
water pressure * and so on. The units used to describe pressure vary wildly, depending upon the application; we speak of pressure using units
such as inches of mercury, millimetres of mercury, bars and millibars, pounds per square inch (psi), atmospheres, and so on.

For scientific purposes, the ** pascal ** is the preferred^{*} unit of pressure.

Scientists define ** pressure ** as `force per unit area' In the SI system of metric units the standard unit of pressure is the
** pascal (Pa) ** , (given the area in square metres and the force in newtons.) The pressure ** P ** on any surface can be calculated by taking the applied force ** F ** and dividing it by the surface area ** A ** over which the force is applied.

Algebraically; | Dimensionally; |

One ** pascal ** (1Pa) is the ** pressure****(P)** exerted by a ** force ****(F)** of one ** newton ** (1N) on a ** surface area****(A)** of one **square metre**(1m^{2}).

Consider the following three cases,

Case I | Case II | Case III | |
---|---|---|---|

The WEIGHT (force) applied to the surface | 32 N | 32N | 32 N |

Sketch | |||

The AREA over which the force is distributed | 1m^{2} | 4m^{2} | 16m^{2} |

The calculated PRESSURE | 32Pa | 8Pa | 2Pa |

HINT: For objects that are on, or near, the Earth's surface, their masses (in kilograms) can be converted to weights (in newtons) by multiplying by 9.8 (or 10 for quick approximations).

* e.g. * A 12kg object pushes down with a force (weight) of about 120N.

* The

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