Pressure ©

'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(1m2).


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 1m2 4m2 16m2
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 pascal is preferred because it integrates easily with the standard SI units of mass, time, distance, acceleration, force, energy, and power; also, students will have a much easier time understanding these basic concepts if they conform to the simple, self-consistent, SI set of units and definitions.
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