Pressure is the force per unit area that is applied on the surface of an object. Pressure is defined as the ratio of the force(F) applied perpendicular to the surface to the area(A) over wich the force is distributed : P= F/A.

A force is an influence that can change the motion of an object and can cause an object with mass to change its velocity A fore has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newton (N).

Atmospheric pressure is the pressure within the atmosphere of Earth. It is cause by the gravitaionnal attraction of the planet on the atmospheric gases above the surface. A column of air with a cross-sectionnal area of 1in² has a weight of about 14.7 lbs, resulting in a pressure of 14.7 psi.