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Pressure is defined in terms of a force applied over an area. In many physics problems we consider idealised situations; typically a single, positive ("pushing") force acting on a surface. For example, an ideal gas is modelled as particles (eg. air molecules) that strike (and elastically bounce away from) the walls of its container. In many idealised contexts like this, pressure cannot be negative: the lowest pressure is zero (eg. in a vacuum, when no gas molecules are present to strike the container walls).