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Suction is the day-to-day term for the movement ofgases orliquids along apressure gradient with the implication that the movement occurs because the lower pressure pulls the gas or liquid. However, the forces acting in this case do not originate from just the lower pressure side, but also from the side of the higher pressure, as a reaction to the pressure difference.
When the pressure in one part of aphysical system is reduced relative to another, thefluid or gas in the higher pressure region will exert aforce relative to the region of lowered pressure, referred to aspressure-gradient force. If all gas or fluid is removed the result is a perfect vacuum in which the pressure is zero. Hence, no negative pressure forces can be generated. Accordingly, from a physics point of view, the objects are not pulled but pushed.
Pressure reduction may bestatic, as in apiston and cylinder arrangement, ordynamic, as in the case of avacuum cleaner when air flow results in a reduced pressure region.
When animals breathe, thediaphragm and muscles around the rib cage cause a change of volume in the lungs. The increased volume of thechest cavity decreases the pressure inside, creating an imbalance with the ambient air pressure, resulting in suction. Similarly, when a straw is used to suck a liquid from a glass into the mouth, the atmospheric pressure on the fluid in the glass pushes the liquid up through the straw along the pressure gradient.
A common semantic mistake in aviation accident reporting is describing people or objects as being 'sucked out' duringrapid decompression events, when physically they are 'blown out' by the higher internal cabin pressure rushing toward the lower ambient pressure outside the plane — the opposite phenomenon to what happens when an object is placed too close to a running jet engine creating the risk of being sucked in.[2][3][4]
