Suction is the day-to-day term for forces experienced by objects that are exposed to the movement of gases or liquids moving along a pressure gradient. Contrary to popular belief, however, the forces acting in this case do not originate from the lower pressure side (the vacuum), but from the side of the higher pressure.

When the pressure in one part of a physical system is reduced relative to another, the fluid or gas in the higher pressure region will exert a force relative to the region of lowered pressure, referred to as pressure-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 sucked but pushed.

Examples

Pressure reduction may be static, as in a piston and cylinder arrangement, or dynamic, as in the case of a vacuum cleaner when air flow results in a reduced pressure region.

When animals breathe, the diaphragm and muscles around the rib cage cause a change of volume in the lungs. The increased volume of the chest 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 into the mouth, the atmospheric pressure pushes the liquid through the straw along the pressure gradient.

A common semantic mistake is made when in case of accidents with spaceships or aircraft in which objects are blown out of the vessel in the case of an uncontrolled decompression which is often wrongly referred to as objects being sucked out.^[2][3]