A suction cup, also sometimes known as a sucker is an object that uses negative fluid pressure of air or water to adhere to nonporous

Pore

- Animal biology and microbiology :* Sweat pore, an anatomical structure of the skin of humans used for secretion of sweat* Canal pore, an anatomical structure that is part of the lateral line sense system of some aquatic organisms...

 surfaces. They exist both as artificially created devices, and as anatomical traits of some animals such as octopi

Octopus

The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...

 and squid

Squid

Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...

.

The working face of the suction cup has a curved surface. When the center of the suction cup is pressed against a flat, non-porous surface, the volume of the space between the suction cup and the flat surface is reduced, which causes the fluid between the cup and the surface to be expelled past the rim of the circular cup. When the user ceases to apply physical pressure to the centre of the outside of the cup, the elastic substance of which the cup is made tends to resume its original, curved shape. Because all of the fluid has already been forced out of the inside of the cup, the cavity which tends to develop between the cup and the flat surface has little to no air or water in it, and therefore lacks pressure. The pressure difference between the atmosphere on the outside of the cup, and the low-pressure cavity on the inside of the cup, is what keeps the cup adhered to the surface.

The length of time for which the suction effect can be maintained depends mainly on how long it takes for fluid to leak back into the cavity between the cup and the surface, equalizing the pressure with the surrounding atmosphere. This depends on the porosity and flatness of the surface and the rim of the cup.

Artificial suction cups are believed to have first been used in the 3rd century, B.C.

Anno Domini

and Before Christ are designations used to label or number years used with the Julian and Gregorian calendars....

, and were made out of gourd

Gourd

A gourd is a plant of the family Cucurbitaceae. Gourd is occasionally used to describe crops like cucumbers, squash, luffas, and melons. The term 'gourd' however, can more specifically, refer to the plants of the two Cucurbitaceae genera Lagenaria and Cucurbita or also to their hollow dried out shell...

s. They were used to suction "bad blood" from internal organs to the surface. Hippocrates

Hippocrates

Hippocrates of Cos or Hippokrates of Kos was an ancient Greek physician of the Age of Pericles , and is considered one of the most outstanding figures in the history of medicine...

 is believed to have invented this procedure.

The modern suction cup was patented in 1882, and is based on the biological suckers on octopus

Octopus

The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...

 arms. The patent has since expired.

To calculate the force of a suction cup, use the formula

where F = force, A = area, P = pressure.

This is derived from the definition of pressure, which is
For example, a suction cup of radius 2.0 cm has an area of (0.020 m)² = 0.0013 square meters. Using the force formula (F = AP), the result is
F = (0.0013 m²)(100,000 Pa) = about 130 newtons, assuming the pressure inside the suction cup is negligible when compared to atmospheric pressure

Atmospheric pressure

Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

 (about 101,000 Pa).

Uses

Suction cups have a number of commercial and industrial applications. In office and household settings, they are commonly used to affix objects (ranging from signs to mugs) to nonporous vertical surfaces such as refrigerator doors and tiled walls. Large suction cups are used to temporarily hold and move large panes of glass and are often utilized when fitting automobile windscreens, raised floor

Raised floor

A raised floor or access floor are types of floor that provide an elevated structural floor above a solid substrate to create a hidden void for the passage of mechanical and electrical services...

 tiles, etc.

Suction cups are also found on Nerf

Nerf

Nerf is a toy brand created by Parker Brothers and currently owned by Hasbro. The acronym NERF stands for "Non-Expanding Recreational Foam". Most of the toys are a variety of foam-based weaponry, but there are also several different types of Nerf toys, such as balls for sports like football,...

 darts and toilet plunger

Plunger

A plunger is a common device that is used to release stoppages in plumbing. The tool consists of a rubber cup with an attached stick "shaft", usually made of wood or bronze. Before use, any objects such as hair in the plug grate should be removed and, if possible the overflow hole should be...

s.http://www.suctioncupmuseum.com/html/history.html

Specialized suction cups have also been utilized by urban climbers to scale buildings with smooth exterior surfaces.