Suction cup
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 surfaces. They exist both as artificially created devices, and as anatomical traits of some animals such as octopi and squid.
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., and were made out of gourds. They were used to suction "bad blood" from internal organs to the surface. Hippocrates is believed to have invented this procedure.
The modern suction cup was patented in 1882, and is based on the biological suckers on octopus 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 (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 tiles, etc.
Suction cups are also found on Nerf darts and toilet plungers.[1]
Specialized suction cups have also been utilized by urban climbers to scale buildings with smooth exterior surfaces.[1]