Sucking
Creating a partial vacuum is called sucking. Infants, and all baby mammals, are born with a sucking reflex, by which they instinctively know to suck for food (or more specifically, milk).More generally, drinking is partly done by sucking, and especially when drinking through a straw.
"Breathing in" involves the expansion of the lungs, with the result that air is sucked in.
It should be noted that while "sucking" seemingly causes attraction of the fluid, there is no physical basis for this: it just reduces the counter-pressure, so that that gets less than the pressure in the fluid. The difference becomes strikingly apparent when it is observed that there is an upper limit to the force that can be applied (unlike with normal pressure which is theoretically unlimited). This also means that "sucking" does not work when there is no pressure at the surface of the fluid being "sucked" (for example a liquid in a vacuum).
A straw or tube (at sea level) that is longer than about 10m (about 33 feet) will not fully raise water even if a complete vacuum is created at the top. Beyond this height liquids must be pumped with extra positive pressure from near the bottom of the tube. A vacuum has the minimum pressure possible, zero.
The fact that trees taller than 10 metre exist shows that water is not simply sucked up from the top. See also Transpirational pull.
Physicists are so disparaging of the term "suck" that they mockingly joke "there is no such thing as gravity: the earth sucks".
See also Implosion, Vacuum cleaner.
The word "suck" has a number of usages in slang:
- Sucking is a term for a form of oral sex.
- Something that "sucks" is bad, poorly made, unworthy, or uninteresting.