To suck water up a straw to a maximum height of 68 mm, what minimum gauge pressure must be produced in the lungs? Note that in order to be able to suck up water, the pressure in the lungs must be lower than the pressure outside by the exact amount of pressure created by the weight of the column of water in the straw.

Answer 1

68 mm = 0.068 m

P= pgd (where p=density)
P= (1000 kg/m^3)g(0.068 m)
P= 666 Pa

and it's negative, I don't know why.

Is this for UTSC? I have the same question, exact wording :D

Answer 2

It is negative because gauge pressures are measured relative to atmospheric pressure. It has to be negative to suck water up the straw.

See
http://www.jiskha.com/display.cgi?id=1259197828
for a similar response.