Posted by Lily on Wednesday, November 25, 2009 at 8:10pm.
To suck water up a straw to a maximum height of 187 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.

Physics  MathMate, Wednesday, November 25, 2009 at 11:55pm
Gauge pressure is the pressure relative to the ambient atmospheric pressure.
A pascal (Pa) is the unit of measurement of pressure equivalent to 1 N (newton) acting on an area of 1 m².
To produce a pressure sufficient to suck water to a height of 187 mm, the pressure is equivalent to the total weight of 187 mm high of water in a container exactly 1 m³ in area.
Let g=acceleration due to gravity, m/s²
Pressure = mg N
=(0.187*1*1) m³ * 1000 kg/m³ * 9.8 m/s²
= 18326 pascals
= 18.326 KPa
Since to suck water, the pressure must be negative, so the required gauge pressure is 18.3 KPa.

Physicscorr  MathMate, Thursday, November 26, 2009 at 12:03am
Sorry, the numerical value was erred by a factor of 10. It should read:
= 1833 pascals
= 1.83 KPa
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